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UNIT 3 CONTENTS CHAPTER 5 Energy and Change CHAPTER 6 Rates of Chemical Reactions UNIT 3 PROJECT Developing a Bulletin About Catalysts and Enzymes e ——rra— UNIT 3 OVERALL EXPECTATIONS m What energy transformations and mechanisms are involved in chemical change? m What skills are involved in determining energy changes for physical and chemical processes and rates of reaction? m How do chemical technologies and processes depend on the energetics of chemical reactions? Look ahead to the project at the end of Unit 3. Start preparing for the project now by listing what you already know about catalysts and enzymes. Think about how catalysts and enzymes affect chemical reactions. As you work through the unit, plan how you will investigate and present a bulletin about the uses of catalysts and enzymes in Canadian industries. In the nineteenth century, railway tunnels were blasted through the Rocky Mountains to connect British Columbia with the rest of Canada. Workers used nitroglycerin to blast through the rock. This compound is so unstable, however, that accidents were frequent and many workers died. Alfred B. Nobel found a way to stabilize nitroglycerin, and make it safer to use, when he invented dynamite. What makes nitroglycerin such a dangerous substance? First, nitro- glycerin, C3H5(NQO3)3(, gives off a large amount of energy when it decomposes. In fact, about 1500 k] of energy is released for every mole of nitroglycerin that reacts. Second, the decomposition of nitroglycerin occurs very quickly—in a fraction of a second. This fast, exothermic reaction is accompanied by a tremendous shock wave, which is caused by the expansion of the gaseous products. Finally, nitroglycerin is highly shock- sensitive. Simply shaking or jarring it can cause it to react. Thus, nitroglycerin’s explosive properties are caused by three fac- tors: the energy that is given off by its decomposition, the rate at which the reaction occurs, and the small amount of energy that is needed to initiate the reaction. In this unit, you will learn about the energy and rates of various chemical reactions. Energy Changes and Rates of Reaction

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Chapter Preview 5.1 The Energy of Physical, Chemical, and Nuclear Changes 5.2 Determining Enthalpy of Reaction by Experiment 5.3 Hess's Law of Heat Summation 5.4 Energy Sources Prerequisite Concepts and Skills Before you begin this chapter, review the following concepts and skills: m writing balanced chemical equations (Concepts and Skills Review) m performing stoichiometric calculations (Concepts and Skills Review) Energy and Change Think about a prehistoric family group building a fire. It may seem as though this fire does not have much in common with a nuclear power plant. Both the fire and the nuclear power plant, however, are technolo- gies that harness energy-producing processes. As you learned in Unit 2, humans continually devise new techno- logies that use chemical reactions to produce materials with useful properties. Since the invention of fire, humans have also worked to devise technologies that harness energy. These technologies depend on the fact that every chemical, physical, and nuclear process is accompanied by a characteristic energy change. Consider the melting of an ice cube to cool a drink, the combustion of natural gas to cook a meal, and the large-scale production of electricity via a nuclear power plant. All societies depend on the energy changes that are associated with these physical, chemical, and nuclear processes. In this chapter, you will study the causes and magnitude of the energy changes that accompany physical changes, chemical reactions, and nuclear reactions. You will see that different processes involve vastly different amounts of energy. You will learn how to calculate the amount of energy that is absorbed or released by many simple physical changes and chemical reactions. This will allow you to predict energy changes without having to carry out the reaction—an important skill to have when dealing with dangerous reactions. Finally, you will examine the efficiency and environmental impact of traditional and alternative energy sources. Enough radiant energy reaches Earth every day to meet the world’s energy needs many times over. Since this is ~the case, why do fossil fuels provide most of Canada’s energy, while solar power supplies only a tiny fraction? " 220 MHR - Unit 3 Energy Changes and Rates of Reaction

