Introduction to Chemistry PDF Past Paper - Dr. A. Bounab

Introduction to chemistry Dr A. Bounab Introduction to Chemistry Contents 1. Classifications of Matter.................................................................

Introduction to chemistry Dr A. Bounab Introduction to Chemistry Contents 1. Classifications of Matter........................................................................................................................................... 3 2. Substances and Mixtures.......................................................................................................................................... 3 3. Elements and Compounds........................................................................................................................................ 4 4. Physical and Chemical Changes............................................................................................................................... 5 5. Measurement............................................................................................................................................................. 5 6. Handling numbers..................................................................................................................................................... 7 6.1. Scientific Notation:........................................................................................................................................... 7 6.2. Significant Figures:........................................................................................................................................... 7 7. Early Experiments to Characterize the Atom............................................................................................................ 8 7.1. Cathodic rays - electron.................................................................................................................................... 8 7.2. Radioactivity..................................................................................................................................................... 9 7.3. The Atomic Model.......................................................................................................................................... 11 8. Summary................................................................................................................................................................. 13 Chemistry is the study of matter and how we can change matter chemically and physically. What is matter? Matter is everything around us that has mass and volume. Matter can be any phase - solid, liquid, or gas. In this unit, we explore the properties, phases, and how we measure matter. Completing this unit should take you approximately 3 hours. Upon successful completion of this unit, mainly title 1 to 6, you will be able to: classify properties of matter and changes of matter as physical or chemical; name and use SI units for length, mass, time, and volume; perform mathematical operations involving significant figures; convert measurements into scientific notation; Title 7 will cover the historical facts and early experiments that lead the characterization of Atoms. References Chang, Raymond. (2008). General chemistry : the essential concepts, (5th ed.). McGraw-Hill. Ebbing, D. D.., & Gammon, S. D.. (2007). General chemistry (9th ed.). Houghton Mifflin Co. Saylor.org Academy (e-learning platform): https://learn.saylor.org/ Khan academy (e-learning platform): https://www.khanacademy.org/ 2024/2025 Page 1 of 13 Introduction to chemistry Dr A. Bounab Google Drive Link : https://drive.google.com/drive/folders/101FJNZngEpXrN_5-YBqY6bOG7a6NCkr2?usp=sharing 2024/2025 Page 2 of 13 Introduction to chemistry Dr A. Bounab 1. Classifications of Matter Commented [BA1]: ‫المادة‬ Matter is anything that occupies space and has mass, and Chemistry is the study of matter and the changes it undergoes. Matter exists in three states: solid, liquid, and gas (Figure 1). Commented [BA2]: ‫الحاالت الثالثة للمادة‬ Solids have definite shape and definite volume. The particles are ordered and close together. Liquids have definite volume and indefinite shape, meaning they take on the shape of the container. A liquid's particles are less ordered, but still relatively close together. Gases, such as the air inside balloons, take the shape and volume of their container. Their particles are highly disordered. The three states of matter can be interconverted without changing the composition of the substance. Upon heating, a solid (for example, ice) will melt to form a liquid (water). (The Commented [BA3]: ‫تسخين‬ temperature at which this transition