Chemistry for Engineers (Midterm) PDF

Chemistry for Engineers (Midterm) Yvonne Louise C. Arriesgado Kristel M. Salalila, LPT Milben Alejandro Bragais  Table of Contents Module 4: The Chemistry Of The Atmosphere 39 Introduction...

Chemistry for Engineers (Midterm) Yvonne Louise C. Arriesgado Kristel M. Salalila, LPT Milben Alejandro Bragais  Table of Contents Module 4: The Chemistry Of The Atmosphere 39 Introduction 39 Learning Objectives 39 Lesson 1.Atmospheric Composition 40 Lesson 2.Greenhouse gases and other atmospheric pollutants 42 Lesson 3. Layers of the Atmosphere 43 Lesson 4. Chlorofluorohydrocarbons (CFCs) 46 Summary 47 Assessment Task 48 References 49 Module 5: Soil Chemistry 50 Introduction 50 Learning Objectives 50 Lesson 1. Defining and Measuring Soil pH 51 Lesson 2. Discovering Ion Exchange-Measuring Soil pH in Different Solutions 52 Summary 53 Assessment Task 54 References 55 Module 6: Chemistry of Water 56 Introduction 56 Learning Objectives 56 Lesson 1. Properties of Water 57 Lesson 2. Water and Environment: Water Pollution 63 Lesson 3. Water Analysis and Treatment 63 Summary 66 Assessment Task 67 References 68 Module 7: Nuclear Chemistry 69 Introduction 69 Learning Objectives 69 Lesson 1. Radioactivity 70 Lesson 2. Nuclear Reactions 71 Lesson 3. Rates of Radioactive Decay 72 Lesson 4. Energy Changes in Nuclear Reactions 73 Summary 76 Assessment Task 77 References 78 List of Figures Module 4: The Chemistry of the Atmosphere 4.1 Percentage composition of the Earth’s Atmosphere 42 4.2 Four layers of Earth’s atmosphere in 3 dimensions as seen diagonally from above the exobase. 44 Module 6: The Chemistry of Water 5.1 Models of the water molecule and Lewis structure 57 5.2 A partially negative and partially positive water molecule. 57 5.3 Solvation of ions in water molecules. 59 5.4 Water molecule arrangement for solid (a), liquid (b) and gas(c) 59 5.5 Hydrogen bonds between water molecules 60 Module 7: Nuclear Chemistry 6.1 Nuclear Fission of U-235 73 6.2 Nuclear Fusion 74 List of Tables 4.1 Composition of the Earth’s Atmosphere; Dry Air near Sea Level 41 6.1 Some Physical and Chemical Properties of Water. 60 6.2 Impurities in water sources. 64  MODULE 4: THE CHEMISTRY OF THE ATMOSPHERE Introduction The atmosphere the different reactions that occurs on Earth is a complex topic but very essential in understanding different phenomena. Learning the composition of Earth’s atmosphere and finding the relationship of these elements and compounds in important in our survival on Earth (Marshall, 2017). The Chemistry of the Earth’s atmosphere has been an important topic in understanding the global changes and how we can maintain the protection provided by the atmosphere to the Earth’s inhabitants. Atmospheric chemistry serves as a foundation to help humans understand the phenomena related to the atmosphere. It focused on the chemical composition and chemical processes in the Earth’s atmosphere (Marshall, 2017). Greenhouse gasses, atmospheric pollutants, and ozone depletion are major concerns that threatens the life on Earth. Increase of greenhouse gasses and common atmospheric pollutants in the atmosphere, as well as ozone depletion are human-induced. Understanding the chemistry of the atmosphere and how it affects humans and the environment is important for the survival of life on Earth (Marshall, 2017). Learning Outcomes At the end of this module, students will be able to: 1. distinguish the composition and relative amounts of the various gases in the Earth’s atmosphere; 2. determine the different chemical reactions that occurs in the different layers of the atmosphere; and 3. explain how greenhouse gases and other atmospheric pollutants affects the Earth. 39 Lesson 1. Atmospheric Composition A. History Carbon dioxide is the main component of the early atmosphere together with small amount of water vapor, methane and ammonia, with the possibility of nitrogen, and little to no oxygen which is similar to Mars and Venus atmosphere (Marshall, 2017). The volcanoes are the source of carbon dioxide, water vapor, and nitrogen gas in the early atmosphere. The icy comets that hit the Earth may have added to the water supply. The earliest life on Earth evolved to survive with little to no oxygen. The atmosphere that we know today has stabilized approximately 200 million years ago, the proportions of different gases have been much the same since then (Marshall, 2017). Approximately 2.7 billion years ago when Algae first produced oxygen, through photosynthesis. Over the next billion years, plants evolved and the percentage of oxygen gradually increased to a level that enabled animals to evolve. Photosynthesis also decreased the percentage of the carbon dioxide of the early atmosphere. Formation of sedimentary rocks and fossil fuels that contain carbon also helped decreased the carbon dioxide in the atmosphere (Chemistry of the Atmosphere, 2020). B. Photosynthesis Photosynthesis is the process of converting light energy (E = h v) to chemical energy and storing it in the chemical bonds of sugar-like molecules performed by plants and algae. Photosynthesis helps the oxygen increase in the atmosphere. Algae and plants produced oxygen through photosynthesis as shown in the equation below (Chieh, 2015). (h v) Light Equation in symbol: 6CO2 + 6H2O C6H12O6+ 6O2 (eq.1) Chlorophyll Light Equation in word: Carbon dioxide + Water Glucose + Oxygen (eq.2) Chlorophyll (six molecules of water plus six molecules of carbon dioxide produce one molecule of sugar plus six molecules of oxygen.) The process of photosynthesis occurs in plants and some algae (Kingdom Protista) which needs only light energy, CO2, and H2O to make sugar. It occurs in the chloroplasts (chloromeans green and plastimeans formed or molded), specifically using chlorophyll (phyll means leaf), the green pigment involved in photosynthesis (Chieh, 2015). 40  C. Composition Earth’s atmosphere is about four-fifths (78.08%) nitrogen, about one-fifth (20.95%) oxygen, and small proportions of various other gases, including carbon dioxide, water vapor, and noble gases (Figure 1). Figure 4.1. Percentage composition of the Earth’s Atmosphere (siencetallis.weebly.com) The Earth’s atmosphere is an extremely complex system. Temperature and pressure changes over a wide range with the altitude. The atmosphere is bombarded by radiation and energetic particles from the Sun which has profound chemical effects, especially on the outer reaches of the atmosphere. Due to Earth's gravitational field, lighter atoms and molecules tend to rise to the top. With all these factors, the composition of the atmosphere is not uniform (Earth’s Atmosphere, 2020). Although traces of many substances are present, about 99 % of the entire atmosphere is composed of N2 and O2. The noble gases and CO2 make up most of the remainder. Table 1 show the composition by mole fraction of dry air near sea level (Earth’s Atmosphere, 2020). 