Question

Write a balanced equation for the standard heat of combustion of ethanol (C2H6O).

Use ∆H°f values from your text to calculate the standard heat of combustion for ethanol.

Liquid water was produced in this reaction.

Answer 1

The balanced equation is written as

CH3CH2OH(g)+3O2(g)→2CO2(g)+3H2O(l)

The standard heat of combustion of ethanol

ΔH=Σ(ΔHbroken)−Σ(ΔHformed)

At a molecular level, ethanol's combustion reaction looks like this

CH3CH2OH(g)+3O2(g)→2CO2(g)+3H2O(g)

So, we need to break

5 C-H bonds; 1 C-O single bond; 1 O-H bond; 3 O=O double bonds; 1 C-C single bond.

And form 6 O-H bonds; 4 C=O double bonds.

Substitute the average bond enthalpies for the type of bonds that can be found in your reactants, ethanol and oxygen, and products, carbon dioxide and water.

As a result, the change in enthalpy for the combustion of ethanol is

ΔH°=[(5⋅411)+(1⋅358)+(1⋅459)+(3⋅494)+(1⋅346)]−[(6⋅459)+(4⋅799)]

ΔH°=4700−5950=-1250 kJ/mol.

The minus sign indicates that the reaction is exothermic.