Consider the table below comparing two exothermic reactions. Methanol and ethanol are popular, eco-friendly fuels. However, which fuel releases more energy when burned? Your understanding of communicating enthalpy changes will help you answer this question.

Fuel	Combustion Reaction Equation	ΔcH°
Methanol	2CH3OH(l) + 3O2(g) → 2CO2(g) + 4H2O(g)	- 1275.8 kJ
Ethanol	C2H5OH(l) + 3O2(g) → 2CO2(g) + 3H2O(g)	- 1234.8 kJ

If you consider the enthalpies of combustion for the balanced combustion reaction, methanol has a higher enthalpy of combustion.

However, is it scientifically sound to conclude that methanol releases more energy when combusted? As you have already probably predicted, it would be wiser to compare molar enthalpies of combustion for each fuel. Recall that Δ_cHm = Δ_cH/n

Per mole, ethanol releases significantly more energy than methanol.

Another way to compare these fuels is to look at the enthalpies of combustion per gram. To do this, you must divide the molar enthalpies of combustion by the molar mass of each fuel.

\( \mathrm { Energy~per~gram~CH_3OH(l) = -637.9 \frac{kJ}{mol} \times \dfrac{1~mol}{32.05~g} = -19.90 \frac{kJ}{g} } \)

\( \mathrm { Energy~per~gram~C_2H_5OH(l) = -1234.8 \frac{kJ}{mol} \times \dfrac{1~mol}{46.08~g}= -26.80 \frac{kJ}{g} } \)

Per gram of fuel, ethanol releases more energy per gram than methanol.