Chemical Energetics

5.1 Enthalpy change, ΔH
Learning outcomes
Candidates should be able to:
1 understand that chemical reactions are accompanied by enthalpy changes and these changes can be
exothermic (ΔH is negative) or endothermic (ΔH is positive)
2 construct and interpret a reaction pathway diagram, in terms of the enthalpy change of the reaction and of
the activation energy
3 define and use the terms:
(a) standard conditions (this syllabus assumes that these are 298K and 101 kPa) shown by ⦵.
(b) enthalpy change with particular reference to: reaction, ΔHr
, formation, ΔHf
, combustion, ΔHc ,
neutralisation, ΔHneut
4 understand that energy transfers occur during chemical reactions because of the breaking and making of
chemical bonds
5 use bond energies (ΔH positive, i.e. bond breaking) to calculate enthalpy change of reaction, ΔHr
6 understand that some bond energies are exact and some bond energies are averages
7 calculate enthalpy changes from appropriate experimental results, including the use of the relationships
q = mcΔT and ΔH = –mcΔT/n

5.2 Hess’s Law
Learning outcomes
Candidates should be able to:
1 apply Hess’s Law to construct simple energy cycles
2 carry out calculations using cycles and relevant energy terms, including:
(a) determining enthalpy changes that cannot be found by direct experiment
(b) use of bond energy data