Chapter 5 Thermochemistry

Chapter 5 Thermochemistry

By Luis Bello

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LEARNING OUTCOMES

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• Define energy, distinguish types of energy, and describe the nature of energy changes that accompany chemical and physical changes

• Distinguish the related properties of heat, thermal energy, and temperature

• Define and distinguish specific heat and heat capacity, and describe the physical implications of both

• Perform calculations involving heat, specific heat, and temperature change

​Thermochemistry is the study of energy changes that occur during chemical reactions and during changes of state. 

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When chemical reactions occur, some chemical bonds are broken, while new chemical bonds form.

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As a result of the rearrangement of atoms, the total chemical potential energy of the system either increases or decreases.

​Types of Energy

​Two basic types of energy exist: potential energy and kinetic energy. 

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Potential energy is stored energy. It has not yet been released, but is ready to go. 

Kinetic energy is energy of motion. It causes work to be done through movement.

​Chemical Potential Energy

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Energy is the capacity for doing work or supplying heat. When you fill your car with gasoline, you are providing it with potential energy. 

Chemical potential energy is the energy stored in the chemical bonds of a substance.

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The various chemicals that make up gasoline contain a large amount of chemical potential energy that is released when the gasoline is burned in a controlled way in the engine of the car.

​The release of that energy does two things. Some of the potential energy is transformed into work, which is used to move the car.

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At the same time, some of the potential energy is converted to heat, making the car’s engine very hot. The energy changes of a system occur as either heat or work, or some combination of both.​

​Heat

Heat is energy that is transferred from one object or substance to another because of a difference in temperature between them. Heat always flows from an object at a higher temperature to an object at a lower temperature. The flow of heat will continue until the two objects are at the same temperature.

Multiple Choice

A cast iron skillet is used to fry bacon. For optimal frying, the pan must be heated to about 178 ^oC from a room temperature of 22.0 ^oC. It is known that 1.58 x 10⁵ J of heat energy are absorbed by the pan to reach the desired temperature and the specific heat of iron is 0.450 J/g ^oC. What must the mass of the skillet be?

• 12.7 kg

• 2.25 kg

• 110 kg

• 1.97 kg