occurs is called the melting point.) Further Commented [BA4]: ‫الجليد‬ heating will convert the liquid into a gas. (This conversion takes place at the Figure 1: Three states of matter Commented [BA5]: ‫درجة الذوبان‬ boiling point of the liquid.) Commented [BA6]: ‫درجة الغليان‬ On the other hand, cooling a gas will cause it to condense into a liquid. When the liquid is cooled further, it will freeze Commented [BA7]: ‫تبريد‬ into the solid form. Commented [BA8]: ‫تكثف‬ Commented [BA9]: ‫تجمد‬ 2. Substances and Mixtures Commented [BA10]: ‫مادة ذات تكوين ثابت‬ Commented [BA11]: ‫خليط‬ A substance is matter that has a definite or constant composition and distinct properties. Examples are water (H2O), silver (Ag), ethanol (C2H5OH), table salt (NaCl), and carbon dioxide (CO2). Substances differ from one another in composition and can be identified by their appearance, smell, taste, and other properties. Commented [BA12]: ‫ الذوق وخواص أخرى‬،‫ الرائحة‬،‫الشكل‬ A mixture is a combination of two or more substances in which the substances retain their distinct identities. Some examples are air, soft drinks, milk, and cement. Mixtures do not have constant composition. Exp: Sea water has different composition from Tap water. Mixtures are either homogeneous or heterogeneous: Commented [BA13]: ‫متجانس أو غير متجانس‬ Sugar dissolves in water, the composition is the same throughout the solution = homogeneous mixture. Commented [BA14]: ‫محلول‬ 2024/2025 Page 3 of 13 Introduction to chemistry Dr A. Bounab Sand mixed with iron filings, the sand grains and the iron filings remain visible and separate (Figure 2). The composition is not uniform = a heterogeneous mixture. Oil + water creates another heterogeneous mixture because the liquid does not have a constant composition. Any mixture, whether homogeneous or heterogeneous, can be created and then separated by physical means into pure components without changing Commented [BA15]: ‫طرق فيزيائية‬ the identities of the components. Thus, sugar can be recovered from a water solution by heating the solution and evaporating it to dryness. Condensing the water vapor will give us back the water component. To separate the iron- sand mixture, we can use a magnet to remove the iron filings from the sand, because sand is not attracted to the magnet (See Figure 2.b). After separation, the components of the mixture will have the same composition and properties as they did to start with. Figure 2: (a) The mixture contains iron filings and sand. (b) A magnet Commented [BA16]: ‫العناصر والمركبات‬ 3. Elements and Compounds separates the iron filings from the mixture. A substance can be either an element or a compound. An element is a substance that cannot be separated into simpler substances by chemical means. Commented [BA17]: ‫طرق كيميائية‬ Chemists use alphabetical symbols to represent the names of the elements. The first letter of the symbol for an element Commented [BA18]: ‫رموز‬ is always capitalized, but the second letter is never capitalized. For example, Co is the symbol for the element cobalt, whereas CO is the formula for carbon monoxide, which is made Commented [BA19]: ‫التركيبة‬ up of the elements carbon and oxygen. Table 1 shows some of the more common elements. Table 1: Some common elements and their symbols Commented [BA20]: ‫يتفاعل‬ Most elements can interact with one or more other elements to form compounds. We define a compound as a substance Commented [BA21]: ‫نسب ثابتة‬ composed of two or more elements chemically united in fixed proportions. Hydrogen gas, for example, burns in oxygen Commented [BA22]: ‫يحترق‬ 2024/2025 Page 4 of 13 Introduction to chemistry Dr A. Bounab gas to form water, a compound whose properties are distinctly different from those of the starting materials. Water is Commented [BA23]: ‫المواد األولية‬ made up of two parts of hydrogen and one part of oxygen. This composition does not change, regardless of whether the water comes from Mediterranean sea, the Nil River in Egypt, or the ice caps on Mars. Unlike mixtures, compounds can be separated only by chemical means into their pure components. Figure 3: Classification of matter 4. Physical and Chemical Changes In chemistry, we often study changes in matter. Two types of changes can occur in matter: physical and chemical. To determine whether you are dealing with a physical or chemical change, ask yourself if you can reverse the process to recover the original material. Physical changes can be reversed, but chemical changes generally cannot. For example, ice melting is a physical change because you can re-freeze the water. However, cooking a steak is a chemical change because you cannot recover the raw meat. Watch this video (on Youtube) to