The Energy of Physical, Chemical, and Nuclear Processes Most physical changes, chemical reactions, and nuclear reactions are accompanied by changes in energy. These energy changes are crucial to life on Earth. For example, chemical reactions in your body generate the heat that helps to regulate your body temperature. Physical changes, such as evaporation, help to keep your body cool. On a much larger scale, there would be no life on Earth without the energy from the nuclear reactions that take place in the Sun. The study of energy and energy transfer is known as thermodynamics. Chemists are interested in the branch of thermodynamics known as thermochemistry: the study of energy involved in chemical reactions. In order to discuss energy and its interconversions, thermochemists have agreed on a number of terms and definitions. You will learn about these terms and definitions over the next few pages. Then you will examine the energy changes that accompany chemical reactions, physical changes, and nuclear reactions. Studying Energy Changes The law of conservation of energy states that the total energy of the universe is constant. In other words, energy can be neither destroyed nor created. This idea can be expressed by the following equation: AEuniverse =0 Energy can, however, be transferred from one substance to another. It can also be converted into various forms. In order to interpret energy changes, scientists must clearly define what part of the universe they are dealing with. The system is defined as the part of the universe that is being studied and observed. In a chemical reaction, the system is usually made up of the reactants and products. By contrast, the surroundings are everything else in the universe. The two equations below show the relationship between the universe, a system, and the system’s surroundings. Universe = System + Surroundings AEyniverse = AEsystem + AEsurroundings =0 From the relationship, we know that any change in the system is accompanied by an equal and opposite change in the surroundings. AEsystem = _AEsurroundings Look at the chemical reaction that is taking place in the flask in Figure 5.1. A chemist would probably define the system as the contents of the flask—the reactants and products. Technically, the rest of the universe is the surroundings. In reality, however, the entire universe changes very little when the system changes. Therefore, the surroundings are considered to be only the part of the universe that is likely to be affected by the energy changes of the system. In Figure 5.1, the flask, the lab bench, the air in the room, and the student who is carrying out the reaction all make up the surroundings. The system is more likely to significantly influence its immediate surroundings than, say, a mountaintop in Japan (also, technically, part of the surroundings). Section Preview/ Specific Expectations In this section, you will m write thermochemical equa- tions, expressing the energy change as a heat term in the equation or as AH m represent energy changes using diagrams m compare energy changes that result from physical changes, chemical reac- tions, and nuclear reactions m communicate your under- standing of the following terms: thermodynamics, thermochemistry, law of conservation of energy, system, surroundings, heat (Q), temperature (T), enthalpy (H), enthalpy change (AH), endothermic reaction, exothermic reac- tion, enthalpy of reaction (AH. ), standard enthalpy of reaction (AH’ ), thermo- chemical equation, mass defect, nuclear binding energy, nuclear fission, nuclear fusion i EETZXEN The solution in the flask is the system. The flask, the laboratory, and the student are the surroundings. Chapter 5 Energy and Change - MHR 221

Heat and Temperature Heat, Q, refers to the transfer of kinetic energy. Heat is expressed in the same units as energy—joules (J). Heat is transferred spontaneously from a warmer object to a cooler object. When you close the door of your home on a cold day to “prevent the cold from getting in,” you are actually preventing the heat from escaping. You are preventing the kinetic energy in your warm home from transferring to colder objects, including the cold air, outside. Temperature, 7, is a measure of the average kinetic energy of the particles that make up a substance or system. You can think of temperature as a way of quantifying how hot or cold a substance is, relative to another substance. K B uc 110 :(\ GETEEE] Celsius degrees and Kelvin degrees are the same size. The Kelvin scale begins at absolute zero. This is the temperature at which the particles in a substance have no kinetic energy. Therefore, Kelvin temperatures are never negative. By contrast, 0°C is set at the melting point of water. Celsius temperatures can be positive or negative. Temperature is measured in either Celsius degrees (°C) or kelvins (K). The Celsius scale is a relative scale. It was designed so that water’s boiling point is at 100°C and water’s melting point is at 0°C. The Kelvin scale, on the other hand, is an absolute scale. It was designed so that 0 K is the temperature at which a substance possesses no kinetic energy. The relationship between the Kelvin and Celsius scales is shown in Figure 5.2, and by the following equation. Temperature in Kelvin degrees = Temperature in Celsius degrees + 273.15 Enthalpy and Enthalpy Change Chemists define the total internal energy of a substance at a constant pressure as its enthalpy, H. Chemists do not work with the absolute enthalpy of the reactants and products in a physical or chemical process. Instead, they study the enthalpy change, AH, that accompanies a process. That is, they study the relative enthalpy of the reactants and products in a system. This is like saying that the distance between your home and your school is 2 km. You do not usually talk about the absolute position of your home and school in terms of their latitude, longitude, and elevation. You talk about their relative position, in relation to each other. The enthalpy change of a process is equivalent to its heat change at constant pressure. 222 MHR - Unit 3 Energy Changes and Rates of Reaction