Basic Sciences - Chemistry As A Central Science - PDF

BASIC SCIENCES- CHEMISTRY AS A CENTRAL UNIT 2 SCIENCE SESSION 1 SESSION 1: BRANCHES OF THE CHEMICAL SCIENCE Welcome to Session 1 of Unit 2. In this session we will...

BASIC SCIENCES- CHEMISTRY AS A CENTRAL UNIT 2 SCIENCE SESSION 1 SESSION 1: BRANCHES OF THE CHEMICAL SCIENCE Welcome to Session 1 of Unit 2. In this session we will look at the various branches in chemistry and their application in real life situation. Objectives By the end of this session, you should be able to list the different branches of chemical science. Now read on… 1.1 Branches of Chemistry The subject of chemistry is divided into three main branches. These are Physical, Inorganic and Organic chemistry. Chemistry is the study of matter and the changes that matter undergoes. Matter is anything that has mass and occupies space. The changes that matter undergoes always involve either again or loss of energy. Energy is nonmaterial and is the ability to do work to accomplish some change. The study of chemistry involves matter, energy and their interrelationship. Matter and energy are at the heart of chemistry. Chemistry is a broad area of study covering every thing from the basic parts of an atom to interaction between huge biological molecules. Because of this, chemistry encompasses a lot or specialties. 1.2 Physical Chemistry In Physical Chemistry we study the mathematical relations between atoms, and also analyse the factors that control atomic combinations, compound formations, and rate of formation of compounds, factors which determine the state of existence and pressure. Physical Chemistry enables us therefore to predict which reactions are possible, the rates at which they will occur and the products which are likely to result. Physical Chemistry always enables us to design suitable processes and to determine in advance the yield of desired and undesired products. Is a discipline that attempts to explain the way in which matter behaves physical chemists develop theoretical concepts and try to prove them experimentally. This helps us to understand how chemical systems behave. It also deals with the effect of the structure of substances on their physical properties. CoDEUCC/ Post-Diploma Progrmme 37  UNIT 2 SESSION 1 BRANCHES OF THE CHEMICAL SCIENCE 1.3 Inorganic Chemistry Inorganic Chemistry is the study of the behaviour and characteristics of compounds of mineral origin. For example, bauxite, iron ore, limestone, feldspar, clays are all minerals. The compounds making up these minerals are referred to as inorganic compounds. Only certain simple compounds of carbons are studies in this branch of chemistry- some of these simple carbon compounds are carbon II Oxide, Carbon IV Oxide and the trioxoarbonate (IV). 1.4 Organic Chemistry Organic Chemistry is the study of the behaviour and characteristics of compounds composed entirely of carbon, hydrogen, and oxygen with occasional occurrences of atoms of sulphur and nitrogen. The study of the properties of some carbon compounds and uses of proteins, amino acids, carbohydrates, sugar and natural products of vegetable origin is organic chemistry. Is the study of matter that consists principally of carbon and Hydrogen. Organic chemists study methods of preparing such diverse substance as plastics, drugs, solvents and a host of industrial chemicals. Is the study of matter that consists of all of the elements other than carbon and hydrogen and their combination. Inorganic chemists have been responsible for the development of unique substances such as semi conductors and high-temperature ceramics for industrial use. 1.5 Analytical Chemistry Apart from these three mentioned branches, we have Analytical Chemistry which enables us to determine what is present, (that is qualitative and quantitative analysis) in a given sample. Quantitative analysis can be by volumetric methods, by instrumental method or by gravimetric methods. A visit to the hospital we are requested to go to the laboratory with urine and faeces. The work done on these samples is what we referred to as analytical chemistry. It involves the analysis of matter to determine its composition and the quantity of each kind of matter that is present. Analytical chemists detect traces of toxic chemicals in water and air. They also develop methods to analyse human body fluids for drugs, poisons and levels of medication. 38 CoDEUCC/ Post-Diploma Programme BASIC SCIENCES- CHEMISTRY AS A CENTRAL UNIT 2 SCIENCE SESSION 1 Self-Assessment Questions Exercise 2.1 1. List the different branches in chemistry. 2. Name any THREE (3) methods by which a given sample can be analyse quantitatively. 3. The branch of chemistry that deals with the study of the behaviour and characteristic of compounds of mineral origin is …………… 4. Give FIVE (5) inorganic compounds of mineral origin. CoDEUCC/ Post-Diploma Progrmme 39  UNIT 2 SESSION 1 BRANCHES OF THE CHEMICAL SCIENCE This is a blank sheet for your short notes on:  issues that are not clear  difficult topics if any. 40 CoDEUCC/ Post-Diploma Programme BASIC SCIENCES – CHEMISTRY AS A CENTRAL UNIT 2 SCIENCE SESSION 2 SESSION 2: CHEMISTRY AS A CENTRAL SCIENCE Welcome to Session 2 of Unit 2. In this session, we shall discuss why chemistry is considered as a central science. Objectives By the end of this session, you should be able to state why chemistry is a central science. Now read on… 2.1 Chemistry as a Central Science Chemistry deals with all the multitude of materials and changes we see around us that make our world so diverse beautiful and at times mysterious. It explains the rusting of some nails, the melting of ice, the digestion of a candy bar, the colours of your vacation slides, the baking of bread and the aging of a person. Chemistry has been declared an essential part of most curriculums. Your interest may be in agriculture, dental hygiene, electrical engineering, geology, biology