see examples of physical and chemical changes, and how we can observe a change to classify it. https://youtu.be/3e8e0d1fWLk (Source: Khan Academy) 5. Measurement Chemists use measurements to compare the properties of different substances. They can make simple measurements by using common devices or instruments: The meterstick measures length; the buret, the pipet, the graduated cylinder, and the volumetric flask measure volume (Figure 4); 2024/2025 Page 5 of 13 Introduction to chemistry Dr A. Bounab the balance measures mass; the thermometer measures temperature. Figure 4: Some common measuring devices found in a chemistry laboratory. A measured quantity is usually written as a number with an appropriate unit. To say that the distance between Algiers and Tamanrasset by car along a certain route is 1930 is meaningless. We must specify that the distance is 1930 kilometers. In science, units are essential to stating measurements correctly. In 1960, the General Conference of Weights and Measures, the international authority on units, proposed a revised metric system called the International System of Units (abbreviated SI, from the French System International d’Unites). Table 2 shows the seven SI base units. All other SI units of measurement can be derived from these base units. Table 2: SI Base Units Like metric units, SI units are modified in decimal fashion by a series of prefixes, as shown in Table 3. We use both metric and SI units. Measurements that we will utilize frequently in our study of chemistry include time, mass, volume, density, and temperature. 2024/2025 Page 6 of 13 Introduction to chemistry Dr A. Bounab Table 3: Prefixes used with SI Units 6. Handling numbers 6.1. Scientific Notation: Scientific notation is a way of expressing very large or very small numbers in a compact form. It is written as: 𝑁 × 10𝑛 , Where: 𝑁 is a number between 1 and 10 (called the coefficient). 10𝑛 is the power of ten. For example, 5000 can be written as 5 × 103 , and 0.00012 can be written as 1.2 × 10−4. Watch this video (Source: Khan Academy) which explains the basics of scientific notation. https://youtu.be/trdbaV4TaAo 6.2. Significant Figures: Significant figures (sig figs) refer to the digits in a number that carry meaningful information about its precision. The rules for identifying significant figures are: Non-zero digits are always significant. Example: 123 has 3 significant figures. Zeros between non-zero digits are significant. Example: 1002 has 4 significant figures. Leading zeros (zeros before the first non-zero digit) are not significant. Example: 0.0056 has 2 significant figures. Trailing zeros in a number with a decimal point are significant. Example: 12.3400 has 6 significant figures. Trailing zeros in a whole number without a decimal point may or may not be significant, depending on how the number is written. Example: 1500 could have 2, 3, or 4 significant figures. 2024/2025 Page 7 of 13 Introduction to chemistry Dr A. Bounab Rules for Calculations: Addition/Subtraction: The result should have the same number of decimal places as the measurement with the fewest decimal places. Multiplication/Division: The result should have the same number of significant figures as the measurement with the fewest significant figures. These rules help ensure that the precision of the result matches the precision of the measured quantities. Watch these two videos (Source: Khan Academy) to learn how to count sig figs for a given quantity. https://youtu.be/eCJ76hz7jPM, and https://youtu.be/eMl2z3ezlrQ After you watch the videos, complete this practice set on this link (Source: Khan Academy). https://www.khanacademy.org/math/arithmetic-home/arith-review-decimals/arithmetic- significant-figures-tutorial/e/significant_figures_1 7. Early Experiments to Characterize the Atom 7.1. Cathodic rays - electron Long before the end of the 19th century, it was well known that applying a high voltage to a gas contained at low pressure in a sealed tube (called a gas discharge tube) caused electricity to flow through the gas, which then emitted light (Figure 5). Researchers trying to understand this phenomenon found that an unusual form of energy was also emitted from the cathode, or negatively charged electrode; this form of energy was called a cathode ray. Figure 5: A Gas Discharge Tube Producing Cathode Rays. 2024/2025 Page 8 of 13 Introduction to chemistry Dr A. Bounab In 1897, the British physicist J. J. Thomson (1856–1940) proved that atoms were not the most basic form of matter. He demonstrated that cathode rays could be deflected, or bent, by magnetic or electric fields, which indicated that cathode rays consist of charged particles (Figure 6). More important, by