Quick questions on Thermochemical equations and enthalpy changes: VCE Chemistry Unit 3

A thermochemical equation is a balanced chemical equation that includes the enthalpy change (ΔH) for the reaction as written. The equation must include the states of all reactants and products because enthalpy changes depend on phase (vapour vs liquid water release different energies, for example).

To find the energy released or absorbed by a non-stoichiometric amount, use the mole ratio.

Thermochemical equations are visualised as energy profile diagrams showing reactants, products, and the activation energy barrier.

Always include states because the same reaction releases different ΔH depending on phase. For example:

Write the thermochemical equation for the complete combustion of propane gas, $\Delta H_c = -2220 \, \text{kJ/mol}$. State the states of all species. [3 marks]

Calculate the heat released when $100 \, \text{g}$ of ethanol burns completely. $\Delta H_c (\text{ethanol}) = -1367 \, \text{kJ/mol}$. [3 marks]

Use Hess's law to determine $\Delta H$ for $\text{C(s)} + 2 \text{H}_2 (\text{g}) \to \text{CH}_4 (\text{g})$ from: $\text{C(s)} + \text{O}_2 \to \text{CO}_2$ ($-394$); $\text{H}_2 + 1/2 \text{O}_2 \to \text{H}_2 \text{O}$ ($-286$); $\text{CH}_4 + 2 \text{O}_2 \to \text{CO}_2 + 2 \text{H}_2 \text{O}$ ($-890$). (a) Write equations 1+2 and reverse 3. (b) Add and simplify.