Hess’ Law and Enthalpy Cycles

Experiments: The experiments in previous section allow enthalpy changes to be determined
directly. Hess’ Law determines it indirectly.
• Difficult to determine standard enthalpy change of combustion directly as…
– Carbon dioxide and water would also be formed.
– Activation energy too high.
– Reactions gives mixture of oxides.
• Hess’ Law: If a reaction can take place by more than one route and the initial and final conditions
are the same. Enthalpy change route 1 = route 2 due to conservation of energy. Have to follow
direction of arrows for a route from reactant to product.
• Enthalpy Cycle: A diagram showing routes from reactant to product to find indirect determination
of an enthalpy change from other known enthalpy changes using Hess’ law. A + B = C. So can
calculate unknowns.
• Indirect Determination of Enthalpy Changes of Reaction and Combustion:
– Remember want to form 1 mole in formation.
– Remember want to combust 1 mole in combustion. Excess oxygen.
– Remember elements are 0 for ∆fH.
– Remember standard states and write them down for every substance.
– Remember to multiply standard enthalpy change of formation by balancing numbers.
– Be flexible with what question has given.
• ∆rH
: Can work out standard enthalpy change of reaction from standard enthalpy change of
formation of reactants and products.
– Construct enthalpy cycle. Arrows upwards. Both routes go to products. Elements on the
– Assign ∆fH values given and sum them on each side.
– ∆rH = ∑ ∆fH products – ∑ ∆fH reactants Or = ∆H.
– ADD not subtract the products and reactants separately. Then subtract sum of product from
sum of reactant.

– Construct enthalpy cycle. Instead of elements, put common combustion products CO2 (g) and
H2O (l). Arrows downwards.
– Assign ∆cH
values and calculate unknown enthalpy change.
– ∆rH = ∑ ∆cH reactants – ∑ ∆cH products. Or = ∆H.
• Can also use ∆cH reactants – ∆cH products to get ∆fH.