Hess's law, energy cycles and bond energies
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The teaching part (1 min) is free to watch. A free account opens the whole lecture: 2 worked examples, 3 practice questions, 4 real past-paper questions with the answers explained, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
What this lecture covers
- Energy you cannot measure directly
- Hess's law
- Using enthalpies of formation
Read the transcript of the teaching part
Last time
Before we start: last time, in Ideal gases and lattice structures, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
Energy you cannot measure directly
You cannot burn graphite to make methane, or measure the heat of forming propane from carbon and hydrogen directly. But you can measure how much heat is given out when each of them burns. Hess's law lets you combine those measured numbers to get the one you cannot measure.
Hess's law
Hess's law: the enthalpy change of a reaction is independent of the route taken, provided the initial and final conditions are the same. It follows from the conservation of energy. Energy cannot be gained or lost by choosing a different route. So we can draw an energy cycle, and add the delta H values round it, with signs, to find one unknown.
Using enthalpies of formation
Route: reactants to elements, then elements to products. The first step reverses the formation of reactants. So delta H reaction equals the sum of delta H f of the products, minus the sum of delta H f of the reactants. Multiply each by its coefficient. Elements in their standard states have a delta H f of zero.
How often it comes up
How often Cambridge has asked each part, counted by ExamYard from every past paper in its question bank: Hess's law: enthalpies of formation and combustion.
The whole chapter: Chemical Energetics, Chemical Energetics.
Chapter 5: Chemical energetics
- Enthalpy changes, calorimetry and pathway diagrams 8 min
- Hess's law, energy cycles and bond energies 9 min
Also for A Level Chemistry 9701
Which topics come up most in A Level Chemistry 9701 past papers, counted from every paper since 2004: Paper 2 topic frequency, Paper 4 topic frequency.