General Chemistry Principles, Patterns, and Applications, 2011

8. [3] As you will learn in Chapter 6 "The Structure of Atoms", hot objects can also lose energy as radiant
energy, such as heat or light. This energy is converted to heat when it is absorbed by another object.
Hence radiant energy is equivalent to heat.
9. [4] The joule is named after the British physicist James Joule (1818–1889), an early worker in the field of
energy.
10. [5] We specify the exact temperatures because the amount of energy needed to raise the temperature of
1 g of water 1°C varies slightly with elevation. To three significant figures, however, this amount is 1.00 cal
over the temperature range 0°C–100°C.
5.2 Enthalpy
L E A R N I N G O B JE C T I V E S
1. To know the key features of a state function.
2. To use Hess’s law and thermochemical cycles to calculate enthalpy changes of chemical reactions.
To study the flow of energy during a chemical reaction, we need to distinguish between a system, the small, welldefined
part of the universe in which we are interested (such as a chemical reaction), and its surroundings, the rest
of the universe, including the container in which the reaction is carried out (). In the discussion that follows, the
mixture of chemical substances that undergoes a reaction is always the system, and the flow of heat can be from the
system to the surroundings or vice versa.
Figure 5.5 A System and Its Surroundings
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