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AQA A Level Chemistry – 3.1 Physical Chemistry

3.1.4
Energetics

Section 3.1.4 covers the heat energy changes that accompany chemical reactions: exothermic and endothermic reactions, enthalpy change at constant pressure, standard conditions and the standard enthalpies of combustion and formation, calorimetry with q = mcΔT, Hess’s law, and mean bond enthalpies. Required practical 2 measures an enthalpy change directly.

Exam Paper
Paper 1 & 2
7405/1 and 7405/2
Specification Points
3.1.4.1 – 3.1.4.4
4 sections covered
Topic Parts
5 of 5 live
Revision notes available
Exam Board
AQA
7405 (2015 onwards)

Revision Notes

Work through 3.1.4 in a structured sequence.

Exo and endo
Enthalpy
Changes
1
Available

Enthalpy Changes and Standard Conditions

Exothermic and endothermic reactions, enthalpy level diagrams, activation energy and standard conditions.

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Definitions
Standard
Enthalpies
2
Available

Standard Enthalpy Changes

The standard enthalpy change definitions and the equation each one refers to.

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q = mcΔT
Calorimetry
3
Available

Calorimetry: Measuring Enthalpy Changes

Calculating ΔH from a temperature change, cooling curve corrections, errors and assumptions.

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Enthalpy cycles
Hess’s
Law
4
Available

Hess’s Law and Enthalpy Cycles

Building enthalpy cycles and calculating ΔH from formation and combustion data.

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Break and make
Bond
Enthalpies
5
Available

Bond Enthalpies

Mean bond enthalpy, calculating ΔH from bonds broken and made, and why the answers differ from Hess values.

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Specification Coverage

3.1.4 Energetics – AQA A Level Chemistry

The following AQA specification points outline the knowledge and skills students are expected to demonstrate for Energetics. Wording follows the AQA A Level Chemistry (7405) specification.

3.1.4 Energetics

3.1.4.1
Enthalpy change (ΔH)
i
Reactions can be endothermic or exothermic.
ii
Enthalpy change (ΔH) is the heat energy change measured under conditions of constant pressure.
iii
Standard enthalpy changes refer to standard conditions, ie 100 kPa and a stated temperature (eg ΔH298⦵).
iv
Define standard enthalpy of combustion (ΔcH⦵).
v
Define standard enthalpy of formation (ΔfH⦵).
3.1.4.2
Calorimetry
i
The heat change, q, in a reaction is given by the equation q = mcΔT, where m is the mass of the substance that has a temperature change ΔT and a specific heat capacity c.
ii
Use this equation to calculate the molar enthalpy change for a reaction.
iii
Use this equation in related calculations.
RP 2
Measurement of an enthalpy change.
3.1.4.3
Applications of Hess’s law
i
Hess’s law.
ii
Use Hess’s law to perform calculations, including calculation of enthalpy changes for reactions from enthalpies of combustion or from enthalpies of formation.
3.1.4.4
Bond enthalpies
i
Mean bond enthalpy.
ii
Use mean bond enthalpies to calculate an approximate value of ΔH for reactions in the gaseous phase.
iii
Explain why values from mean bond enthalpy calculations differ from those determined using Hess’s law.