Hess’s Law - Chemistry

1. Fundamental Concepts

Hess’s Law is based on the Law of Conservation of Energy. Its core principle is: the enthalpy change (ΔH) of a chemical reaction depends only on the initial state (reactants) and final state (products) of the reaction, not on the reaction path/steps. The total enthalpy change remains the same whether the reaction proceeds in a single step or multiple steps. ΔH is a state function, with the common unit of kJ/mol.

When a reaction is reversed, the sign of ΔH is flipped while the numerical value remains unchanged (e.g., if ΔH = +50 kJ/mol for the forward reaction, ΔH = -50 kJ/mol for the reverse reaction).
When a chemical equation is multiplied by a coefficient n, ΔH is also multiplied by the same coefficient n (e.g., ΔH for 2A → 2B is 2×ΔH for A → B).
The total ΔH of a reaction = the algebraic sum of ΔH values of all individual step reactions (the step reactions must combine to form the overall reaction).
It only applies to enthalpy change calculations under constant pressure, with other energy changes except volume work neglected.

Given: ① C(s) + O₂(g) → CO₂(g) ΔH₁ = -393.5 kJ/mol

Calculate the ΔH for the reaction: 2C(s) + 2O₂(g) → 2CO₂(g)

Answer: ΔH = 2×(-393.5) = -787.0 kJ/mol

Given: ① N₂(g) + O₂(g) → 2NO(g) ΔH₁ = +180.7 kJ/mol

② 2NO(g) + O₂(g) → 2NO₂(g) ΔH₂ = -113.1 kJ/mol

Calculate the ΔH for the reaction: N₂(g) + 2O₂(g) → 2NO₂(g)

Answer: ΔH = ΔH₁ + ΔH₂ = 180.7 - 113.1 = +67.6 kJ/mol

Given: ① C(s) + O₂(g) → CO₂(g) ΔH₁ = -393.5 kJ/mol

② H₂(g) + ½O₂(g) → H₂O(l) ΔH₂ = -285.8 kJ/mol

③ C₂H₄(g) + 3O₂(g) → 2CO₂(g) + 2H₂O(l) ΔH₃ = -1411.0 kJ/mol

Calculate the ΔH for the reaction: 2C(s) + 2H₂(g) → C₂H₄(g)

Step: 2×① + 2×② - ③

Answer: ΔH = 2×(-393.5) + 2×(-285.8) - (-1411.0) = +52.4 kJ/mol

Write the overall reaction: Identify the reactants, products and their coefficients for the target reaction.
Adjust the step reactions: Scale the coefficients or reverse the direction of the known step reactions to match the overall reaction, and label the corresponding changes in ΔH.
Combine the step reactions: Add the adjusted step reactions together to cancel out all intermediate products, ensuring the result is identical to the overall reaction.
Calculate the total ΔH: Find the algebraic sum of the ΔH values of the adjusted step reactions to get the ΔH of the overall reaction.
Verify: Check that all intermediate products are completely canceled, and the coefficients and physical states of substances match those in the overall reaction.