or one of many other areas of study. Why do some many diverse subjects share an essential tie to chemistry? The answer is that chemistry by its very nature is the central science. In any area of human activity that deals with some aspect of the material world, concern invariably arises about the fundamental nature of the materials involved; their composition and endurance, how they interact with their environment, and how they undergo change. This interest is present whether the material involved is a polymer used to coat electronic components, the pigments used by a renaissance painter or the blood cells of a child born with sickle cell anemia. It is likely that chemistry will play an important role in your future profession. You will be a more versatile and a creative person if you understand the chemical concepts as needed. The relationship of chemistry to professional goals however is not the sole reason to study the subject. Because chemistry is central to our lives and intimately tied to almost every aspect of our contact with the material world, this science is an integral part of our culture. The role of chemistry in our lives goes much deeper than the obvious products of chemical research that we use: items like plastic bags, batteries, photographic films and computer chips. CoDEUCC/ Post-Diploma Programme 41  UNIT 2 SESSION 2 CHEMISTRY AS A CENTRAL SCIENCE Engineering Medicine Home Economics Agriculture CHEMISTRY Physic Geology Biochemistry Pharmacy Figure 1.0. Central Position of Chemistry As shown in figure 1.0 chemistry occupies a central position amongst the basic sciences as well as its links with many other subjects. Chemistry plays an important role in many scientific occupations and has a wide variety of application. Engineers, medical doctors, agriculturalist, pharmacist and many other career scientists have to study topics in chemistry as part of their training. For example, engineering; – chemical engineer, medical doctor – medicinal chemistry, agriculture – soil chemistry and biochemistry, pharmacist – natural product (organic chemistry) etc. Chemistry through its chemical industry plays an important role in the economy of a nation. Many developed countries though have little natural resources have advanced their chemical industries; because they depend mostly on developing countries for their raw materials. 2.2 The Importance of Chemistry Chemistry forms the roots chemical industry. The aim of this industry, since the time of our early ancestors, has been to better the standard of living by improving on the natural resources of the world. Quality of cloths has improved and continues to do so. This is partly due to the development of new materials such a s polyester and nylon; it is also partly due to the development of dye-stuffs and detergents. Plastics that find many uses are now becoming cheaper because of the increase in demand and supply. Some successes have been made by chemists in an attempt to discover other kinds of fuel to replace petrol, kerosene, firewood, charcoal and others. The chemical industry helps in food production by producing fertilizers, selective weed-killers and pesticides. 42 CoDEUCC/Post-Diploma Programme BASIC SCIENCES – CHEMISTRY AS A CENTRAL UNIT 2 SCIENCE SESSION 2 For health, a whole range of drugs have been produced. Some of these drugs help in fighting against malaria and disease-causing bacteria. Efforts are being made today to produce chemicals with avoid any damage or pollution to the environment. Chemists are also trying to find ways to prevent shortages of any natural resources that are important to the chemical industry. Chemistry has a central position among the basic sciences as well as its links with many other subjects. Because of this, a knowledge of chemistry is very useful for the understanding of some topics in some school subjects such as home economics, agricultural science and geography. Chemistry plays an important role in many scientific occupations and has a wide variety of applications. Engineers, medical doctors, agriculturalists and many other career scientists have to study topics in chemistry as part of their training. Chemistry through its chemical industry, plays a big part in the economy of a nation. When the goods produced can satisfy home consumption and there is excess to export, the exported goods bring in foreign exchange that can be used to pay for services and goods that are imported. For this reason the chemical industry is said to play a part in the balance of payments of some countries. Finally, the experimental nature of the subject helps in the preparation of young people for life. It has been observed by specialists in the teaching of chemistry that a chemistry students does not only learn chemistry principles from experiments but other skills. Some of the skills learnt are: critical attitude and observation, open- mindedness, tolerance, patience, cooperative and hardworking attitudes, honesty and carefulness. Self-Assessment Questions Exercise 2.2 1. State FIVE (5) professional areas that uses the knowledge of chemistry. 