gen-define-hess-s-law-and-give-one-short-worked-example-0ec3

Define Hess's law and give one short worked example.

# Hess's Law **Definition:** Hess's Law states that the total enthalpy change of a reaction is independent of the route taken. In other words, if a reaction can occur by multiple pathways, the overall enthalpy change is the same whether it happens in one step or several steps. This works because enthalpy is a state function—it depends only on the initial and final states, not on the path between them. ## Worked Example **Problem:** Calculate ΔH for the reaction: C(s) + ½O₂(g) → CO(g) **Given data:** 1. C(s) + O₂(g) → CO₂(g) ΔH₁ = -394 kJ/mol 2. CO(g) + ½O₂(g) → CO₂(g) ΔH₂ = -283 kJ/mol **Solution:** We need to manipulate the given equations to get our target equation. - Keep equation 1 as is: C(s) + O₂(g) → CO₂(g) ΔH₁ = -394 kJ/mol - Reverse equation 2: CO₂(g) → CO(g) + ½O₂(g) -ΔH₂ = +283 kJ/mol Adding these equations: C(s) + O₂(g) + CO₂(g) → CO₂(g) + CO(g) + ½O₂(g) Canceling CO₂(g) from both sides: **C(s) + ½O₂(g) → CO(g)** **ΔH = ΔH₁ + (-ΔH₂) = -394 + 283 = -111 kJ/mol**