41  Table 4.1. Composition of the Earth’s Atmosphere; Dry Air near Sea Level (wps.prenhall.com). Component* Content (mole fraction) Molecular Weight Nitrogen (N) 0.78084 28.013 Oxygen (O2) 0.20948 31.998 Argon (Ar) 0.00934 39.948 Carbon dioxide (CO2) 0.000355 44.0099 Neon (Ne) 0.00001818 20.183 Helium (He) 0.00000524 4.003 Methane (CH4) 0.000002 16.043 Krypton (Kr) 0.00000114 83.80 Hydrogen (H) 0.0000005 2.0159 Nitrous oxide (N2O) 0.0000005 44.0128 Xenon (Xe) 0.000000087 131.30 *Ozone, sulfur dioxide, nitrogen dioxide, ammonia, and carbon monoxide are present as trace gases in variable amounts. Lesson 2. Greenhouse gases and other atmospheric pollutants A. Greenhouse gases Greenhouse gases, at a certain level, is very important to maintain life on Earth. Water vapor, carbon dioxide, and methane are greenhouse gases. These gases maintain the temperature in the atmosphere that is high enough to support life (Chemistry of the Atmosphere, 2020). Carbon dioxide (CO2), among other gases in the atmosphere, absorb infrared radiation. Though it is only about 0.04% of the atmosphere, it is one of the most important gases that support life here on Earth. Together with Methane, carbon dioxide absorbs heat energy and prevent it from escaping into space. This process keeps the Earth warmer that is not totally a bad thing at a certain level. Too much of these gases though, will lead to global warming (Chemistry of the Atmosphere, 2020). There are human activities that contribute to the increase of greenhouse gases in the atmosphere like rice farming and cattle ranching. Both this activity produces methane gas, so as the number of rice field and cattle ranch increases, the amount of methane in the atmosphere also increases. On the other hand, burning fossil fuels releases CO2back into the atmosphere. Land 42 clearing for farming and logging also releases CO2as well as reducing the vegetation that perform photosynthesis which also helps in reducing CO2 (Chemistry of the Atmosphere, 2020). B. Common Atmospheric Pollutants The major source of atmospheric pollutants is the combustion of fuels. Like coal, most fuels contain carbon and / or hydrogen and may also contain some sulfur. When the fuel is burned, gases like carbon dioxide, water vapor, carbon monoxide, sulfur dioxide, and oxides of nitrogen are released into the atmosphere. Particulates in the atmosphere are from the solid particles and unburned hydrocarbons. These particulates cause global dimming and health problems for humans. Sulfur dioxide(SO2) and oxides of nitrogen also causes respiratory problems in humans and causes acid rains. Lastly, the carbon monoxide (CO) is known to be a toxic gas that is not easily detected because it is colorless and odorless. It is also harmful to humans and the environment (Chemistry of the Atmosphere, 2020). Many of these pollutants have a direct impact on humans. Ozone O3causes irritation of eyes, lungs, and nose; shortness of breath and coughing particularly when exercising. Children and asthmatics are the most vulnerable to these irritations caused by O3. Formaldehyde (HCHO) also causes irritation and allergies and possibly carcinogenic. Nitrogen dioxide(NO2)has negative impact on respiratory system particularly on children. Carbon monoxide (CO) binds to hemoglobin in blood and impairs ability for oxygen transport that causes headache, fatigue, and, at high concentrations, death(Richter, 2019). Lesson 3. Layers of the Atmosphere There are four layers of the atmosphere according to the variation of temperature. The division is made based on the temperature variations as the altitude increases. This is bases on the study of the atmospheric scientists, P. Crutzen, M. Molina and F. S. Rowland, who were awarded with the Nobel Prize in Chemistry in 1995for their work in atmospheric chemistry, particularly concerning the formation and decomposition of ozone (Chieh, 2015). The four layers of the atmosphere are listed below and is shown in Figure 2 (from NASA, lifted from Chieh, 2015). 1. Ionosphere (Aurora) or Thermosphere 3. Stratosphere 2. Mesosphere 4. Troposphere 43  Figure 4.2. Four layers of Earth’s atmosphere in 3 dimensions as seen diagonally from above the exobase. Image is from NASA, lifted from Chieh, 2015 (chem.libretexts.org). Layers drawn to scale, objects within the layers are not to scale. Aurorae shown at the bottom of the thermosphere can actually form at any altitude in this atmospheric layer. Thermosphere and ionosphere can be found above 100 km where the temperature increases from 200 K at 100 km to 500 K at 300 km. The temperature increases as the altitude increases. In the outer space, most particles consist of single atoms like H, He, and O among others. At lower altitude, 200 - 100 km, diatomic molecules like N2, O2, and NO are present. The ionosphere is full of electrically charged ions. The UV rays ionizes these gases (Chieh, 2015). The major reactions are: In the ionosphere: O+hv→O++e− (eq.3) N+hv→N++e− (eq.4) 44  In the neutral thermosphere:N+O2→NO+O(eq.5) N+NO→N2+O(eq.6) O+O→O2(eq.7) The ionosphere and exosphere is located beyond the neutral thermosphere. These layers are of interest for space exploration and space sciences. In the outer space, the atmosphere is more like a plasma than a gas. Below the thermosphere is the mesosphere, 100 - 50 km, in which the temperature decreases as the altitude increase. In this region, OH, H, NO, HO2, O2, and O3 are common (Chieh, 2015). The most prominent chemical reactions are: H2O+hν→OH+H (eq.8) H2O2+O→OH+OH (eq.9) Below the mesosphere is the stratosphere. In this layer, the temperature increases as the altitude increase from 10 km to 50 km (Chieh, 2015). In this region, the following reactions are common: NO2→NO+O(eq.10) N2O→N2+O(eq.11) H2+O→OH+H(eq.12) CH4+O→OH+CH3(eq.13) In the stratosphere, air flow is horizontal. A high concentration of ozone is present in the thin ozone layer in the upper stratosphere which is primarily responsible for absorbing the ultraviolet radiation from the sun (Chieh, 2015). The ozone is generated by these reactions: O2+hν→O+O(eq.14) O2+O→O3(eq.15) Ozone is an unstable form of oxygen with of three oxygen atoms bonded together. It is formed due to photo reaction. Most of the ozone in the atmosphere is in the stratosphere of the atmosphere, with only about 8% in the lower troposphere. Dobson Unit (DU) is used to measure ozone level. It is named after G.M.B. Dobson, who investigated the ozone between 1920 and 1960. One Dobson Unit (DU) is equal to 0.01 mm thickness of ozone at STP (standard temperature and pressure) when all the ozone in the air column above an area is collected and spread over the entire area. Thus, 100 DU is 1 mm thick (Chieh, 2015). 45  When an oxygen molecule receives a photon (h\nu), it dissociates into monoatomic (reactive) atoms. These atoms attack an oxygen molecule to form ozone, O3. As shown in equation 14 and 15 above. The region of rising and falling packets of air is in the troposphere where all weather takes place. The air pressure at the top of the troposphere is only 10% of that at sea level or 0.1 atmospheres. Tropopause is the thin buffer zone between the troposphere and the next layer (Chieh, 2015). The major components of the atmosphere in the region close to the surface of the Earth are N2 (78%), O2 (21%), Ar (1%) with variable amounts of H2O, CO2, CH4, NO2, NO2, CO, N2O, and O3. About 8% of the total ozone in the atmosphere is in the troposphere (Chieh, 2015). Lesson 4. Chlorofluorohydrocarbons (CFCs) CFCs or freon are made up of carbon, hydrogen, fluorine, and chlorine. The three digits, related to the molecular formulas, is used in DuPont to distinguish their product. Before the usage of freon, Chemist Roy J. Plunkett discovered tetrafluoroethylene resin while researching refrigerants at DuPont. It was known by its trade name, Teflon. Plunkett's discovery was found to be extremely