There are two ways to express the ∆H value associated with a balanced chemical equation.
Thermochemical Equations Versus ∆H Notation
A thermochemical equation includes the energy change as an integral part of the equation.
In an endothermic reaction, the energy absorbed by the reactants to form the products appears on the reactant side of the equation as though the energy itself is a reactant. Of course, energy is not a form of matter, so this cannot be strictly true. Energy (enthalpy) changes are extensive properties. These are properties that depend on the quantity of material reacting and forming in a reaction. For this reason, the unit for an enthalpy change is kJ/mol. Consequently, if twice as many species react, the magnitude of the enthalpy change should be doubled. In a thermochemical equation, the mole in the denominator of the enthalpy unit actually refers to a mole of the reaction. The following equation shows the decomposition of water: 572 kJ/mol + 2 H2O(l) ➝ 2 H2(g) + O2(g) It indicates that 572 kJ of energy are required per mole of this reaction. A mole of this reaction involves the decomposition of 2 mol of H2O(l) to form 2 mol of H2(g) and 1 mol of O2(g). Consequently, if 1 mol of water is decomposed, the energy requirement will be half as much, as represented in the following thermochemical equation. 286 kJ/mol + H2O(l) ➝ H2(g) + ½ O2(g) In an exothermic reaction, the energy released by the reactants as the products are formed appears on the product side of the equation. According to what we’ve learned about bond energy, it should come as no surprise that the equation for the formation of water is simply the reverse of that for the decomposition, including the same magnitude of the heat change. 2 H2(g) + O2(g) ➝ 2 H2O(l) + 572 kJ/mol
H2
hydrogen
catho
O2
anod
e
de
The energy released by this reaction is exploited in fuel cells (Figure 4.4.3). The prototype fuel cell was used in the Apollo spacecrafts. These cells were continuously fed with hydrogen and oxygen gas and had the unique advantage of producing drinking water along with energy. The International Space Station continues to rely on fuel cells for energy production in space today.
-
oxygen
+
H H H
H
H H
H
Water
– +
(a)
(b)
H2 excess hydrogen for recirculation
H
catalyst
H
H
H
O O
O
electrolyte
H
H
catalyst
H H
O
electricity
O
O
O
O O
H H O H O H H2O
water
Figure 4.4.3 The energy required to decompose 2 mol of water in an electrolysis apparatus (a) is identical to the energy released in
the formation of 2 mol of water in a fuel cell (b). Fuel cells may be the most rapidly expanding form of energy production today.
192 Chapter 4 Expressing and Measuring Chemical Change
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