Question

The heats of combustion of $C{H}_4$ and $C_2{H}_6$ are -890.3 and -1559.7kJ mol-1 respectively. Which of the two has greater efficiency of fuel per gram?

• A. methane
• B. ethane
• C. equal
• D. none

Answer 1

The correct option is A methane

The fuel efficiency can be predieted from the amount of heat evolved for every gram of fule consumed.

(i) The combustion of methane is as follows :

$C{H}_{4\left(g\right)}+{20}_{2\left(g\right)}\to C{O}_{2\left(g\right)}+2H_2{O}_{\left(1\right)}$

$DHc=890.3KJmol-1$

Molar mass of $C{H}_4=16$

Heat produced per gram $=\frac{890.3}{16}=55.64KJ{g}^{-1}$

(ii) The combustion of ethane is as follows :

$C_2{H}_{6\left(g\right)}+\frac{7}{2}{O}_{2\left(g\right)}\to 2C{O}_{2\left(g\right)}+3H_2{O}_{\left(1\right)}$

$D{H}_c=1559.7KJmo{l}^{-1}$

Molar mass of $C_2{H}_6=30$

Heat produced per gram = $\frac{1559.7}{30}=51.99KJ{g}^{-1}$

Thus, methane has greater fuel efficiency then ethane.