heat-tolerant and stick-resistant. It was then introduced in 1949 after years of research. His continued research eventually led to the usage of chlorofluorohydrocarbons known as CFCs or freon as refrigerants (Chieh, 2015). It has been discovered that CFCs is depleting the ozone. The depletion of ozone is due to the presence of chlorine in the troposphere, and eventually their migration to the stratosphere. A major source of chlorine is Freons: CFCl3 (Freon 11), CF2Cl2 (Freon 12), C2F3Cl3 (Freon 113), C2F4Cl2 (Freon 114). Freons decompose in the troposphere. For example: CFCl3→CFCl2+Cl ( eq.16) CF2Cl3→CF2Cl+Cl (eq.17) The chlorine atoms catalyze the decomposition of ozone, Cl+O3→ClO+O2(eq.18) and ClO molecules further react with O generated due to photochemical decomposition of ozone: O3+hν→O+O2(eq.19) 46  ClO+O→Cl+O2 (eq.20) O+O3→O2+O2(eq.21) The net result or reaction is 2O3→3O2(eq.22) The U.S. including about two-dozen other countries signed the Montreal Protocol in 1987 to phase out the use of CFCs. Since 2010, there is a global ban on the use of CFCs. Summary Atmospheric stratification describes the structure of the atmosphere. The atmosphere structure is divided it into distinct layers describing each specific characteristic such as temperature or composition. The layers of atmosphere by temperature, includes thermosphere, mesosphere, stratosphere, and troposphere. Atmosphere composition, specifically at dry air composition, by volume, is roughly 78.08% nitrogen, 20.95% oxygen, 0.93% argon, 0.036% carbon dioxide, and small amounts of other gases. The atmosphere is very important as it protects life on Earth by absorbing ultraviolet solar radiation and warming the surface through heat retention or the greenhouse effect. Specifically, the ozone protects the Earth from the harmful UV radiation from the sun, however, human activities have caused its destruction. The ozone layer is continuously recovering and with the knowledge in atmospheric chemistry, we can refrain from using products that can harm or disrupt the recovery of the ozone layer. 47  Assessment Task 4 1. Discuss the different layers of the atmosphere and the different chemical reactions occurring in each layers. 2. Discuss the ozone layer. Where is it located in the atmosphere? What is the molecular structure and how is it formed? 3. Discuss the anthropogenic activities that cause pollution in the atmosphere and how it affects humans and the environment. 48  References Chemistry of the Atmosphere. (2020). Retrieved 22 October 2020, from https://sciencetallis.weebly.com/9-chemistry-of-the-atmosphere.html Chieh, C. (2015). Atmosphere. Retrieved 23 October 2020, from https://chem.libretexts.org/Bookshelves/Environmental_Chemistry/Supplemental_Modul e s_(Environmental_Chemistry)/Atmospheric_Chemistry/Atmosphere Chieh, C. (2015). Photosynthesis. Retrieved 23 October 2020, from https://chem.libretexts.org/Bookshelves/Environmental_Chemistry/Supplemental_Modul e s_(Environmental_Chemistry)/Atmospheric_Chemistry/Photosynthesis Earth’s Atmosphere (2020). Chapter 18, Section 1. Retrieved 23 October 2020, from https://wps.prenhall.com/wps/media/objects/3083/3157782/blb1801.html Earth’s Atmosphere | Introduction to Chemistry. (2020). Retrieved 22 October 2020, from https://courses.lumenlearning.com/introchem/chapter/earths-atmosphere/ Marshall, F. (2017). Chemistry of the atmosphere - Gojimo. Retrieved 22 October 2020, from http://www.gojimo.com/chemistry-of-the-atmosphere/ National Research Council. 1999. Charter 5 Changes in the Chemistry of the Atmosphere. Global Environmental Change: Research Pathways for the Next Decade. Washington, DC: The National Academies Press. doi: 10.17226/5992. Richter, A. (2019). Atmospheric Chemistry. Retrieved 23 October 2020, from http://www.iup.unibremen.de/doas/lectures/richter_atmospheric_chemistry_02.pdf 49  MODULE 5: SOIL CHEMISTRY Introduction Soil is a mixture of inorganic and organic solids, air and water. Soil chemistry involves the chemical reactions and processes between these components and particularly focuses on investigating the fate of contaminants and nutrients within soils. Knowledge of soil chemistry allows scientists to monitor, control and predict the effects of pollutants in the environment. Chemical knowledge combined with understandings from the Earth sciences, physics and biology are needed to understand, prevent and remediate environmental issues with soils (Rate, Mcelnea, and Bucat, 2014 and 2011). Learning Outcomes At the end of this module, students will be able to: 1. discuss soil chemistry; 2. explain how to measure soil pH in different solutions; and 3. determine the process of defining and measuring soil pH. 50 Lesson 1. Defining and Measuring Soil pH (Rate, Mcelnea, and Bucat, 2011 and 2014) According to Rate, Mcelnea, and Bucat (2011 and 2014), the pH of a solution is used to describe how acidic or alkaline a substance is. Pure water is neutral and has a pH of 7. Substances with a pH of less than 7 are considered acidic (e.g. vinegar) and those greater than 7 are alkaline (eg. ammonia). In solutions, pH is a measure of hydrogen ion activity and is defined as -log10{H+}, where {H+} is hydrogen ion activity. As pHis measured on a logarithmic scale, the difference between each level is 10 times, so the difference between 3 levels would be 10 x 10 x 10 = 1000 times (e.g. a substance with pH 4 is 1000 times more acidic than a substance with pH 7). They also stated that in soils, pH is also a useful concept and is defined as -log10{H+} in a solution which is in chemical equilibrium with soil particles. In practice, soil pH is measured by shaking a known mass of soil (to make a suspension) in a defined volume of water or dilute salt solution, and then the pH is measured using a calibrated pH electrode. Soil pH In the book Environmental Chemistry used by the authors Rate, Mcelnea, and Bucat (2011 and 2014) they discussed that soil pH is an important property that affects a surprisingly large range of chemical, physical and biological processes in soils. Through an intricate and interlinked set of chemical reactions and other biophysical processes, soil pH itself depends on other soil properties, such as the amount and type of minerals present, the organic matter content, and the dynamics of water and oxygen. Soil pH is important because of the effect that is has on the accessibility of essential elements, or nutrients, in soils. For example, many elements can be taken up by plants more easily if the soil pH is nearneutral to marginally acidic (about pH 6-7). Outside this pH range, plants may be deficient in some elements, or some elements may become toxic. SOIL pH BUFFERING Also stated by the authors, some of the solid particles in soils are surprisingly chemically reactive. Depending on their composition, soils can resist or buffer changes in pH, because of different reactions between soil particles and acidic or alkaline water. Of the materials present in soils, two of the most important are the clay minerals and organic matter (humus), which also have the important property of existing as very small particles. These particles are often less than 0.001 mm or 10-6 m. Soil particle size is usually measured in microns (µm), 1 µm = 10-6 m. 51  Furthermore, clay minerals from the phyllosilicate group are very important in soils. Most naturally occurring clay minerals have a crystal structure with a