2. What role does chemistry play in the economies of countries? CoDEUCC/ Post-Diploma Programme 43  UNIT 2 SESSION 2 CHEMISTRY AS A CENTRAL SCIENCE This is a blank sheet for your short notes on:  issues that are not clear; and  difficult topics, if any. 44 CoDEUCC/Post-Diploma Programme BASIC SCIENCES – CHEMISTRY AS A CENTRAL UNIT 2 SCIENCE SESSION 3 SESSION 3: MATTER AND ATOMIC STRUCTURE In this session, we shall discuss what chemistry is. We shall also discuss what substances or matter is made up of. Objectives By the end of this session, you should be able to: (a) state what matter is; (b) distinguish between electrons and neutrons; and (c) define relative atomic mass of a substance. Now read on… 3.1 Matter and Atomic Structure The science that deals with the composition and properties of matter is called chemistry. Matter is anything that occupies space and has mass. This could be illustrated by blowing air into a balloon and taking its weight and afterwards taking the weight of the empty balloon. The mass of air will be the difference in mass between the inflated balloon and the deflated balloon. Therefore air, water, trees, cement and gold are all examples of matter since they all have masses and occupy some space. Although each of these kinds of matter is different, they are similar in one fundamental way. They are all made up of moving subunits called atoms. Over Ninety-two different kinds of atoms are found in nature. Each one forms a specific kind of matter known as an element. Gold, oxygen and mercury are examples of elements. Each has a different kind of atom making up its structure. There are three states of matter namely solid, liquid and gas. The only substance that can exist in all three states is water. Solid water is referred to as ice; the liquid water is the one we drink whiles the gaseous water is referred to as water vapour. All matter is made up of very tiny particles that cannot be seen by the naked eye. However, sometimes their effects on their surroundings can be seen or felt. The study of chemistry mainly involves how matter behaves and how it interacts with other matter to give new substances. Some of the particles that make up matter are atoms, ions and molecules. These atoms, molecules and ions are the building blocks or units of matter. CoDEUCC /Post-Diploma Programme 45  UNIT 2 MATTER AND ATOMIC STRUCTURE SESSION 3 3.1.1 The Atomic Theory The idea that matter consists of tiny particles is very, very old. It was first put forward by the Greek thinker Democritus in 500 BC. For centuries the theory met with little success. People were not prepared to believe in particles which they could not see. The theory was revived by a British chemist called John Dalton in 1808. Dalton called the particles atoms from the Greek word for ‘cannot be split’. According to Dalton’s atomic theory, all forms of matter consist of atoms. The atomic theory explained many observations which has puzzled scientists. Why are some substances solid, some liquid and others gaseous? When you heat them, why do solids melt and liquids change into gases? In this topic, you will learn how the atomic theory provides answers to these questions and many others. An atom is the smallest particle of an element that always keeps the properties of that element. Atoms are the building blocks of matter. Atoms are made up of particles called electrons, protons and neutrons. The electron is a particle of energy which carries a negative charge. The protons carry a positive charge. The protons and the neutrons are concentrated in the nucleus which is at the center of the atom. The nucleus is spherical in nature and is surrounded by a much wider circles or series or concentric circles, which described the orbits or path of the rotating electrons, usually represents it. The material world is constructed entirely of atoms and molecules. Since a single piece of concrete block is itself made up of sand, cement and water bonded together by chemical energy, so is the atom also made up of some kind of basic components. These are electrons, protons, neutrons, etc. All things are composed of atoms. Atoms are too small for visual detection. The nucleus carries a net positive charge and the electrons carry a net negative charge. The sum all the positive charges equals the sum of all the negative charges, and the atom as a whole is electrically neutral. 46 CoDEUCC / Post-Diploma Programme BASIC SCIENCES – CHEMISTRY AS A CENTRAL UNIT 2 SCIENCE SESSION 3. Nucleus e- Orbit of electron 3.1.2 Evidence of Particles in Matter The process by which particles of substances spread uniformly into any available space is called diffusion. Diffusion can only occur when matter is made up of small and separate particles and is not continuous. The particles are also able to move between the particles of other substances. Diffusion of particles of substances helps humans to detect pleasant and unpleasant scents and taste. Another piece of evidence for the existence of particles in matter is the processes of crystallization. During crystallization millions of particles of a substance come together from its solution to form crystals. The crystal grows bigger as more particles leave the solution to form the solid. 3.1.3 Protons, Neutrons and Electrons The work of Marie and Pierre Curie, Rutherford and other scientists showed that atoms are made up of smaller particles. These subatomic particles differ in mass and in electrical charge. They are called protons, neutrons and electrons. Sub-atomic particles Particle Mass Charge (in atomic mass units) Proton 1 +1 Neutron 1 0 Electron 0.0005 -1 Protons and neutrons both have the same mass. We call this mass one atomic mass unit, one u (1.000 u = 1.67 x 10-27 kg). The mass of an atom depends on the number of protons and neutrons it contains. The electron in an atom contribute very little to its mass. The number of protons and neutrons together is called the mass number. CoDEUCC /Post-Diploma Programme 47  UNIT 2 MATTER AND ATOMIC STRUCTURE SESSION 3 Electrons carry a fixed quantity of negative electric charge. This quantity is usually written as –e. A proton carries a fixed charge equal and opposite to that of the electron. The charge on a proton can be written as +e. Neutrons are uncharged particles. The whole atom is electrically neutral because the number of electrons in an atom is equal to the number of protons. The number of protons (which is also equal to the number of electrons in a neutral atom) is called either the atomic number or the proton number. You can see that Number of neutrons = Mass number - Atomic (proton) number For example, an atom of potassium has a mass of 39 u and an atomic (proton) number of 19. The number of electrons is 19, the