All changes, whether chemical or physical, are accompanied by a change in energy. Each reacting molecule possesses a certain amount of energy due to the nature of chemical bonds. So does each product molecule. As the bonds of the reacting molecules break and the new bonds of the products form, energy is released or absorbed, depending on whether the reactants have higher or lower energy than the products.
We can measure energy changes in several ways. The two kinds of energy change
of most interest to us are: (1) the change in free energy (
G), which is the energy available to do useful work (discussed in Chapter 13),
and (2) the change in enthalpy (
H), which is the heat energy absorbed or released by the reaction and measured
at constant pressure. Most chemical reactions take place under the constant
pressure of the atmosphere. The energy released or absorbed by such reactions
is the change in enthalpy,
H, which can be shown as
In reporting values of
H, a superscript is used to show the temperature at which the measurements were
made. For example, the symbol
H 0°C shows that the change in enthalpy was measured at 0°C. If no temperature
is shown, the enthalpy change was measured at 25°C. All changes are measured
at one atmosphere pressure.
The value of H given
with an equation refers to that particular equation. When the enthalpy change
was measured, the physical states of the components were those stated in the
equation. If the physical states are different, there will be a different enthalpy
change. This difference is illustrated by the next two equations for the formation
of water. They differ in enthalpy change. In the first, gaseous water is formed,
and in the second, liquid water is formed; the difference between their enthalpy
changes reflects the difference in energy content between a gas and a liquid.
(See Chapter 9 for more discussion of
this point.)
The enthalpy change given for a reaction also depends on the coefficients used in the equation for the reaction. Thus, if the equation for the formation of water is written
the enthalpy change is twice what it was in the previous equation for the formation of gaseous water when the coefficient of water was 1. This last problem can be resolved by doing as we do in several equations where we report the enthalpy change per mole of one component of the reaction, thus removing any ambiguity in interpretation.
A. Endothermic and Exothermic Reactions
A reaction that absorbs energy is an endothermic reaction; its enthalpy change
( H) is positive. The
enthalpy of the products of the reaction is greater than that of the reactants.
Energy is absorbed from the surroundings. The following reactions are endothermic.
A reaction that releases energy is an exothermic reaction; its enthalpy change is negative. The enthalpy of the products is less than that of the reactants. Energy is released to the surroundings. The following reactions are exothermic.
Notice that the decomposition of water (equation b) is endothermic and requires the input of 285.8 kJ energy per mole of water decomposed. The reverse reaction, the formation of one mole of water from hydrogen and oxygen (equation d), is exothermic and releases 285.8 kJ energy. The amount of energy is the same, but the sign of the energy change is different.
Another example is the relationship between energy change and the direction of a reaction is the formation and decomposition of glucose. Glucose (C6H12O6) is formed from carbon dioxide and oxygen in the cells of green plants in the process called photosynthesis. Photosynthesis is an endothermic reaction. The source of the energy for the formation of glucose is light (radiant energy), usually from the sun.
Thus, green plants have the remarkable ability to trap the energy of sunlight and use that energy to produce glucose from carbon dioxide and water. The energy is stored in the glucose. Animal and plant cells have the equally remarkable ability to metabolize glucose and use the energy released to maintain body temperature or do biological work, such as contracting muscles or thinking.
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Example
Solution a. The enthalpy change is positive; the reaction is endothermic. The reverse reaction is:
b. The enthalpy change is negative; the reaction is exothermic. The reverse reaction is:
c. The enthalpy change is negative; the reaction is exothermic. The reverse reaction is:
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B. The Stoichiometry of Energy Changes
The energy change associated with a reaction is a stoichiometric quantity and
can be treated arithmetically, as were mass changes in Section 8.4. For many
reactions, enthalpy changes have been determined and tabulated in the chemical
literature. The changes listed in such sources apply only to the form of the
equation they accompany, as explained previously.
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Example
Solution Equation
Wanted:
Given
Conversion factors
Arithmetic equation
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Example Solution Equation
Wanted
Given
Conversion factors Glucose, mass to moles: 180 g glucose = 1 mol glucose Arithmetic equation
Answer
Answer
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