charge imbalance; the anion charge is greater than the cation charge. The charge deficit is balanced by cations (e.g. Na+, Ca2+, H+) that do not form part of the clay structure, but instead are attracted weakly to the surface of clay particles by electrostatic forces. Soil organic matter generally consists of very large molecules, which have weakly acidic functional groups (mainly carboxylic acid, R–COOH) as part of their chemical structure. Depending on the pH of the soil, these groups dissociate, leaving a negatively charged (anionic) particle (having many R–COO- groups). This negative charge can also be balanced by any cation. The weakly associated cations are in rapid equilibrium, and can ‘swap’ for other cations in a process known as ion exchange. Atoms are made up of protons, neutrons and electrons. The protons and neutrons are found in the center of the atom, called the nucleus. The electrons move around the nucleus in orbitals. Each particle carries a different electrical charge: neutrons are neutral, protons have a positive charge and electrons have a negative charge. Ions When an atom loses or gains electrons, it is called an ion. A cation is an atom (or group of atoms) that has lost electrons and therefore has a net positive charge (e.g. H+, NH4+). An anion is an atom (or group of atoms) that has gained electrons and therefore has a net negative charge (e.g. O2-, OH-). Lesson 2. Discovering Ion Exchange – Measuring Soil Ph In Different Solutions According to Rate, Mcelnea, and Bucat (2011 and 2014), soil pH can be measured in several different solutions, each one giving a slightly different result. Two of the most common solutions are deionised water (which is readily available) and 0.01 mol/L CaCl2 solution (used because it is thought to, very approximately, mimic the composition of the pore water in soils). The existence of ion exchange on clays and other small soil particles means that the ionic composition of a solution used for pH measurement will affect the result. In most cases, the higher the ionic strength of the measurement solution, the lower the measured pH (soils are variable and complex, so a pH decrease is not always observed). A higher concentration of some ions (e.g. Ca2+) perturbs the ion exchange equilibrium so that hydrogen ions are released into the aqueous phase of the suspension. 52 Acid Sulfate Soils According to Rate, Mcelnea, and Bucat (2011 and 2014) acid sulfate soils are a significant environmental issue and more than one scholarly article has described acid sulfate soils as ‘the nastiest soils in the world. Acid sulfate soils (ASS) form following human disturbance of soils or sediments, which contain sulfide minerals, commonly the iron disulfide mineral (pyrite or FeS2 ). Pyrite and other sulfide minerals such as mackinawite (FeS) and greigite (Fe3 S4 ) are chemically stable under reducing conditions, such as where there is very little oxygen. In this state these soils are known as potential acid sulfate soils (PASS). In the presence of oxygen, however, sulfide minerals oxidise to produce sulfuric acid, forming actual acid sulfate soils (AASS). Furthermore, if no alkaline mineral (e.g. calcite, CaCO3) is present to neutralise the acid formed, the soil becomes permanently acidic. This acidification starts off a cascade of other effects; other soil minerals may dissolve, releasing toxic ions such as Al3+ and some trace elements into waterways (Rate, Mcelnea, and Bucat, 2011 and 2014). Summary The pH of a solution is used to describe how acidic or alkaline a substance is. Pure water is neutral and has a pH of 7. Substances with a pH of less than 7 are considered acidic. Substances greater than 7 are alkaline. Soil pH is an important property that affects a surprisingly large range of chemical, physical and biological processes in soils. Some of the solid particles in soils are surprisingly chemically reactive. Atoms are made up of protons, neutrons and electrons. The protons and neutrons are found in the center of the atom, called the nucleus. Soil pH can be measured in several different solutions, each one giving a slightly different result. Acid sulfate soils are a significant environmental issue and more than one scholarly article has described acid sulfate soils as ‘the nastiest soils in the world. 53  Assessment Task 7-1 1. What do you think are the effects of acid in soil? Give 3 and include your sources. 2. Wild Watering (adopted from Environmental Chemistry (n.d.) retrieved from https://www.researchgate.net/publication/258521390_Soil_Chemistry) Procedure: Investigate the importance of clean water by ‘watering’ some seedlings or plants with different liquids. Think of different liquids to water your chosen plants with, for example: water (control), milk, cola, soy sauce, vinegar, soapy water, etc. Make sure that all other variables (e.g. light, amount of liquid, plant type, soil type, etc.) remain the same. Research and discuss the possible impacts of chemical runoff on soil and the environment. 54  Reference Rate, A.W., Mcelnea, A.E., and Bucat, R (2011 and 2014). React to Chemistry-a resource book of Ideas for National Science Week 2011 (pg.34-38). Australian Science Teachers Association. Retrieved from https://www.researchgate.net/publication/258521390_Soil_Chemistry 55  MODULE 6: The Chemistry of Water Introduction Water is one of the most essential compounds that exist in nature and sustains life. Without it, basically the earth is a dry, lifeless planet. Around 70 percent of the earth is made up of water, thus, it serves as a coolant of our planet and helps moderate the earth’s temperature. (Zumdahl, 1998). Our bodies and our cells are mostly composed of fluids and water as well maintaining the body temperature. Most living organisms require water as it is important component in chemical and biological systems such as photosynthesis, cellular respiration and metabolism. Rain and freshwater resource are generated through the process called water cycle. Water is called the “universal solvent” since it is naturally found in most places and it is able to dissolve different substances. There are three main sectors that use water such as industrial like factories, resorts and pools, and malls; agricultural- such as used in irrigation, water systems and plant propagation, hydroponics and domestic- such as used in cleaning, cooking, washing and bathing. Water is also a source of a clean, renewable energy in a form of hydroelectric power. Water in its gaseous from called the steam is high in kinetic energy and therefore is used to power steam turbines. Learning Outcomes At the end of this module, students should be able to: 1. Characterize and identify the chemical and physical properties of water; 2. Explain water pollution and its causes; 3. Discuss some of the methods of water analysis and treatment; and 4. Propose methods of rehabilitating and preserving local water resource 56 Lesson 1. The Water Molecule and its Properties A. The Water molecule A single water molecule is formed by polar covalent bonds of two hydrogen atoms bonded to one oxygen atom- hence the molecular formula is H2O. Covalent bonds involve the sharing of electrons between the atoms. For water, both oxygen and hydrogen share the electrons to form a bond. However, the sharing of electrons is unequal. 