The standard heat of formation, H f, is defined as the enthalpy change for the f

Question

The standard heat of formation, Hf, 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 Hf=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:

Then the standard heat of reaction for the overall reaction is

Hrxn = Hf(products) Hf(reactants)

=2(33.2) [2(90.2)+0]

=-114 kJ

Part A

For which of the following reactions is Hrxn equal to Hf of the product(s)?

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

Check all that apply.

Li(s)+12Cl2(g)LiCl(s)

Part B

The combustion of propane, C3H8, occurs via the reaction

C3H8(g)+5O2(g)3CO2(g)+4H2O(g)

with heat of formation values given by the following table:

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

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

Explanation / Answer

Part A

In this reaction H2(g)+12O2(g)H2O(g) Hrxn to Hf of the product.

Because Hf of H2 &O2 are 0 kJ / mole

PART B

You calculate Hc from standard enthalpies of formation:

Hoc = Hf(p) Hf(r)

Hoc = 3 x (-393.5) + 4 (-241.8) - (-104.7)

Hoc = - 1180.5 - 967.2 + 104.7

Hoc = -2043 kj / Mole

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