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.)