105° a. 3D b. ball and stick c. Lewis structure Figure1. Models of the water molecule and Lewis structure Water is a polar molecule, meaning it has both partial positive and partial negative poles. This is due to the presence of a highly electronegative oxygen atom withdrawing the electron cloud away from the two hydrogen atoms. The partially-charged poles are symbolized by Debye (δ). In the following illustration, the oxygen atom attracts the electron cloud toward itself making it more negative and thus obtaining a partially negative Debye. The ability of an atom to attract the electrons to itself is called electronegativity. On the other hand, the hydrogen atoms since they are less electronegative, they tend to lose the electron cloud away thus making them partially positive (Zumdahl, 1998). Figure2. A partially negative and partially positive water molecule. 57  Water undergoes the process called autoionization. Due to its polar character, molecules of water easily interact with each other forming ions. In equilibrium, water forms an – OH hydroxide ion which is the base and an H3O+ or H+ hydronium ion which is the acid. Thus, say in a glass of water, this reaction occurs simultaneously and in equilibrium (Zumdahl, 1998). H-O-H + H-O-H H3O+ + -OH Equation 1. Autoinization reaction of water. The corresponding equilibrium expression for this reaction is: K = [H3O+] [OH- ] [H2O ] where K = equilibrium constant [ ] = molar concentration [H O] Kw = [H+] [OH- ] where Kw = equilibrium constant for water pH is negative log of activity of the Hydrogen ion = - log[H+] In an equilibrium reaction, positive ions in solution must be balanced by negative ions [H+] = [OH- ] Kw = [H+] [OH- ] (pH) and substituting [H+] for [OH- ], Kw = [H+]2 Taking the negative logarithm –log [H+] = –½log Kw Just as pH represents –log [H+] it is convenient to use pK to represent –log Kw pH = -log[H+] = -½log Kw = ½ pKw In dilute aqueous solution at 25ºC, Kw = 10-14, the pH of pure water is 7.0 As mentioned earlier, water is the common solvent called as the universal solvent since it can dissolve many substances. The process of dissolving solutes is called solvation (Zumdahl, 1998). In solvation, the solute particles are surrounded by the water molecules in an orientation following its charges. Thus, the ions or particles are being hydrated. Say, in a solution containing water and calcium carbonate, CaCO3, the positive calcium ions are solvated by water molecules in such a way that the partially negative oxygen atoms are attached. While for the negative carbonate ion, the partially positive hydrogen ions are attracted. 58  Figure3. Solvation of ions in water molecules. B. Properties of Water Water at room temperature is a tasteless, colorless liquid. One interesting characteristic of water is it can exist in three states without changing its molecular identity-only its energy and particle attraction. The change in phase of water is mainly due to two factors: temperature and pressure (Bauer, Birk, Marks, 2016). Water, at very low temperature and low kinetic energy, can exist as solid in a form of ice, snow and permafrost found in the glaciers. At room temperature, water exists in its liquid state. At very high temperature and high kinetic energy, water exists as gas. a b c Figure4. Water molecule arrangement for solid (a), liquid (b) and gas(c) Based from the figures above, water in the solid phase has molecules that are close to each other. Here there is a strong intermolecular force of attraction. However, the energy of the molecules is low. At low temperature, water expands and increases it volume because it forms a crystal lattice. Each water molecule is attached to four water molecules making solid water rigid and stable. Ice crystals are arranged in hexagonal orientation thus increasing its volume. The density of water at 0 °C is 0.9167 g/ml (Zumdahl, 1998). At room temperature, water is in liquid state. In this phase, water is at the highest density compared to the other phases. Liquid water density is 0.9998 g/ml (Bauer, Birk, Marks, 2016). Since the solid water is less dense than the liquid water, ice floats on water. 59  As mentioned, water is considered as the universal solvent. This is primarily due to its polar nature. According to the solubility rules, “like dissolves like”. Therefore, substances or compounds having the same polarity can be mixed or can be dissolved. One classic example is oil and water. Oil is a nonpolar compound while water is a polar compound. Thus, due to their differences in polarity, they are unable to mix or be dissolved. The following table lists some of the physical properties of water. Table 1. Some Physical and Chemical Properties of Water Molecular formula H2O Molecular weight 18.01528 g/mol Molecular Geometry Bent, V-shaped Boiling Point 100 °C Freezing Point 0 °C Density 1 g/mL or 1000kg/m3 at 4 °C Polarity Polar pH 7.00 1. Hydrogen Bonds What is special about water is that its molecules are able to form strong intermolecular forces of attraction called the hydrogen bonds. (Zumdahl, 1998). The hydrogen bonds occur in between the hydrogen atom of one water molecules connecting to the O-H group of another water molecule. Since the water molecule is charged, the bonds are formed through the attraction of charges. Remember that opposite charges attract each other. The partially positive hydrogen atom is attracted to the partially negative oxygen atom of the other water molecule Figure 5. Hydrogen bonds between water molecules 60  2. High Specific Heat Specific heat is defined as the amount of heat one gram of a substance must absorb or lose to change its temperature by one degree Celsius. For water, this amount is one calorie. Water has the highest specific heat capacity of any substance that naturally exist as liquid at room temperature and pressure caused by strong hydrogen bonding. The specific heat of water is 4.184 J/ g °C and this is equivalent to 1 cal (Bauer, Birk, Marks, 2016). This means that for example, for 1 gram of water in order to increase the temperature, say from 55 °C to 56 °C, 4.184 J of heat is needed. Now, to decrease the temperature of a 1 g of a substance by 1°C, you need to remove 4.184 J of heat. 3. High Heat of vaporization Vaporization is the process of phase change from liquid to gas (Zumdahl, 1998). This occurs at the surface of the water wherein the water molecules escape into gas. Vaporization in water involves an addition of heat. Heat increases the kinetic energy of the molecules thus breaking the hydrogen bonds. Heat of vaporization is the amount of energy required to change one gram of a liquid substance to a gas. Water has a high heat of vaporization equivalent to 586 cal. Thus, 586 cal is needed to completely convert 1 gram of water into gas. This property enables water to be used as an inexpensive coolant for car engines, in factories and other industries. Water can absorb more heat before turning into vapor compared to other substances. The cooling effect of evaporation is observed in perspiration in a form of sweat. A substantial amount of energy is absorbed by the sweat on our skin as it evaporates making the body much cooler. This maintains our body temperature called as homeostasis (Bauer, Birk, Marks, 2016). 4. Molar heat of vaporization Related to heat of vaporization, molar heat of vaporization is the amount of heat needed to evaporate 1 mole of a given amount of liquid at its boiling point. The strong intermolecular forces of attraction in water in the form of hydrogen bonds results in higher heat of vaporization. The molar heat of vaporization of water is 40.7 KJ/mol (Zumdahl, 1998). 