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Thermochemistry

Learn about energy transformations, heat flow, exothermic and endothermic processes, and units for measuring heat flow in this chapter on thermochemistry.

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Thermochemistry

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  1. Thermochemistry Chapter 17 Chemistry

  2. Section 1 - The Flow of Energy — Heat and Work Introduction • The temperature of lava from a volcano ranges from 550°C to 1400°C. • As lava flows, it loses heat and begins to cool. • You will learn about heat flow and why some substances cool down or heat up more quickly than others.

  3. 17.1 1. Energy Transformations • Energy is the capacity to do work or supply heat. • Energy has neither mass nor volume. • Thermochemistry is the study of energy changes that occur during chemical reactions and changes in state. • The energy stored in the chemical bonds of a substance is called chemical potential energy. • The kinds of atoms and their arrangement in the substance determine the amount of energy stored. • Heat, represented by q, is energy that transfers from one object to another because of a temperature difference between them. • Heat always flows from a warmer object to a cooler object. When fuel is burned in a car engine, chemical potential energy is released and is used to do work.

  4. 2. Exothermic and Endothermic Processes • In studying energy changes, you can define a system as the part of the universe on which you focus your attention. • The surroundings include everything else in the universe. • Together the system and its surroundings make up the universe. • The law of conservation of energystates that in any chemical or physical process, energy is neither created nor destroyed.

  5. Exothermic and Endothermic Processes (cont.) • An endothermic processis one that absorbs heat from the surroundings. • In an endothermic process, the system gains heat as the surroundings cool down. • q has a positive value because heat is flowing into the system. • An exothermic processis one that releases heat to its surroundings. • In an exothermic process, the system loses heat as the surroundings heat up. • q has a negative value because heat is flowing out of the system.

  6. 17.1 3. Units for Measuring Heat Flow • Heat flow is measured in two common units, the calorie and the joule. • A calorie (cal) is defined as the quantity of heat needed to raise the temperature of 1g of pure water by 1⁰C. • The word calorie is written with a small c except when referring to the energy contained in food. • The energy in food is usually expressed in Calories. • The joule is the SI unit of energy • One joule of heat raises the temperature of 1g of pure water by 0.2390⁰C 1 J = 0.2390 cal 4.184 J = 1 cal

  7. 4. Heat Capacity and Specific Heat • The heat capacity of an object depends on both its mass and its chemical composition. • The greater the mass the greater the heat capacity. • The amount of heat needed to increase the temperature of an object exactly 1°C is the heat capacityof that object. • The specific heat capacity, or simply the specific heat (C), of a substance is the amount of heat it takes to raise the temperature of 1 g of the substance 1°C • Unit for specific heat is J/g∙⁰C or cal/g∙⁰C

  8. Heat Capacity and Specific Heat (cont.) • Example: • Step 1: • Step 2:

  9. END OF SECTION 1

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