Standard Heats of Formation Worksheet

Standard Heats of Formation

Read each question carefully and provide your best answer. Show all your work for calculation problems.

What is Enthalpy?

Enthalpy (H) is a thermodynamic property of a system. It is the sum of the internal energy (U) of the system and the product of its pressure (P) and volume (V). In simpler terms, it's a measure of the total heat content of a system. When a chemical reaction occurs, the change in enthalpy (ΔH) represents the heat absorbed or released during the reaction at constant pressure.

Standard Enthalpy of Formation (ΔH°f)

The standard enthalpy of formation (ΔH°f) is the enthalpy change when one mole of a compound is formed from its constituent elements in their standard states at 25°C (298 K) and 1 atm pressure. The standard enthalpy of formation for an element in its standard state is defined as zero.

1. Which of the following statements best describes the standard enthalpy of formation (ΔH°f)?

2. What is the standard enthalpy of formation (ΔH°f) for a pure element in its standard state?

3. The standard state for most elements is defined at a temperature of   and a pressure of  .

4. A reaction with a negative ΔH is called an   reaction, meaning it   heat.

5. According to Hess's Law, the total enthalpy change for a reaction is   of the pathway taken.

6. Using the following standard enthalpies of formation, calculate the enthalpy change (ΔH°) for the combustion of methane (CH4):

CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)

ΔH°f [CH4(g)] = -74.8 kJ/mol

ΔH°f [CO2(g)] = -393.5 kJ/mol

ΔH°f [H2O(l)] = -285.8 kJ/mol

7. Explain how Hess's Law is applied in calculating the enthalpy change of a reaction.

8. The standard enthalpy of formation for a compound is always a negative value.

9. A reaction that absorbs heat from its surroundings is an endothermic reaction.