61  5. Adhesion and Cohesion Adhesion is the attraction between water molecules and molecules of other substances or objects. Adhesion is observed when water “climbs” up the tube placed in a glass of water: notice that the water appears to be higher on the sides of the tube than in the middle. This is because the water molecules are attracted to the charged glass walls of the tube more than they are to each other and, therefore, adhere to the tube. This type of adhesion is called capillary action. This process is also involved in the movement of water and nutrients from the root systems in the soil to other parts of plants above the ground, as well as the movement of blood through the veins in the human body (Zumdahl, 1998). When you observe water inside a graduated cylinder or a tube, you will observe a meniscus. A meniscus can be concave or convex but in the case of water, it is a concave meniscus. The lower meniscus is the result of a greater attraction between the water molecules than that of the water and the walls of the graduated cylinder or tube. (Bauer, Birk, Marks, 2016). Cohesion happens when water molecules are attracted to other water molecules (because of hydrogen bonding), keeping the molecules together. You can observe this in droplets of water on glass surfaces or the dew hanging on the tip of a leaf. Cohesion is related to surface tension. Surface tension is the energy needed to break through the surface of a liquid. The higher surface tension means, a high energy is required to break through the surface. Surface tension enables water to act like a huge membrane-like structure. MINI- EXPERIMENT Try this in your home! Pour water on a shallow basin. Carefully, place a dry needle horizontally on the surface and let it float. Why did it float? Place a drop of dishwashing liquid on your finger ps. Touch the water near the needle with the dishwashing liquid. What happened? You observed that the needle float on water. This is due to surface tension. The water molecules on the surface are like “blanket” pushing the needle upward. Upon the addition of dishwashing liquid, the water-water interactions are disrupted and broken. This caused the lowering the surface tension of water thus the needle sinks. 62 Lesson 2. Water and the Environment: Water Pollution About 71% of the Earth’s surface is covered by water, most of which is in oceans and unavailable for human consumption due to its high salinity. (https://www.nationalgeographic.org/encyclopedia/water-worlds). Approximately 97% of all water is saline and 2% is fresh water held in ice caps and glaciers. Therefore, at least 99% of all water is generally unsuitable for human use because of salinity (ocean water) and location (ice caps and glaciers). However, less than 1% of total water as fresh water is available for consumption (http://water.usgs.gov/edu/earthwherewater.html). Water pollution is one of the biggest crises in the world. Yearly, around 3.6 million people die from water-related disease worldwide (https://www.theworldcounts.com/challenges/planetearth/freshwater/deaths-from-dirty- water/story). Water pollution occurs when undesirable, toxic substances enter the bodies of water such as lakes, rivers, ponds and other water resources. These contaminants can be chemical in the form of substances such as pesticides and oil, inorganic metals such as lead, mercury and chromium or can be biological such as the coliform bacteria, E.coli from human feces and viruses. There are also radioactive contaminants that came from radioactive sources such as the nuclear wastes. These contaminants are harmful to the body and can cause gastrointestinal diseases, mutation, poisoning and even death. In other words, pollution decreases the quality of water (http://inside.mines.edu/~epoeter/_GW/17WaterChem1/WaterChem1pdf). Lesson 3. Water Analysis and Treatment Of the available fresh water, approximately 97% is groundwater stored deep below the surface of the Earth and only 1.4% is surface water in rivers and lakes. (http://water.usgs.gov/edu/earthwherewater.html). Due to the very limited resource of freshwater as potable drinking and usable water, treatment of water is applied. This is to ensure that the water that we use is clean and safe for human consumption and usage. Since water is mostly found underground and on open, uncontrolled bodies of water, there are possibilities of that it contain impurities or contaminants. The following table lists the common contaminants or impurities found in water sources. 63  Table 2. Impurities in water sources. Source Cations (Positive ions) Anions (Negative ions) Contact of water with Ca+2, Fe+2 Mg+2, Mn+2 K+, Na+, Zn+2 HCO3 - , CO3 -2 Cl- , F- , NO3- minerals, soils, and rocks PO4 -3, OH - , SO4 -2 H2BO3 - The atmosphere (rain) H+ HCO3 - , Cl- , SO4 -2 Decomposi on of organic materNH4 +, H+, Na+ Cl- , HCO3 - , OH - , NO2 - NO3 - HS, in the environment Organic radicals Municipal, industrial, and Inorganic ions, including Inorganic ions, organic agricultural sources and other a variety of heavy metals molecules human ac vity (Pb and Hg) The presence of ions such as Ca2+ and Mg2+ and carbonates makes the water difficult to react with laundry detergents and soaps. This is called hardwater. Hardwater is the term for water sources that contains more cations or minerals. The reaction of minerals with soaps results in the formation of white-calcified solid materials that is found in the faucets, showers and sinks called as hardwater scum. On the other hand, a softwater is water that has low dissolved minerals. The following equation shows the reaction of hard water with soap. 2 C17 H35COO- Na+(s) + CaCl2 2 C17 H35COO- Ca2+(s) + 2NaCl Soap calcium chloride white precipitate salt Hard water has some benefits such as it supplies important minerals in the body such as magnesium and calcium for a stronger teeth and it prevents the corrosion and dissolution of metals. 1. Common Types of water analysis (https://sheltonswater.com/blog/top-7methods- of-water-treatment) a. Temperature and pH testing Testing the temperature helps determine the rate of biochemical reaction in an aquatic environment and indeed whether they are able to occur at all. If the water temperature is too elevated, this can limit the water’s ability to hold oxygen and decrease organisms’ capacity to resist particular pollutants. To measure the acidity of water, pH testing is used. Most aquatic organisms are only able to survive within a pH range of 6 to 8. b. Chloride test and dissolved oxygen test Chloride is usually present in fresh and salt water. However, its levels can be derived as a result of minerals dissolving and industrial pollution. In dissolved oxygen test, it measures the 64 amount of oxygen dissolved in water. Without this, aquatic life is unable to conduct cellular respiration and is thus a key indicator of water health. c. Turbidity test Measures the amount of particulate matter that is suspended in the water, or more simply, how clear the water is. If high levels of turbidity are present, photosynthesis is affected as light is unable to penetrate, increasing water temperature. d. Metals Testing that indicates the presence of a suite of metals which are not naturally occurring in water. Heavy metals (Aluminium, Antimony, Arsenic, Beryllium, Bismuth, Copper, Cadmium, Lead, Mercury, Nickel, Uranium, Tin, Vanadium and Zinc) can find their way into water bodies through natural processes or human activities such as mining, processing of minerals, use of metals as containers and transportation through metallic pipelines. Heavy metals are known to harm kidneys, liver, nervous system and bone structure. 