Topic 5
Chemical Energetics
Topic 5 covers the energy changes that accompany chemical reactions: exothermic and endothermic reactions, reaction pathway diagrams and activation energy, standard conditions of 298 K and 101 kPa, the enthalpy changes of reaction, formation, combustion and neutralisation, calculating enthalpy changes from experimental results, bond energies, and Hess’s law energy cycles. Lattice energy and Born-Haber cycles follow at A Level in Topic 23.
Revision Notes
5 revision notes, in the order the specification teaches them.
Enthalpy Changes and Standard Conditions
Exothermic and endothermic reactions, reaction pathway diagrams, activation energy and standard conditions.
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Standard Enthalpy Changes
The standard enthalpy change definitions and the equation each one refers to.
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Calorimetry: Measuring Enthalpy Changes
Calculating ΔH from a temperature change, cooling curve corrections, errors and assumptions.
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Hess’s Law and Enthalpy Cycles
Building enthalpy cycles and calculating ΔH from formation and combustion data.
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Bond Energies
Bond energy, calculating ΔH from bonds broken and made, and why the answers differ from Hess values.
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Enthalpy Changes Map
Cambridge International AS & A Level Chemistry · Topic 5 Chemical energetics: Enthalpy changes
The whole section on one A4 page: ΔH and its sign, pathway diagrams and Eₐ, standard conditions and the standard enthalpy changes.
Calorimetry Map
Cambridge International AS & A Level Chemistry · Topic 5 Chemical energetics: Calorimetry
The whole section on one A4 page: q = mcΔT, reactions in solution, burning fuels, errors and assumptions, uncertainty and the cooling-curve correction.
Hess's Law Map
Cambridge International AS & A Level Chemistry · Topic 5 Chemical energetics: Hess's law
The whole section on one A4 page: Hess's law, formation and combustion cycles, combining equations, the Hess's law practical and evaluating the result.
Bond Energies Map
Cambridge International AS & A Level Chemistry · Topic 5 Chemical energetics: Bond energies
The whole section on one A4 page: bond energy, bond energy, ΔH from bonds broken and made, working back, limitations and which bond breaks first.
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Revision Game
Match reagents to products, with your board's reagents and conditions.
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Specification
Every specification point in this topic, in the exam board's own words. Rate how confident you are with each one.
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Self-assessment · 19 points
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5.1.1
5.1.2
- a
construct and interpret a reaction pathway diagram, in terms of the enthalpy change of the reaction
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5.1.3
- a
define and use the term standard conditions (this syllabus assumes that these are 298 K and 101 kPa) shown by ⦵
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5.1.4
- 5.1.4
understand that energy transfers occur during chemical reactions because of the breaking and making of chemical bonds
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5.1.5
- 5.1.5
use bond energies (ΔH positive, i.e. bond breaking) to calculate enthalpy change of reaction, ΔHr
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5.1.6
5.1.7
5.2.1
5.2.2
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