Question

The standard heat of formation, ΔH∘f, is defined as the enthalpy change for the formation of one mole of substance from its constituent elements in their standard states. Thus, elements in their standard states have ΔH∘f=0. Heat of formation values can be used to calculate the enthalpy change of any reaction.

Consider, for example, the reaction

2NO(g)+O2(g)⇌2NO2(g)

with heat of formation values given by the following table:

Substance   ΔH∘f

(kJ/mol)

NO(g)   90.2

O2(g)   0

NO2(g)   33.2

Then the standard heat of reaction for the overall reaction is

ΔH∘rxn===ΔH∘f(products)2(33.2)−114 kJ

Part A

For which of the following reactions is ΔH∘rxn equal to ΔH∘f of the product(s)?

You do not need to look up any values to answer this question.

Check all that apply.

   Na(s)+12F2(l)→NaF(s)

   2H2(g)+O2(g)→2H2O(g)

   H2O2(g)→12O2(g)+H2O(g)

   Na(s)+12F2(g)→NaF(s)

   2Na(s)+F2(g)→2NaF(s)

   H2(g)+12O2(g)→H2O(g)

Part B

The combustion of heptane, C7H16, occurs via the reaction

C7H16(g)+11O2(g)→7CO2(g)+8H2O(g)

with heat of formation values given by the following table:

Substance   ΔH∘f

(kJ/mol)

C7H16 (g)   -187.9

CO2(g)   −393.5

H2O(g)   −241.8

Calculate the enthalpy for the combustion of 1 mole of heptane.

Express your answer to four significant figures and include the appropriate units.

Answer 1