2. Types of Water treatment (https://ade.group/laboratory-analysis-water-testing) a. Coagulation / Flocculation In coagulation, liquid aluminum sulfate or alum and/or polymer is added to raw or untreated water. Then, the mixture causes the dirt particles in the water to coagulate or stick together. As a result, larger particles are formed called flocs. These flocs can easily removed by filtration or settling. b. Sedimentation In sedimentation, the flocs undergo the treatment process in sedimentation basins. The water moves slowly, making the heavy floc particles settle to the bottom. Then, the accumulated flocs that are found at the bottom is known as sludge. The sludge is carried on to drying lagoons and are removed by filtration. c. Filtration In filtration, water passes through a filter, which is made to take away particles from the water. Such filters are composed of gravel and sand or sometimes pulverized carbon or activated carbon. 65  d. Disinfection Addition of disinfectant such as Chlorine is applied since it is very effective in killing microorganisms. e. pH Correction To adjust pH levels, lime is combined with filtered water. This, also, stabilizes naturally soft water so corrosion can be minimized in the water distribution system and plumbing of customers. Summary Water is a naturally occurring compound essential to life. There are three main sectors that use water. These are industrial, agricultural and domestic. A single water molecule (H2O) is formed by polar covalent bonds of two hydrogen atoms bonded to one oxygen atom. Water at room temperature is a tasteless, colorless liquid. One interesting characteristic of water is it can exist in three states without changing its molecular identity-only its energy and particle attraction. Water is a polar molecule, it has both partial positive and partial negative poles. This is due to the presence of a highly electronegative oxygen atom withdrawing the electron cloud away from the two hydrogen atoms. The partially-charged poles are symbolized by Debye (δ). Water molecules interact via hydrogen bonding. The ability of an atom to attract the electrons to itself is called electronegativity. Water undergoes the process called autoionization. In equilibrium, water forms an –OH hydroxide ion which is the base and an H3O+ or H+ hydronium ion which is the acid. 66  The change in phase of water is mainly due to two factors: temperature and pressure. Some of the properties of water are high specific heat, high heat of vaporization, molar heat of vaporization, adhesion and cohesion including surface tension. Water pollution is one of the major environmental problems that can be of natural causes or human-made or anthropogenic causes. Some methods of water testing are temperature and pH testing, Chloride test and dissolved oxygen test, Turbidity test and Metal tests. Some of the methods of water treatment are Coagulation / Flocculation, Sedimentation, Filtration, Disinfection and pH Correction. Assessment Task 1. Discuss the different properties of water. 2. Identify one natural water resource in your community. Conduct a simple survey and research about that water resource. 3. Propose ways on how prevent water pollution of your chosen natural resource and its water treatment. 67  References Bauer, Richard., Birk, James., Marks, Pamela. General Chemistry (Books I and II). 2016. Mc-Graw Hill Education. Zumdahl, Steven S. Chemical Principles. 3rd ed. 1998. Houghlin-Millfin Co. Deaths from dirty water and related diseases. Retrieved 6 October 2020 from https://www.theworldcounts.com/challenges/planet-earth/freshwater/deaths-from- dirtywater/story Top 7 Methods of Water Treatment. 7 November 2015 [Blog post]. Retrieved 3 October 2020, from https://sheltonswater.com/blog/top-7-methods-of-water-treatment Water Chemistry 1. Retrieved 3 October 2020, from http://inside.mines.edu/~epoeter/_GW/17WaterChem1/WaterChem1pdf. Water Quality testing. Retrieved 3 October 2020, from https://ade.group/laboratoryanalysis-water-testing Water worlds. National Geographic Resource Library. 2020. Retrieved 6 October 2020, from https://www.nationalgeographic.org/encyclopedia/water-worlds/ 68  MODULE 7: NUCLEAR CHEMISTRY Introduction Nuclear Chemistry, a branch of Chemistry, studies the changes which occur in the nucleus of various elements in the environment. Such changes then result in radioactivity and nuclear power. With the vast application and therefore deemed imperative, there is a need for everyone to grasp the concept of radioactivity and nuclear power (Nuclear Chemistry, n.d.) Learning Outcomes At the end of this module, students will be able to: 1. Identify how the concentration of radioactive material changes with time; and 2. Determine nuclear binding energies and the amount of energy released in a nuclear reaction. 69 Lesson 1. Radioactivity Radioactivity is the process where unstable atomic nuclei release energetic subatomic particles. It was first discovered by a French scientist Henri Becquerel in 1869, after which the SI unit for radiation, Becquerel, was named (Sofian, 2016) According to Bauer, Birk, and Marks (2019), radiation is a transmitted energy that comes from a source and travels through matter or space. Two types of radiation: a) Electromagnetic forms including visible light, infrared and ultraviolet, as well as gamma rays and X-rays. b) Particulate radiation is mass given off from unstable atoms with the energy of motion. (e.g. beta particle and alpha particles) Unstable atoms have an excess of matter, energy, or both. If they spontaneously give off this excess as ionizing radiation, they are radioactive. An element is composed of atoms that have the same number of protons in their nuclei. The atoms merely rearrange and/or transfer electron from one element to another. In contrast, nuclear reactions involve changes in the composition of atomic nuclei. The changes usually result in changes in the number of protons in the nuclei, thereby changing the identity of the elements. The study of nuclear chemistry, then, focuses on the proton and neutron in the nucleus. These nuclear particles are called nucleons. Isotopes – atoms with the same number of protons (atomic number Z), but different number of neutrons (N) and mass numbers (A). Can be expressed as A zX An atom of an isotope is called nuclide and has a nucleus specified by its mass number and number of protons and neutrons. Nuclear Decay Out of 3000 known nuclides, only 250 are stable. The rest decompose over some period of time, releasing radiation in the process of creating new nuclides. Nuclear Chemistry (n.d.) states that some atoms are unstable. The emission of particles or electromagnetic radiation is one way to gain stability. 