same as the number of protons. Te number of neutrons in the atom is 39 – 19 = 20 3.1.4 Relative Atomic Mass Hydrogen is the smallest atom. It consists of one proton and one electron. Chemists compared the masses of other atoms with that of a hydrogen atom. They use relative atomic mass. The relative atomic mass, A. of calcium is 40. This means that one calcium atom is 40 times as heavy as one atom of hydrogen. Mass of one atom of the element Relative atomic mass of an element = Mass of one atom of hydrogen A hydrogen atom weights 1.7 x 10-24g; the heaviest atoms weigh 5 x 10-22g. 3.1.5 Elements Gold, copper, and iron are typical metallic elements. Like all metallic elements, they conduct electricity. Many of the metallic substances we use are not elements; they are alloys. Steel, brass, bronze, gunmetal, solder and many others are alloys. An alloy is a combination of two or more metallic elements and sometimes non- metallic elements also. Silicon, carbon and chlorine are not-metallic elements. Some of their characteristics are typical of non-metallic elements,. Diamond is an example being shiny; most non-metallic elements are dull. Graphite is the only non-metallic element that conducts electricity. Silicon is one of the few semiconductors of electricity. 48 CoDEUCC / Post-Diploma Programme BASIC SCIENCES – CHEMISTRY AS A CENTRAL UNIT 2 SCIENCE SESSION 3 There are 92 elements found on Earth. A further 14 elements have been made by scientists. Table 1 is a summary of the characteristics of metallic and non-metallic elements. You will see that there are many differences. Metallic and non-metallic elements also differ in their chemical reactions. Table 1: Characteristics of metallic and non-metallic elements Metallic elements Non-metallic elements Solids, except for mercury which is a Solids and gases, except for bromine liquid which is a liquid Hard and dense Most of the solid elements are softer than metals, but diamond is very hard A smooth metallic surface is shiny, but Most are dull, but diamond is many metals tarnish in air, e.g. iron rusts brilliant The shape can be changed by Many solid non-metallic elements hammering: they are malleable. They break easily when you try to change can be pulled out into wire form: they are their shape. Diamond is the ductile. exception in being hard and strong. Conduct heat, although highly polished Poor thermal conductors surfaces reflect heat Good electrical conductors Poor electrical conductors, except for graphite. Some are semi-conductors, e.g. silicon. Make a pleasing sound when struck: as Are not sonorous. sonorous Examples Name an element which fits each of these descriptions? (a) a solid metallic element (b) a liquid metallic element (c) a solid non-metallic element (d) a gaseous non-metallic element (e) a hard solid element (f) a soft solid element (g) a shiny element (i) a dull element. 3.1.7 Differences in Structures of Different Elements How do these Differences Arise? The reason lies in the different arrangements of atoms in the different elements. In many elements the atoms are bonded together in groups called molecules. Oxygen, chlorine and many non-metallic elements consist of individual molecules. There are strong bonds between the atoms in the molecules, but between molecules there is only a very weak attraction. The molecules move about independently, and these elements are gaseous. CoDEUCC /Post-Diploma Programme 49  UNIT 2 MATTER AND ATOMIC STRUCTURE SESSION 3 Sulphur is a yellow solid. There are two forms of sulphur, which form differently shaped crystals. The crystals of rhombic sulphur are octahedral; those of monoclinic sulphur are needle-shaped. Rhombic and monoclinic sulphur are allotropes of sulphur: the only difference between them is the shape of their crystals. The reason why the crystals are shaped differently is that the sulphur molecules are packed into different arrangements in the allotropes. Check Point (a) What kinds of structures do the atoms of carbon form? (b) Why does this structure make diamond a hard substance? (c) What uses of diamond depend on its hardness? (d) Explain how its structure makes graphite less hard than diamond. (e) What are diamond and graphite called? Self-Assessment Questions Exercise 2.3 1. Calculate the atomic mass and number of neutrons in each of the following. (a) 1224Mg (b)11 24 Na (c) 126C (d ) 246C (e) 1735Cl ( f ) 1737Cl 2. Define Matter. 3. Give FIVE (5) examples of matter. 4. How many different kinds of atoms are found in nature? 5. An inflated balloon was found to weigh 5.65g. If the weight of the deflated balloon is 3.66g. Calculate the mass of air. 6. Complete the table below: PARTICLE MASS CHARGE [ In Atomic Mass Unit] Proton b c a 1 d electron 0.0005 e 50 CoDEUCC / Post-Diploma Programme BASIC SCIENCES- CHEMISTRY AS A CENTRAL UNIT 2 SCIENCE SESSION 4 SESSION 4: SYMBOLS: FORMULAS: EQUATIONS Hello and welcome to Session 4 of Unit 2. This session will look at chemical symbols, formulae and chemical equations as used in chemistry. I hope you would enjoy reading it. Objectives By the end of this session, you should be able to: (a) write the chemical formula of some common chemicals; (b) explain what a chemical reaction is; and (c) write chemical equations for burning of some common substances. Now read on… 4.1 Chemical Symbols For every element there is a symbol. For example, the symbol for sulphur is S. The letter S stands for one mole of sulphur atom. Sometimes, two letters are needed. The letters Si stand for element silicon: the symbol for silicon is Si. The symbol of an element is a letter or two letters which stand for one atom of the element. In some cases the letters are taken from the Latin name of the element, Ag from argentums (silver) and Pb from plumbum (lead) are examples.  