Example 5.10: Using Enthalpy of Combustion

As Figure 21 suggests, the combustion of gasoline is a highly exothermic process. To approximate the amount of heat produced by burning 1.00 L of gasoline, we will do the calculation for 1.00 L of isooctane \(\ce{(C8H18)}\), a common component of gasoline. The density of isooctane is 0.692 g/mL.
Figure 21:The combustion of gasoline is very exothermic. (credit: modification of work by “AlexEagle”/Flickr)

Solution

\(V_{\mathrm{isooctane}}\) \(= 1.00\ \mathrm{L}\)


\(ρ_{\mathrm{isooctane}}\) \(= 0.692\ \frac{\mathrm{g}}{\mathrm{mL}}\)


\(q_{\mathrm{approx}}\) = ?


Plan: 1) \(m_{\mathrm{isooctane}}\) 2) \(n_{\mathrm{isooctane}}\) 2) \(q_{\mathrm{rxn}}\)
\(m_{\mathrm{isooctane}}\) \(= V_{\mathrm{isooctane}} \cdot ρ_{\mathrm{isooctane}}\)

\(\ \ \ =1.00\ \mathrm{L} \cdot 0.692\ \frac{\mathrm{g}}{\mathrm{mL}}\)

\(\ \ \ =692.\ \mathrm{g}\)


\(n_{\mathrm{isooctane}}\) \(= \dfrac{m_{\mathrm{isooctane}}}{M_{\mathrm{\ce{C8H18}}}}\)

\(\ \ \ =\dfrac{692.\ \mathrm{g}}{114.23\ \frac{\mathrm{g}}{\mathrm{mol}}}\)

\(\ \ \ =6.06\ \mathrm{mol}\)


Table 2 gives enthalpy of combustion of isooctane as -5460 kJ/mol.

\(ΔH_{\mathrm{combustion}}\) \(= -5460\ \frac{\mathrm{kJ}}{\mathrm{mol}}\)


\(q_{\mathrm{approx}}\) \(= n_{\mathrm{isooctane}} \cdot ΔH_{\mathrm{combustion}}\)

\(\ \ \ =6.06\ \mathrm{mol} \cdot (-5460\ \frac{\mathrm{kJ}}{\mathrm{mol}})\)

\(\ \ \ =-3.31\times 10^{4}\ \mathrm{kJ}\)


The combustion of 1.00 L of isooctane produces 33,100 kJ of thermal energy. (This amount of energy is enough to melt 99.2 kg, or about 218 lbs, of ice.)