Standard Heats of Formation Worksheet

Standard Heats of Formation

Read each question carefully and answer to the best of your ability. Show all work for calculations.

1. The standard heat of formation (ΔH°f) is the enthalpy change when one mole of a compound is formed from its   in their standard states.

2. For any element in its standard state, the standard heat of formation is defined as  .

3. Hess's Law states that the total enthalpy change for a reaction is the   of the enthalpy changes for the individual steps.

1. Which of the following conditions represents standard state for a substance?

2. The standard heat of formation for O₂ (g) is:

1. Explain why the standard heat of formation of an element in its most stable form is zero.

2. How can Hess's Law be used to calculate the enthalpy change of a reaction that is difficult to measure directly?

Using the following standard heats of formation:

ΔH°f [CH₄(g)] = -74.8 kJ/mol

ΔH°f [CO₂(g)] = -393.5 kJ/mol

ΔH°f [H₂O(l)] = -285.8 kJ/mol

Calculate the enthalpy change (ΔH°rxn) for the combustion of methane:

CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l)

1. An exothermic reaction has a positive enthalpy change (ΔH).

2. The standard heat of formation of a compound is always negative.