The symbols of some common elements Element Symbol Element Symbol Element Symbol Aluminium Al Gold Au Oxygen O Barium Ba Hydrogen H Phosphorus P Bromine Br Iodine I Potassium K Calcium Ca Iron Fe Silver Ag Carbon C Lead Pb Sodium Na Chlorine Cl Magnesium Mg Sulphur S Copper Cu Mercury Hg Tin Sn Fluorine F Nitrogen N Zinc Zn 4.2 Formula For every compound there is a formula. The formula of a compound contains the symbols of the elements present and some numbers. The numbers show the ratio in which atoms are present. The compound carbon dioxide consists of molecules. CoDEUCC/Post-Diploma Programme 51  UNIT 2 SESSION 4 SYMBOLS: FORMULAS: EQUATIONS Each molecule contains one atom of carbon and two atoms of oxygen. The formula of the compound is CO2. The 2 below the line multiplies the O in front of it. To show three molecules of carbon dioxide you write 3CO2. Sand is impure silicon dioxide (also called silicon (IV) oxide). It consists of macromolecules, which contain millions of atoms. There are twice as many oxygen atoms as silicon atoms in the macromolecule. The formula or silicon dioxide is therefore SiO2. The formulas of some of the compounds mentioned are:  Water, H2O (two H atoms and one O atom; the 2 multiples the H in front of it).  Sodium chloride, NaCl (one Na; one Cl).  Silver oxide, Ag2O (two Ag: one O).  Iron (II) sulphide, FeS (one Fe; one S). The formula for aluminum oxide is Al2O3. This tells you that the compound contains two aluminum atoms for every three oxygen atoms. The numbers below the line multiply the symbols immediately in front of them. The formula for calcium hydroxide is Ca(OH)2. The 2 multiplies the symbols in the brackets. There are 2 oxygen atoms, 2 hydrogen atoms and 1 calcium atom. To write 4 Ca(OH)2 means that the whole of the formula is multiplied by 4. It means 4 calcium, 8 oxygen and 8 hydrogen atoms. Table 2 lists the formulas of some common compounds.  The formulas of some common compounds Compound Formula Water H2O Carbon monoxide CO Carbon dioxide CO2 Sulphur dioxide SO2 Hydrogen chloride HCl Hydrochloric acid HCl(aq) Sulphuric acid H2SO4(aq) Nitric acid HNO3(aq) Sodium hydroxide NaOH Sodium chloride NaCl 52 CoDEUCC/Post-Diploma Programme BASIC SCIENCES- CHEMISTRY AS A CENTRAL UNIT 2 SCIENCE SESSION 4 Sodium sulphate Na2SO4 Sodium nitrate NaNO3 Sodium carbonate Na2CO3 Sodium hydrogen carbonate NaHCO3 Calcium oxide CaO Calcium hydroxide Ca(OH)2 Calcium chloride CaCl2 Calcium sulphate CaSO4 Calcium carbonate CaCO3 Calcium hydrogen carbonate Ca(HCO3)2 Copper (II) oxide CuO Copper (II) sulphate CuSO4 Aluminium chloride AlCl3 Aluminium oxide Al2O3 Ammonia NH3 Ammonium chloride NH4Cl Ammonium sulphate (NH4)2SO4 4.3 Chemical Equation In a chemical reaction, the starting materials, the reactants, are changed into new substances, the products. The atoms present in the reactants are not changed in any way, but the bonds between the atoms change. Chemical bonds are broken and new chemical bonds formula. Chemical symbols and formulas give us a nice way of showing what happens in a chemical reaction. A convenient way of describing a chemical reaction is by a chemical equation. Examples 1 illustrate how chemical reactions are represented by equations. Example 1: Copper and sulpur combine to form copper sulphide. Writing a word equation for the reaction. Copper + Sulphur Copper sulphide The arrow stands for the direction of the reaction Writing the symbols for the elements and the formula for the compound gives the chemical equation: Cu + S CuS CoDEUCC/Post-Diploma Programme 53  UNIT 2 SESSION 4 SYMBOLS: FORMULAS: EQUATIONS Adding the state symbols Cu(s) + S(s) CuS(s) 4.4 Why is this Called an Equation? The two sides are equal. On the left hand side, we have one atom of copper and one atom of sulphur; on the right hand side, we have one atom of copper and one atom of sulphur combined as copper sulphide. The atoms on the left hand side and the atoms on the right hand side are the same in kind and in number. Example 2 Calcium carbonate decomposes when heated to give calcium oxide and carbon dioxide. The word equation is Calcium carbonate Calcium oxide + Carbon dioxide The chemical equation is CaCO3(s) CaO(s) + CO2(g) Example 3 Carbon burns in oxygen to form the gas carbon dioxide. The word equation is Carbon + Oxygen Carbon dioxide We must use the formula O2 for oxygen because oxygen consists of molecules which contain two oxygen atoms. The chemical equation is C(s) + O2(g) CO2(g) Example 4 Magnesium burns in oxygen to form the solid magnesium oxide. Magnesium + Oxygen Magnesium oxide Mg(s) + O2 (g) MgO(s) There is something wrong here! The two sides are not equal. The left hand side has two atoms of oxygen; the right hand side has only one. Multiplying MgO by 2 on the right hand side should fix it Mg(s) + O2(g) 2MgO(s) 54 CoDEUCC/Post-Diploma Programme BASIC SCIENCES- CHEMISTRY AS A CENTRAL UNIT 2 SCIENCE SESSION 4 Now there are two oxygen atoms on both sides, but there are two magnesium atoms on the right hand side and only one on the left hand side. Multiply Mg by 2. 2Mg(s) + O2(g) 2MgO(s) The equation is now a balanced chemical equation. Check up. On the left hand side, number of Mg atoms = 2; number of O atoms = 2. On the right hand side, number of Mg atoms =2; number of O atoms = 2. The equation is now balanced. Check Point 1. Refer to the table of elements at the end of the book, (i.e. the Periodic Table). Write down the names and symbols of the elements with atomic numbers 8, 10, 20, 24, 38, 47, 50, 80, 82. Say what each of these elements is used for. 2. Give the meanings of the following symbols(s) of the state symbols (s), (I), (g), (aq). 3. Try writing balanced chemical equations for the following reactions. (a) Zinc and sulphur combine to form zinc sulphide. (b) Copper reacts with oxygen to form copper (II) oxide. (c) Sulphur and oxygen form sulphur dioxide. (d) Magnesium carbonate decomposes to form magnesium oxide and carbon dioxide. (e) Hydrogen and copper (II) oxide form copper and water. (f) Carbon and carbon dioxide react to form carbon monoxide. (g) Magnesium reacts with sulphuric acid to form hydrogen and magnesium sulphate. (h) Calcium reacts with water to form hydrogen and solution of calcium hydroxide. (i) Zinc reacts with steam to form hydrogen and zinc oxide. (j) Aluminium and chlorine react to form aluminium chloride. 