Energetics

Energetics

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Content

• Enthalpy changes: ∆H, of formation; combustion; hydration; solution; neutralisation; atomisation; bond energy; lattice energy; electron affinity

• Hess’ Law, including Born-Haber cycles

• Entropy and Free Energy

Learning Outcomes

Candidates should be able to:

(a) explain that most chemical reactions are accompanied by energy changes, principally in the form of heat usually associated with the breaking and forming of chemical bonds; the reaction can be exothermic (∆H negative) or endothermic (∆H positive)

(b) construct and interpret an energy profile diagram, in terms of the enthalpy change of the reaction and of the activation energy (see also Section 8)

(c) explain and use the terms:

(i) enthalpy change of reaction and standard conditions, with particular reference to: formation; combustion; hydration; solution; neutralisation; atomisation

(ii) bond energy (∆H positive, i.e. bond breaking) (see also Section 2)

(iii) lattice energy (∆H negative, i.e. gaseous ions to solid lattice)

(d) calculate enthalpy changes from appropriate experimental results, including the use of the relationship:

heat change = mc∆T

(e) explain, in qualitative terms, the effect of ionic charge and of ionic radius on the numerical magnitude of a lattice energy

(f) apply Hess’ Law to construct simple energy cycles, e.g. Born-Haber cycle, and carry out calculations involving such cycles and relevant energy terms (including ionisation energy and electron affinity), with particular reference to:

(i) determining enthalpy changes that cannot be found by direct experiment, e.g. an enthalpy change of formation from enthalpy changes of combustion

(ii) the formation of a simple ionic solid and of its aqueous solution

(iii) average bond energies

(g) explain and use the term entropy

(h) discuss the effects on the entropy of a chemical system by the following:

(i) change in temperature

(ii) change in phase

(iii) change in the number of particles (especially for gaseous systems)

(iv) mixing of particles

[quantitative treatment is not required]

(i) predict whether the entropy change for a given process or reaction is positive or negative

(j) state and use the equation involving standard Gibbs free energy change of reaction, ∆G⦵ : ∆G ⦵ = ∆H⦵ – T∆S⦵ [the calculation of standard entropy change, ∆S⦵ , for a reaction using standard entropies, S⦵ , is not required]

(k) state whether a reaction or process will be spontaneous by using the sign of ∆G⦵

(l) understand the limitations in the use of ∆G⦵ to predict the spontaneity of a reaction

(m) predict the effect of temperature change on the spontaneity of a reaction, given standard enthalpy and entropy changes