5 Chemical energeticsAS Level

5.2 Hess's law

2 of 2 objectives have lessons · 0 studied · 0 secure

Lessons

Hess's law: the route does not matterEnthalpy is a state function, so the total enthalpy change depends only on where you start and finish — which lets you add up reactions you cannot do in a calorimeter. 7 min, not started
Formation cycles: going round by the elementsRoute any reaction through the elements instead, and the sum of formation enthalpies gives an enthalpy change no calorimeter could measure. 8 min, not started
Combustion cycles: burning your way to an answerCombustion enthalpies are easy to measure, so a cycle that burns everything to CO₂ and H₂O can find formation enthalpies that no direct experiment can reach. 7 min, not started
Bond energies: break first, then formEvery reaction can be imagined as pulling all the reactant bonds apart into gaseous atoms, then making the product bonds — which turns bond enthalpies into a Hess cycle. 7 min, not started

Syllabus objectives

2 objectives

What Cambridge says you should be able to do, in our words. Codes are the topic number plus the objective’s position.

5.2.1

Apply Hess's law to construct simple energy cycles.

Taught in: Bond energies: break first, then form, Combustion cycles: burning your way to an answer, Formation cycles: going round by the elements, Hess's law: the route does not matter · 10 questions

5.2.2

Carry out calculations using cycles and relevant energy terms, including: (a) determining enthalpy changes that cannot be found by direct experiment; (b) use of bond energy data.

Taught in: Bond energies: break first, then form, Combustion cycles: burning your way to an answer, Formation cycles: going round by the elements · 14 questions