What Is Hess’s Law? A Level Chemistry Explained
Learn what Hess's Law means in A Level Chemistry, how enthalpy cycles work, and how to calculate an enthalpy change without mixing up reaction directions.
Hess’s Law states that the enthalpy change of a reaction is the same whatever route the reaction takes, provided the starting and final substances, including their physical states, are the same. Enthalpy change is the heat energy transferred at constant pressure. Because it depends only on the start and end points, you can calculate a reaction’s enthalpy change by adding the changes for reactions that make an alternative route.
Using an enthalpy cycle
An enthalpy cycle shows two routes between the same substances. If you know the enthalpy changes along one route, their sum gives the enthalpy change for the other. This is useful when a reaction’s enthalpy change is difficult to measure directly.
For example, suppose you want the enthalpy change for forming carbon dioxide from carbon monoxide:
CO(g)+12O2(g)→CO2(g) \mathrm{CO(g) + \tfrac{1}{2}O_2(g) \rightarrow CO_2(g)} CO(g)+21O2(g)→CO2(g)Assume the standard enthalpies of formation are ΔHf∘(CO2(g))=−393.5 kJ mol−1\Delta H_f^\circ(\mathrm{CO_2(g)})=-393.5\,\mathrm{kJ\,mol^{-1}}ΔHf∘(CO2(g))=−393.5kJmol−1 and ΔHf∘(CO(g))=−110.5 kJ mol−1\Delta H_f^\circ(\mathrm{CO(g)})=-110.5\,\mathrm{kJ\,mol^{-1}}ΔHf∘(CO(g))=−110.5kJmol−1. A standard enthalpy of formation is the enthalpy change when one mole of a compound forms from its elements in their standard states.
The formation route for carbon dioxide starts with carbon (graphite) and oxygen gas. The route through carbon monoxide starts with those same elements, but needs an additional amount of oxygen to reach carbon dioxide. Oxygen gas has zero standard enthalpy of formation because it is an element in its standard state. Subtracting the carbon monoxide route from the carbon dioxide route therefore gives:
ΔH∘=−393.5−(−110.5)=−283.0 kJ mol−1 \Delta H^\circ=-393.5-(-110.5)=-283.0\,\mathrm{kJ\,mol^{-1}} ΔH∘=−393.5−(−110.5)=−283.0kJmol−1The negative result means the reaction releases heat.
Common exam mistake
Check the direction of every arrow in your cycle. If you reverse a reaction, reverse the sign of its enthalpy change. If you multiply an equation to get the amounts you need, multiply its enthalpy change by the same number. Always check that both routes begin and end with exactly the same substances, amounts and physical states.