measuring the extent of the deflection of the cathode rays in magnetic or electric fields of various strengths, Thomson was able to calculate the mass-to-charge ratio of the particles. These particles were emitted by the negatively charged cathode and repelled by the negative terminal of an electric field. Because like charges repel each other and opposite charges attract, Thomson concluded that the particles had a net negative charge; these particles are now called electrons. Most relevant to the field of chemistry, Thomson found that the mass-to-charge ratio of cathode rays is independent of the nature of the metal electrodes or the gas, which suggested that electrons were fundamental components of all atoms. Figure 6: Deflection of Cathode Rays by an Electric Field. As the cathode rays travel toward the right, they are deflected toward the positive electrode (+), demonstrating that they are negatively charged. Subsequently, the American scientist Robert Millikan (1868–1953) carried out a series of experiments using electrically charged oil droplets, which allowed him to calculate the charge on a single electron. With this information and Thomson’s mass-to-charge ratio, Millikan determined the mass of an electron: 𝑚𝑎𝑠𝑠 × 𝑐ℎ𝑎𝑟𝑔𝑒 = 𝑚𝑎𝑠𝑠 𝑐ℎ𝑎𝑟𝑔𝑒 It was at this point that two separate lines of investigation began to converge, both aimed at determining how and why matter emits energy. The video below shows how JJ Thompson used such a tube to measure the ratio of charge over mass of an electron Watch this Video from Davidson College demonstrating Thompson's e/m experiment: https://youtu.be/o1z2S3ME0cI 7.2. Radioactivity The second line of investigation began in 1896, when the French physicist Henri Becquerel (1852–1908) discovered that certain minerals, such as uranium salts, emitted a new form of energy. Becquerel’s work was greatly extended by Marie Curie (1867–1934) and her husband, Pierre (1854–1906); all three shared the Nobel Prize in Physics in 1903. Building on the Curies’ work, the British physicist Ernest Rutherford (1871–1937) performed decisive experiments that led to the modern view of the structure of the atom. While working in Thomson’s laboratory shortly after Thomson 2024/2025 Page 9 of 13 Introduction to chemistry Dr A. Bounab discovered the electron, Rutherford showed that compounds of uranium and other elements emitted at least two distinct types of radiation. One was readily absorbed by matter and seemed to consist of particles that had a positive charge and were massive compared to electrons. Because it was the first kind of radiation to be discovered, Rutherford called these substances α particles. Rutherford also showed that the particles in the second type of radiation, β particles, had the same charge and mass-to-charge ratio as Thomson’s electrons; they are now known to be high-speed electrons. A third type of radiation, γ rays, was discovered somewhat later and found to be similar to the lower-energy form of radiation called x-rays, now used to produce images of bones and teeth. Figure 7: Effect of an Electric Field on α Particles, β Particles, and γ Rays. These three kinds of radiation—α particles, β particles, and γ rays—are readily distinguished by the way they are deflected by an electric field and by the degree to which they penetrate matter. As Figure 7 illustrates, α particles and β particles are deflected in opposite directions; α particles are deflected to a much lesser extent because of their higher mass-to-charge ratio. In contrast, γ rays have no charge, so they are not deflected by electric or magnetic fields. Figure 8 shows that α particles have the least penetrating power and are stopped by a sheet of paper, whereas β particles can pass through thin sheets of metal but are absorbed by lead foil or even thick glass. In contrast, γ-rays can readily penetrate matter; thick blocks of lead or concrete are needed to stop them. Figure 8: Relative Penetrating Power of the Three Types of Radiation. Cartoon of gamma, alpha, and beta rays. The beta ray is stopped by paper. The alpha ray is stopped by 0.5 centimeter lead. The gamma ray is stopped by 10 centimeter lead. 2024/2025 Page 10 of 13 Introduction to chemistry Dr A. Bounab 7.3. The Atomic Model Once scientists concluded that all matter contains negatively charged electrons, it became clear that atoms, which are electrically neutral, must also contain positive charges to balance the negative ones. Thomson proposed that the electrons were embedded in a uniform sphere that contained both the positive charge and most of the mass of the atom, much like raisins in plum pudding or chocolate chips in a cookie (Figure 9). Figure 9: Thomson’s Plum Pudding or