Question 9.9: Ethanol, C2H5OH, is used to introduce oxygen into some blend......
Ethanol, C_2H_5OH, is used to introduce oxygen into some blends of gasoline. It has a heat of combustion of 1366.8 kJ/mol. What is the heat of formation of ethanol?
Strategy We know the relationship between the heat of combustion of a reaction and the heats of formation of the substances involved. We will need a balanced chemical equation for the combustion, and we must recognize that the combustion of ethanol yields the same products as hydrocarbon combustion. We can write the balanced equation and then use Equation 9.12 to determine the desired quantity. In this case, we know the heat of reaction and will be solving for one of the heats of formation.
\Delta H^{\circ}=\sum_{i}\nu_{i}\Delta H_{f}^{\circ}\,(\mathrm{product})_{i}-\sum_{j}\nu_{j}\Delta H_{f}^{\circ}\,(\mathrm{reactant})_{j} (9.12)
C_2H_5OH(\ell) + 3\ O_2(g) → 2\ CO_2(g) + 3\ H_2O(\ell)\qquad\quad ΔH^\circ = –1366.8 kJ
Using Equation 9.12:
–1366.8 kJ = 2 mol (–393.5 kJ/mol) + 3 mol (–285.8 kJ/mol) – ΔH_f^\circ[C_2H_5OH(\ell)] – 3 mol (0 kJ/mol)
Rearranging and solving gives \Delta H_f^\circ[C_2H_5OH(\ell)] = –277.6 kJ/mol
Analyze Your Answer By now, we have seen enough heats of formation to know that values in the hundreds of kJ/mol seem fairly typical. Notice that it is very important to handle signs carefully when solving this type of problem: the fact that we are adding and subtracting quantities that can be positive or negative allows many opportunities for errors.
Check Your Understanding Incomplete combustion of hydrocarbons leads to the generation of carbon monoxide rather than carbon dioxide. As a result, improperly vented furnaces can poison people who live in an affected building because of the toxicity of CO. Calculate the heat of reaction for the incomplete combustion of methane, CH_4(g), to yield liquid water and CO(g).