Enthalpy changes, calorimetry and pathway diagrams
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The teaching part (2 min) is free to watch. A free account opens the whole lecture: 1 worked example, 4 practice questions, 3 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
- Hot packs and cold packs
- Enthalpy change
- Enthalpy change definitions
- Writing the equations
Read the transcript of the teaching part
Last time
Before we start: last time, in Hess's law, energy cycles and bond energies, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
Hot packs and cold packs
A hand warmer gets hot when you crack it, but an instant cold pack gets cold. Both are just chemical reactions. One gives out energy to its surroundings, the other takes energy in. Chemists measure that energy as an enthalpy change, in kilojoules per mole.
Enthalpy change
Enthalpy change, delta H, is the heat energy change at constant pressure, in kilojoules per mole. An exothermic reaction gives out heat. The temperature of the surroundings rises, and delta H is negative. An endothermic reaction takes in heat. The surroundings cool, and delta H is positive. Standard conditions are 298 kelvin and 101 kilopascals, with substances in their standard states.
Enthalpy change definitions
Standard enthalpy change of formation: the enthalpy change when one mole of a compound is formed from its elements in their standard states, under standard conditions. Standard enthalpy change of combustion: the enthalpy change when one mole of a substance is completely burned in oxygen, under standard conditions. Enthalpy change of neutralisation: the enthalpy change when an acid and a base react to form one mole of water. Always write the equation with one mole of the substance in question, and with state symbols.
Writing the equations
Formation of water: H two gas plus a half O two gas goes to H two O liquid. Formation of carbon dioxide: C solid plus O two gas goes to CO two gas, minus 393.5 kilojoules per mole. Formation of iron(III) oxide: 2 Fe solid plus one and a half O two gas goes to Fe two O three solid. Formation of carbon monoxide: C solid plus a half O two gas goes to CO gas.
How often it comes up
How often Cambridge has asked each part, counted by ExamYard from every past paper in its question bank: Enthalpy change definitions · Measuring enthalpy changes: q = mcΔT.
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.