Thermochemistry: study guide

Key ideas

Thermochemistry follows the heat exchanged in chemical and physical changes. The enthalpy change ΔH is negative for exothermic reactions, which release heat, and positive for endothermic ones.

• Heat to change temperature is q = mcΔT. Water has c = 4.18 J/(g·°C).
• Hess’s law: the enthalpy change of a reaction is the same whether it takes one step or several, so equations and their ΔH values can be added.
• Using formation enthalpies, ΔH°_rxn = ΣΔH°f(products) − ΣΔH°f(reactants), with elements in their standard state at zero.
• With bond enthalpies, ΔH ≈ energy to break bonds in the reactants minus energy released forming bonds in the products.

Key equations

• q = mcΔT
• ΔH°_rxn = ΣΔH°f(products) − ΣΔH°f(reactants)
• ΔH ≈ Σ bonds broken − Σ bonds formed
• q = mL (phase change)

Worked example

Burning methane: CH₄ + 2 O₂ → CO₂ + 2 H₂O(l). With ΔH°f of −74.8 for CH₄, −393.5 for CO₂ and −285.8 kJ/mol for H₂O(l): ΔH°_rxn = [−393.5 + 2(−285.8)] − (−74.8) = −890.3 kJ per mole of methane. In a calorimeter, 100.0 g of water warming from 20.0 °C to 25.0 °C absorbs q = 100.0 × 4.18 × 5.0 = 2090 J.

Common mistakes

• Forgetting to multiply ΔH°f by the stoichiometric coefficient.
• Reversing the products-minus-reactants order, which flips the sign of ΔH.
• Using ΔH for one mole of reaction without scaling when the equation is doubled or reversed.

Practice

Test yourself with practice questions on this topic: Thermochemistry