4. Write balanced chemical equations for the following reactions. (a) Zinc and sulphur combine to form zinc sulphide, ZnS (b) Copper and chlorine combine to form copper (II) chloride, CuCl2. (c) Sulphur burns in oxygen to form sulphur dioxide, SO2. (d) Magnesium carbonate decomposes to form magnesium oxide and carbon dioxide. (e) Calcium burns in oxygen to form calcium oxide. CoDEUCC/Post-Diploma Programme 55  UNIT 2 SESSION 4 SYMBOLS: FORMULAS: EQUATIONS 5. How many different types of atoms are present in the following? Indicate also the sum total of the different atoms in each compound. (a) CaCl2 (b) 3CaCl2 (c) Cu(OH)2 (d) 5Cu(OH)2 (e) H2SO4 (f) 3H2SO2 (g) 2NaNO3 (h) 3Cu(NO3)2. Self-Assessment Questions Exercise 2.4 1. How many different types of atoms are present in the following? (a) CaCl2 (b) H2S04 (c) Cu(OH)2 (d) NaNO3. 2. Write balance chemical equations for the following reactions. (a) Sulphur burns in Oxygen to form sulphur dioxide, SO2. (b) Calcium burns in Oxygen to form calcium oxide. (c) Copper and chlorine combine to form copper (ii) chloride. 3. Complete the table below: COMPOUND FORMULA Calcium Chloride a Calcium hydroxide b c NH3 d NH4Cl Copper (ii) sulphate d 56 CoDEUCC/Post-Diploma Programme BASIC SCIENCES- CHEMISTRY AS A CENTRAL UNIT 2 SCIENCE SESSION 5 SESSION 5: MOLECULES, ELEMENTS AND COMPOUNDS In this session, the composition of some compounds and molecules shall be discussed. The differences between a mixture and a compound shall be discussed. Objectives By the end of this session, you should be able to: (a) distinguish among elements, molecules and compounds; and (b) distinguish between a mixture and a compound. Now read on… 5.1 Molecules, Elements and Compounds Although atoms are the basic building blocks of matter they do not exist in isolation. The smallest number of atoms of the same substance capable of independent existence is referred to as a molecule. However the inert gases, e.g. helium, neon, argon etc. exist independently as atoms. For example the smallest number of atoms of oxygen and hydrogen capable of independent existence are two. Thus in the natural state you will never find atomic hydrogen and atomic oxygen. Just as a single piece of building block has no meaningful existence except when a number of them have been combined together to form a single room which has capable meaningful existence or utility, so one can by analogy refer to a room as a molecule. Compounds are chemical combinations of atoms. 5.1.1 Molecules Molecules are also compounds and they may be combinations of the same element or different atoms of different elements. The formation of compounds involves the absorption or release of energy. Processes of compound formation in which energy is released or evolved are said to be exothermic (heat releasing). Compound formation involving the absorption of energy in the form of heat usually is said to be endothermic. Compounds cannot be easily separated into their constituent atoms by simple physical processes. Their constituent atoms are not even identifiable in compounds. 5.1.2 Compounds The very distress rocket contains the metallic element magnesium. When it is heated – when the fuse is lit – magnesium burns in the oxygen of the air. It burns CoDEUCC/Post-Diploma Programme 57  UNIT 2 MOLECULES, ELEMENTS AND COMPOUNDS SESSION 5 with a brilliant white flame. A white powder is formed. This powder is the compound, magnesium oxide. A change which results in the formation of a new substance is called a chemical reaction. A chemical reaction has taken place between magnesium and oxygen. The elements have combined to form a compound. Magnesium + Oxygen Magnesium oxide Element + Element Compound A compound is a pure substance which contains two or more elements chemically combined. A compound of oxygen and one other element is called an oxide. Making a compound from its elements is called synthesis. Chemical reactions can synthesize compounds, and chemical reactions can also decompose (split up) compounds. Some compounds can be decomposed into their elements by heat. An example is silver oxide. The chemical reaction that takes place is Silver oxide Heat Silver + Oxygen Compound Elements Splitting up a compound by heat is called thermal decomposition. Some compounds can be decomposed into their elements by the passage of a direct electric current. An example is sodium chloride (common salt) which is a compound of sodium and chlorine. A compound of chlorine with one other element is called a chloride. Pass a direct electric current Sodium chloride through the molten compound Sodium + Chlorine Compound Elements The chemical reaction that occurs when a compound is split up by means of electricity is called electrolysis. Water is a compound. It can be electrolysed to give the elements hydrogen and oxygen. Water is a compound of hydrogen and oxygen. You could call it hydrogen oxide. It is possible to make water by a chemical reaction between hydrogen and oxygen. 