Cracking the AP Chemistry Exam

Part IV

Content Review for the AP Chemistry Exam

Chapter 7

Big Idea #5: Laws of Thermodynamics and Changes in Matter

HEAT OF FORMATION, ∆H°_f

Heat of formation is the change in energy that takes place when one mole of a compound is formed from its component pure elements under standard state conditions. Heat of formation is almost always calculated at a temperature of 25°C (298 K).

Remember: ∆H°_f for a pure element is defined as zero. This is even true for elements that, in their pure state, appear as diatomic molecules (such as oxygen and hydrogen).

· If ∆H°_f for a compound is negative, energy is released when the compound is formed from pure elements, and the product is more stable than its constituent elements. That is, the process is exothermic.

· If ∆H°_f for a compound is positive, energy is absorbed when the compound is formed from pure elements, and the product is less stable than its constituent elements. That is, the process is endothermic.

If the ∆H°_f values of the products and reactants are known, ∆H for a reaction can be calculated.

∆H° = Σ ∆H°_f products − Σ ∆H°_f reactants

Let’s find ∆H° for the reaction below.

2 CH₃OH( g) + 3 O₂( g) → 2 CO₂( g) + 4 H₂O( g)

∆H° = Σ ∆H°_f products − Σ ∆H°_f reactants

∆H° = [(2)(∆H°_f CO₂) + (4)(∆H°_f H₂O)] − [(2)(∆H°_f CH₃OH) + (3)(∆H°_f O₂)]

∆H° = [(2)(−394 kJ) + (4)(−242 kJ)] − [(2)(−201 kJ) + (3)(0 kJ)]

∆H° = (−1,756 kJ) − (−402 kJ)

∆H° = −1,354 kJ/mol