Chemical Thermodynamics

Hess's law: the total enthalpy change for a reaction is the same whether it occurs in one step or many, provided the initial and final states are the same.

This works because enthalpy (H) is a state function — it depends only on the current state, not the path taken to reach it.

Practical use: if a reaction is hard to measure directly, you can compute its ΔH by adding/subtracting easier reactions whose ΔH values are known.

Worked example. Find ΔH for: C(s) + ½O₂ → CO(g)

Direct measurement is hard (CO₂ also forms). But we know:

• Reaction A: C(s) + O₂(g) → CO₂(g), ΔH = −393 kJ/mol
• Reaction B: CO(g) + ½O₂(g) → CO₂(g), ΔH = −283 kJ/mol

Reverse B and add to A:

• A: C + O₂ → CO₂ (−393)
• −B: CO₂ → CO + ½O₂ (+283)
• Sum: C + ½O₂ → CO (−110 kJ/mol)

So ΔH = −110 kJ/mol.

Key formulas:

• ΔH_rxn = Σ ΔH_f(products) − Σ ΔH_f(reactants)
• ΔH for reverse reaction = −ΔH for forward reaction
• ΔH for reaction × n = n × ΔH (scale by stoichiometry)
• Standard enthalpy of formation (ΔH_f°) of an element in its standard state = 0