Chocolate Chip Cookie Model of the Atom. In this model, the electrons are embedded in a uniform sphere of positive charge Diagram of the plum-pudding model, with spheres of negatively charged electrons in a larger sphere of positively charged matter. In a single famous experiment, however, Rutherford (Nobel Prize in Chemistry in 1908) showed unambiguously that Thomson’s model of the atom was incorrect (Figure 10). Figure 10: A Summary of Rutherford’s Experiments. Rutherford aimed a stream of α particles at a very thin gold foil target (Figure 10a) and examined how the α particles were scattered by the foil. Gold was chosen because it could be easily hammered into extremely thin sheets, minimizing the number of atoms in the target. If Thomson’s model of the atom were correct, the positively-charged α particles 2024/2025 Page 11 of 13 Introduction to chemistry Dr A. Bounab should crash through the uniformly distributed mass of the gold target like cannonballs through the side of a wooden house. They might be moving a little slower when they emerged, but they should pass essentially straight through the target (Figure 10b ). To Rutherford’s amazement, a small fraction of the α particles were deflected at large angles, and some were reflected directly back at the source (Figure 10c ). According to Rutherford, “It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.” Schematic of Rutherford's gold foil experiment. Inset A: Radium fires a stream of alpha particles onto thin gold foil, showing particle deflection onto surrounding photographic film. Inset B: What Rutherford would have expected if Thomas' model were correct: alpha particles continuing through gold foil with no deflection. Inset C: What Rutherford actually observed: alpha particles deflected by nuclei of gold atoms. The Nuclear Atom: The Nuclear Atom, YouTube(https://youtu.be/eqoyZuv1tWA) Rutherford suggested that both the mass and positive charge are concentrated in a tiny fraction of the volume of an atom, which he called the nucleus. The historical development of the different models of the atom’s structure is summarized in Figure 11and Figure 12 Figure 12: A Summary of the Historical Development of Models of the Components and Structure of the Atom. Rutherford established that the nucleus of the hydrogen atom was a positively charged particle, for which he coined the name proton in 1920. He also suggested that the nuclei of elements other than hydrogen must contain electrically neutral particles with approximately the same mass as the proton. The neutron, however, was not discovered until 1932, when Figure 11: The Evolution of Atomic Theory James Chadwick (1891–1974, a student of Rutherford; Nobel Prize in 2024/2025 Page 12 of 13 Introduction to chemistry Dr A. Bounab Physics, 1935) discovered it. As a result of Rutherford’s work, it became clear that an α particle contains two protons and neutrons, and is therefore the nucleus of a helium atom. Summary timeline of the evolution of atomic theory. Shows events at 1803 with Dalton's original proposal, 1904 with Thomson's model, 1911 with Rutherford's experiment, 1913 with Bohr's model, and 1926 with the current orbital model of the atom. Rutherford’s model of the atom is essentially the same as the modern model, except that it is now known that electrons are not uniformly distributed throughout an atom’s volume. Instead, they are distributed according to a set of principles described by Quantum Mechanics. Figure 11 shows how the model of the atom has evolved over time from the indivisible unit of Dalton to the modern view taught today. 8. Summary Chemists study matter and the substances of which it is composed. All substances, in principle, can exist in three states: solid, liquid, and gas. The interconversion between these states can be affected by a change in temperature. The simplest substances in chemistry are elements. Compounds are formed by the combination of atoms of different elements. Atoms are the ultimate building blocks of all matter. The modern atomic theory establishes the concepts of atoms and how they compose matter. Atoms consist of negatively charged electrons around a central nucleus composed of more massive positively charged protons and electrically neutral neutrons. SI units are used to express physical quantities in all sciences, including chemistry. Numbers expressed in scientific notation have the form 𝑁 × 10𝑛 , where N is between 1 and 10 and n is a positive or negative integer. Scientific notation helps us handle very large and very small quantities. Most measured quantities are inexact to some extent. The number of significant figures indicates the exactness of the measurement. 2024/2025 Page 13 of 13