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OCR A Level Chemistry A – Module 3 Periodic Table and Energy

3.2.1
Enthalpy Changes

Section 3.2.1 covers enthalpy changes: exothermic and endothermic reactions, enthalpy profile diagrams and activation energy, standard conditions and standard states, the enthalpy changes of reaction, formation, combustion and neutralisation, determining enthalpy changes from q = mcΔT, average bond enthalpies, and Hess’ law cycles using combustion, formation and unfamiliar data.

Exam Paper
Paper 1 & 3
H432/01 and H432/03
Specification Points
3.2.1 (a) – (h)
8 learning outcomes
Topic Parts
5 of 5 live
Revision notes available
Exam Board
OCR A
H432 (2015 onwards)

Revision Notes

Work through 3.2.1 in a structured sequence.

Exo and endo
Enthalpy
Changes
1
Available

Enthalpy Changes and Standard Conditions

Exothermic and endothermic reactions, enthalpy profile 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

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

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Specification Coverage

3.2.1 Enthalpy Changes – OCR A Level Chemistry A

The following OCR A learning outcomes state what candidates should be able to do. Wording is taken from the OCR A Level Chemistry A (H432) specification.

3.2.1 Enthalpy changes

3.2.1(a)
explanation that some chemical reactions are accompanied by enthalpy changes that are exothermic (ΔH, negative) or endothermic (ΔH, positive)
3.2.1(b)
construction of enthalpy profile diagrams to show the difference in the enthalpy of reactants compared with products
3.2.1(c)
qualitative explanation of the term activation energy, including use of enthalpy profile diagrams
3.2.1(d)
explanation and use of the terms:
i
standard conditions and standard states (physical states under standard conditions)
ii
enthalpy change of reaction (enthalpy change associated with a stated equation, ΔrH)
iii
enthalpy change of formation (formation of 1 mol of a compound from its elements, ΔfH)
iv
enthalpy change of combustion (complete combustion of 1 mol of a substance, ΔcH)
v
enthalpy change of neutralisation (formation of 1 mol of water from neutralisation, ΔneutH)
3.2.1(e)
determination of enthalpy changes directly from appropriate experimental results, including use of the relationship: q = mcΔT
3.2.1(f)
bond enthalpies:
i
explanation of the term average bond enthalpy (as the breaking of 1 mol of bonds in gaseous molecules)
ii
explanation of exothermic and endothermic reactions in terms of enthalpy changes associated with the breaking and making of chemical bonds
iii
use of average bond enthalpies to calculate enthalpy changes and related quantities (see also 2.2.2 f)
3.2.1(g)
Hess’ law for construction of enthalpy cycles and calculations to determine indirectly:
i
an enthalpy change of reaction from enthalpy changes of combustion
ii
an enthalpy change of reaction from enthalpy changes of formation
iii
enthalpy changes from unfamiliar enthalpy cycles
3.2.1(h)
the techniques and procedures used to determine enthalpy changes directly and indirectly