58 CoDEUCC/Post-Diploma Programme BASIC SCIENCES- CHEMISTRY AS A CENTRAL UNIT 2 SCIENCE SESSION 5 The compound can be synthesised from its elements. Water Hydrogen + Oxygen Electrolysis decomposes the compound into elements Check Point 1. In a chemical change, a new substance is formed in a physical change, no new substance is formed. Say which of the following changes are chemical changes (a) Evaporating water. (d) Cracking an egg (b) Electrolysing water (e) Boiling an egg (c) Melting wax 2. Which of the following brings about a chemical change? (a) Heating magnesium (b) Heating silver oxide (c) Heating sodium chloride (d) Passing a direct electric current through copper wire. (e) Passing a direct electric current through molten salt. 5.1.3 Mixtures and Compounds There are a number of differences between a compound and a mixture of elements. A mixture can contain its components in any proportions. A compound has a fixed composition. It always contains the same elements in the same percentages by mass. You can mix together iron filings and powdered sulphur in any proportions, from 1% iron and 99% sulphur to 99% iron and 1% sulphur, you get a mixture of sulphur and iron. A compound called iron(II) sulphide always contains 64% iron and 36% sulphur by mass. CoDEUCC/Post-Diploma Programme 59  UNIT 2 MOLECULES, ELEMENTS AND COMPOUNDS SESSION 5  Differences between mixtures and compounds Mixtures Compounds A mixture can be separated into A chemical reaction is needed to split its parts by methods such as a compound into simpler compounds distillation and dissolving or into its elements No chemical change takes place When a compound is made, a which a mixture is made chemical reaction takes place, and often heat is given out or absorbed. A mixture behaves in the same A compound does not have the way as its components. characteristics of its elements. It has a new set of characteristics A mixture can contain its A compound always contains its components in any proportions elements in fixed proportions by mass; for example, calcium carbonate (marble) always contains 40% calcium, 12% carbon and 48% oxygen by mass. Check Point 1. Group the following into mixtures, compounds and elements: rain water, sea water, common salt, gold dust, aluminum oxide, ink, silicon, air. 2. Name an element which can be used for each of the following uses: surgical knife, pencil ‘lead’, wedding ring, saucepan, crowbar, thermometer, plumbing, electrical wiring, microcomputer circuit, disinfecting swimming pools, fireworks, artists’ sketching material. 3. What are the differences between a mixture of iron and sulphur and the compound iron sulphide? 4. (a) What happens when you connect a piece of copper wire across the terminals in a battery? What happens when you disconnect the wire from the battery? Is the copper wire the same as before or has it changed? (b) What happens when you hold a piece of copper in a Bunsen flame for a minute and then switch off the Bunsen? Is the copper wire the same or different? (c) What type of change or changes occurs in (a) and (b)? 60 CoDEUCC/Post-Diploma Programme BASIC SCIENCES- CHEMISTRY AS A CENTRAL UNIT 2 SCIENCE SESSION 5 Self-Assessment Questions Exercise 2.5 1. What is a chemical reaction? 2. Which of the following is a chemical process? (i) Dissolving common salt in water (ii) Burning a piece of paper (iii) Grinding a cube sugar into a smaller particle. (iv) Cooking. 3. What is a chemical compound? 4. What is electrolysis? CoDEUCC/Post-Diploma Programme 61  UNIT 2 MOLECULES, ELEMENTS AND COMPOUNDS SESSION 5 This is a blank sheet for your short notes on:  issues that are not clear  difficult topics if any. 62 CoDEUCC/Post-Diploma Programme BASIC SCIENCES- CHEMISTRY AS A CENTRAL UNIT 2 SCIENCE SESSION 6 SESSION 6: MAKING USE OF RADIOACTIVITY Welcome to the last Session of Unit 2. In this session, we shall discuss why some substances undergo changes naturally and also consider the importance of radioactivity. Objectives By the end of this session, you should be able to: (a) explain what a radioactive substance is; (b) list some radioactive substance; and (c) state two importance of radioactive substance. Now read on… 6.1 Making Use of Radioactivity 6.1.1 Carbon-14 Dating Carbon is made up of the isotopes carbon-12, carbon-13 and carbon-14. The isotope carbon-14 is radioactive. It has a half-life of 5700 years. Carbon-14 is present in the carbon dioxide which living trees use in photosynthesis. After a tree dies, it can take in no more carbon-14. The carbon-14 already present decays slowly, carbon-12 does not change. The ratio of the amount of carbon-14 left in the wood to the amount of carbon-14 in living trees can be used to tell the age of the wood. Animals take in carbon-14 in their food while they are alive. After their death, the proportion of carbon-14 in their bones tells how long it is since they died. 6.1.2 Radio-Isotopes Radioactive isotopes differ enormously in their half-lives. Some are listed in table 8.2.  Some radioactive isotopes Isotope Radiation Half-life Isotope Radiation Half-life Uranium-238  5000million years Iodine-131  8 days Uranium-235  700 million years Sodium-24  15 hours Plutonium-239 ,  24000 years Bromine-82 ,  36 hours Carbon-14  5700 years Uranium-239  24 minutes Strontium-90  59 years Strontium-93 ,  8 minutes Caesium-137 ,  30 years Barium-143  12 seconds Cobalt-60 5 years Polonium-213 4 x 10-6s ,   Scientists have found many uses for radioactive isotopes. CoDEUCC /Post-Diploma Programme 63  UNIT 2 SESSION 6 MAKING USE OF RADIOACTIVITY 6.1.3 Medical Uses Radioactivity can be used to penetrate the body and kill cancerous tissue. Cobalt- 60 and Caesium-137 are often used for this purpose. They emit -rays. The dose of radiation must be carefully calculated to destroy cancerous tissue and leave healthy tissue alone. Why is a -emitter used, rather than an – or a –emitter for this job? Radiation is used to destroy germs on medical instruments. It is more convenient than boiling, and it is also more efficient. Cobalt-60 is often used. Why is a –emitter better than an emitter of –or –radiation for this job? The thyroid gland in the throat takes iodine from food and stores it. To find out whether the thyroid is working correctly, a patient is given food containing iodine-131, a –emitter. The radioactive iodine can be detected as it passes through the body. The half-life of iodine-131 is 8 days. After a few weeks, there will be little left in the patient’s body. Can you explain why a –emitter is better than a –emitter for this purpose? 