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) − ΔH∘f(reactants)

=2(33.2) − [2(90.2)+0]

=-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.

Check all that apply.

2Na(s)+F2(g)→2NaF(s)

Na(s)+12F2(l)→NaF(s)

C(s,graphite)+O2(g)→CO2(g)

BaCO3(s)→BaO(s)+CO2(g)

Na(s)+12F2(g)→NaF(s)

CO(g)+12O2(g)→CO2(g)

Part B

The combustion of pentane, C5H12, occurs via the reaction

C5H12(g)+8O2(g)→5CO2(g)+6H2O(g)

with heat of formation values given by the following table:

Substance	ΔH∘f (kJ/mol)
C5H12 (g)	-119.9
CO2(g)	−393.5
H2O(g)	−241.8

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

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

ΔH∘f(products)

Answer 1