70 Lesson 2. Nuclear Reactions(Bauer, Birk, and Marks, 2019) Radioactivity arises from the transformation of one nuclide into another, often resulting in the emission of a particle from a nucleus. Nuclear decay is a spontaneous process in which as unstable nuclide emits radiation and changes into a new nuclide. Equations for Nuclear Reactions Let’s try some example provided by Bauer, Birk, and Marks (2019) to get a feel for nuclear equation: 90Th ------------------22888Ra + 42α 23190Th 232 ------------------23191Pa + 0-1β- 92U + 42α -----------23994Pu + 3 10n 238 Unlike chemical reactions, the number of each type of atom is not conserved In nuclear reactions. Two conditions must be met to balance a nuclear equation: 1. Conservation of mass number 2. Conservation of nuclear charge (atomic number) Particles emitted in a nuclear transformation are identifiable by checking these two conservation conditions. Alpha radiation Uranium 238 undergoes alpha decay in attempt to be stable 23892U ---23490Th + 42He daughter nuclide: the nuclide formed from the decay Parent nuclide: the original nuclide undergoing decay Beta radiation Consists of a stream of beta particles which are high speed electrons. Represented by the symbol 0-1e Iodine-131 is an example of a radioactive isotope that undergoes Beta emission. (an antineutrino also emitted) 13153I 13154Xe + 0-1e 71 Notice the atomic number increases. Beta emission results in the conversion of a neutron ( 10n ) to a proton ( 11p) 1 0n 11p + 0-1e (the electron comes from the neutron being converted NOT from the electron cloud) Gamma radiation High energy photons (electromagnetic radiation of a short wavelength) Gamma radiation does not change the mass or atomic number and is represented as 00. It almost always accompanies other radioactive emission because it represents the energy lost when the remaining nucleons reorganize into more stable arrangements. Generally you do not show gamma rays when writing nuclear equations Positron emission A positron is a particle that has the same mass as an electron but opposite charge. The positron is represented as 0 1e. Carbon-11 is an example of a particle that undergoes positron emission. 116C 115B + 01e Notice the atomic number goes down. Positron emission is the effect of converting a proton to a neutron. 11p10n + 01e Electron capture Capture by the nucleus on an inner shell electron of the electron cloud. Rubidium-81 does this 81 37Rb + 0-1e(orbital electron) 8136Kr Electron capture has the effect of positron emission, converting a proton to a neutron 11p10n + 01e Lesson 3. Rates of Radioactive Decay So far, we have been discussion about the types of particles involved in a nuclear reaction, but according to Bauer, Kirk, and Marks (2019) another important characteristic is how fast the reaction proceed. They also stated that the some nuclides need to have a short half-life – the time required for half of a sample of nuclide to decay to a different nuclide (Bauer, Birk, and Marks, 2019). gt = g0 (½) n Example: The half-life of cobalt-60 is 5.3 years. How much of a 1.000 mg sample will remain after 15.9 years? 72 Dating Because the half-life of any nuclide is constant, the amount of substance remaining in an artifact can serve as a nuclear clock to determine ages of objects. When an animal is alive it maintains a caron-14 to carbon-12 ratio identical to that in the atmosphere. When it dies, this ratio decreases. By measuring this ratio and contrast it to the atmosphere, we can get an approximate age. Radioactive decay is a first order process. Remember, half-life of a first order process is T1/2 = 0.693/k where k is the decay constant. We can determine the k constant by using the rate equation of a first order reaction ln (Nt/No) = -kt Example: If we start with 1.000 grams of Sr-90, 0.953 grams will remain after 2.0 years. (a) what is the half-life of strontium-90? (b) How much strontium-90 will remain after 5.00 years? (28.8 yr, 0.887g) Lesson 4. Energy Changes in Nuclear Reactions (Nuclear Chemistry, n.d.) E = mc2 A very familiar equation that shows mass and energy change are proportional. If a system loses mass, it loses energy (exothermic) and if it gains mass, it gains energy (endothermic). The c2 shows a small mass loss can cause a large energy loss. This is why conservation of mass seems to hold in reactions. For example, combustion of one mole methane loses 9.9 x 10-9 grams. In nuclear reactions, this mass change are much greater, 50,000 times greater than methane combustion. Look at the following 73 23892U 23490Th + 42He The nuclei in this reaction have the following masses: 92U = 238.0003 amu, 23490Th = 233.9942 amu and 42He = 4.0015 amu The m 238 is products minus reactants is 233.9942 + 4.0015 – 238.0003 = -0.0046g Change in energy would be: E = (-0.0046 g)(1kg/1000g) (3.00 x 108 m/s)2 = -4.1 x 10 11 kg-m2 /s2 = -4.1 x 1011 J Example: How much energy is lost or gained when Co-60 undergoes betw decay? Mass of Co60 is 59.9338 amu, Mass of Ni-60 is 59.9308 amu. =(-2.7 x 1011 J) Nuclear Binding Energies Scientists discovered in the 1930’s that the masses of nuclei combined are always less than these nucleons individually. Mass of 2 protons 2(1.00728 amu) Mass of 2 neutrons 2(1.00867 amu) Total = 4.03190 amu The mass of a Helium-4 nucleus is 4.00150 causing a mass defect of 0.0304 amu. The origin of this mass defect is some of the mass is converted to binding energy which binds the nucleons together in the nucleus. Energy then needs to be added to separate these nucleons to overcome this binding energy. This energy added to break the nucleons apart is called nuclear binding energy. 74 Nuclear Fission Figure 7.1 Nuclear Fission of U-235 Source: Chang (1998) Retrieved from: Nuclear Chemistry (n.d.) https://www.sas.upenn.edu/~mcnemar/apchem/nuclear.pdf Nuclear Fusion Figure 6.2 Nuclear Fusion retrieved from Nuclear Chemistry (n.d.) https://www.sas.upenn.edu/~mcnemar/apchem/nuclear.pdf 75  Summary: Radioactivity is the process where unstable atomic nuclei release energetic subatomic particles. Radioactivity was discovered by a French scientist Henri Becquerel in 1869. Two types of radiation: Electromagnetic forms and Particulate radiation. Unstable atoms have an excess of matter, energy, or both. If they spontaneously give off this excess as ionizing radiation, they are radioactive. Isotopes – atoms with the same number of protons but different number of neutrons and mass numbers. Atom of an isotope is called nuclide and has a nucleus specified by its mass number and number of protons and neutrons. Nuclear decay is a spontaneous process in which as unstable nuclide emits radiation and changes into a new nuclide. Some nuclides need to have a short half-life – the time required for half of a sample of nuclide to decay to a different nuclide. Because the half-life of any nuclide is constant, the amount of substance remaining in an artifact can serve as a nuclear clock to determine ages of objects. If a system loses mass, it loses energy (exothermic) and if it gains mass, it gains energy (endothermic). Scientists discovered in the 1930’s that the masses of nuclei combined are always less than these nucleons individually. 76  Assessment Task 7-1 1. Provide 5 examples where nuclear chemistry is being applied. Include description/explanation and sources for each example. 2. Briefly discuss one biological effect of radiation. Include your source(s). 77  References: Bauer R.C., Birk, J.P., Marks, P.S. (2019) General Chemistry (Books I and II). 2nd ed. McGraw-Hill. New York, NY. Nuclear Chemistry (n.d.) What is Nuclear Chemistry, Rates of Radioactive Decay, and Energy Changes in Nuclear Reactions https://www.sas.upenn.edu/~mcnemar/apchem/nuclear.pdf 78