6.1.4 Industrial Uses This research worker is measuring engine wear. The pistons of this engine have been in nuclear reactor. Some of the metal atoms have become radioactive. As the engine runs, the pistons wear away and radioactive atoms the engine wears, the more radioactive the oil becomes. You can tell how well lubricating oil reduces engine wear by timing the uptake of radioactivity. The detector measures the amount of radiation passing through the foil. If the foil is too thick, the detector reading drops. The detector send a message to the rollers, which move closer together to make the foil thinner. Why must the source be a  - emitter, not an  – or – emitter? Why does it need to have a long halflife? Can you suggest a suitable isotope. 6.1.5 Agricultural Research This research worker is studying the uptake of fertilizers by plants. He has used a fertilizer containing phosphorus – 32. This isotope is a -emitter with a half-life of 14 days. By measuring the radioactivity of the leaves, the scientists can find out how much fertilizer has reached them. 6.2 Radioactivity and food Food spoilage is a serious problem. About 20% of the world’s food is lost through spoilage. The major cause is the bacteria, moulds and yeasts which grow on food. Some bacteria produce waste products which are toxic to people and cause the 64 CoDEUCC/Post-Diploma Programme BASIC SCIENCES- CHEMISTRY AS A CENTRAL UNIT 2 SCIENCE SESSION 6 symptoms of food poisoning (sickness and diarrhoea). There are thousands of cases of food poisoning every year and some are fatal. Now there is an answer. Irradiation of food with  -rays kills 99% of disease- carrying organisms. These include salmonella, which infects a lot of poultry, and Clostridium, the cause of botulism, which is often fatal. Spices, which are likely to contain micro-organisms as they are imported from tropical countries, can be irradiated with no loss of flavour. Irradiating potatoes is useful because it stops them sprouting without affecting the taste. The treatment is not suitable for all foods. Red meats turn brown and develop an unpleasant taste, eggs develop a smell, shrimps turn black and tomatoes go soft. Some people fear that irradiated food will be radioactive. In fact, foods contain a natural low level of radioactivity, and the treaatment increases this level only slightly. The dose of radiation which the food receives is carefully calculated. By the time the food is eaten, the extra radioactivity has decayed. The best proof that irradiation is safe is that you can not detect it. 6.3 The Dangers of Radioactivity We make good use of radioactivity. However, large doses of radiation are dangerous. Exposure to a high dose of radiation burns the skin. Delayed effects are damage to the bones and the blood. People who are exposed to a lower level of radiation for a long time may develop leukaemia (a disease of the blood cells) and cancer. When radioactive elements get inside the body, they are very dangerous. They irradiate the body organs near them, and the risk of cancer is very great. Even a an emitter of -rays, the rays with least penetrating power, can do immense damage if it gets inside the body. Fairly low doses of radioactivity can damage human genes. This may result in the birth of deformed babies. People who work with radioactive sources take precautions to protect themselves.  They didn’t know what the symbol meant In the city of Goiana in Brazil in 1987, two junk collectors broke into a disused medical clinic and stole a heavy cylinder object which they took to be lead. They saw the metal cylinder to junk yard. It emitted an eerie blue light from narrow slits. The manager of junk yard hammered the head off the cylinder. Inside lay a capsule containing a powdery blue substance which stuck to the skin and glowed. It was caesium- 137 a radioactive CoDEUCC /Post-Diploma Programme 65  UNIT 2 SESSION 6 MAKING USE OF RADIOACTIVITY isotope which is used in cancer therapy. Massive doses of the -radiation from caesium-137 cause leukaemia, bleeding, sterility and cataract. The dealer did not know this: he thought the bluish powder was so pretty and shinny that he gave away bit of it to his neighbours. Almost immediately, those who touched it became sick and feverish. One man stored some under his bed because he because he wanted to see it glow in the dark. Several children rubbed it on their bodies, and their skin became blistered and burnt. Check Point 1. Sodium-24 is a radioisotope used in medicine. Its half-life is 15 hours. A solution containing 8.0 mg of the isotope is prepared. What mass of isotope remains after (a) 15 hours (b) 30 hours (c) 5 days? 2. Cobalt-60 is a radioisotope made by placing cobalt in a nuclear reactor. It has a half-life of 5 years. The activity of a piece of cobalt-60 is 32.0 kBq. How long would it take for its activity to fall to (a) 16.0 kBq (b) 1.0 kBq? 3. In figure 8.2B, how long would it take for the count rate to fall from 800 c.p.m to 100 c.p.m? 4. The following measurements were made in an experiment using a Geiger- MÜller tube near a radioactive source. Time (hours) 0 0.5 1.0 1.5 2.0 2.5 Count (c.p.m) 510 414 337 276 227 188 The background count was 30 c.p.m. (a) The initial count rate from the source alone was 480 c.p.m (510 – 30). This is the corrected count rate. Work out the corrected count rates for the other readings. (b) Plot a graph of the count rate (on the vertical axis) against time. (c) Use your graph to find the half-life of the source. Self-Assessment Questions Exercise 2.6 1. What is a radioactive substance? 2. What is the importance of carbon dating? 3. What is meant by “half-life of a radioisotope”? 4. What are isotopes (i) State the isotopes of carbon? (ii) Which of them is/are radioactive? (iii) What is the half-life of the radioactive isotope of carbon? (iv) Where can radioactive isotopes of carbon be found. 66 CoDEUCC/Post-Diploma Programme