Topic 12
Entropy and
Energetics

understand that, since endothermic reactions can occur spontaneously at room temperature, enthalpy changes alone do not control whether reactions occur

understand entropy as a measure of disorder of a system in terms of random dispersal of molecules

understand entropy as a measure of disorder of a system in terms of random dispersal of energy quanta between molecules

understand that entropy of a substance increases with temperature

understand that entropy increases as solid changes to liquid

understand that entropy increases as liquid changes to gas

understand that perfect crystals at zero kelvin have zero entropy

be able to interpret the natural direction of change as the direction of increasing total entropy

be able to interpret gases spreading spontaneously through a room as an example of increasing total entropy

understand why entropy changes occur during: changes of state

understand why entropy changes occur during: dissolving of a solid ionic lattice

understand why entropy changes occur during: reactions in which there is a change in the number of moles from reactants to products

understand that total entropy change of any reaction is the sum of entropy change of system and entropy change of surroundings

know the expression ΔStotal = ΔSsystem + ΔSsurroundings

be able to calculate the entropy change of the system for a reaction, ΔSsystem, given the entropies of the reactants and products

be able to calculate entropy change in the surroundings using ΔSsurroundings = −ΔH/T

be able to calculate ΔStotal using ΔSsurroundings = −ΔH/T

understand that the feasibility of a reaction depends on: the balance between ΔSsystem and ΔSsurroundings, so that even endothermic reactions can occur spontaneously at room temperature

understand that the feasibility of a reaction depends on: temperature, as higher temperatures decrease the magnitude of ΔSsurroundings so its contribution to ΔStotal is less Students should be able to calculate the temperature at which a reaction is feasible. Students may also use ΔG = ΔH - TΔSsystem in answers, although this approach is not a requirement of the specification.

understand that reactions can occur as long as ΔStotal is positive even if one of the other entropy changes is negative

understand thermodynamic stability

understand kinetic stability

distinguish between thermodynamic stability and kinetic stability

be able to define the terms: standard enthalpy change of atomisation, ΔatH

be able to define the terms: electron affinity

be able to define the terms: lattice energy (as the exothermic process for the formation of one mole of an ionic solid from its gaseous ions)

be able to construct Born-Haber cycles

be able to carry out calculations related to Born-Haber cycles

understand that a comparison of the experimental lattice energy value (from a Born-Haber cycle) with the theoretical value (obtained from electrostatic theory) in a particular compound indicates the degree of covalent bonding

understand that polarisation of anions by cations leads to some covalency in an ionic bond, based on evidence from the Born-Haber cycle

be able to define the term 'enthalpy change of solution, ΔsolH'

be able to define the term 'enthalpy change of hydration, ΔhydH of an ion'

be able to use energy cycles to calculate enthalpy change of solution of an ionic compound using enthalpy change of hydration and lattice energy

be able to use energy level diagrams to calculate enthalpy change of solution of an ionic compound using enthalpy change of hydration and lattice energy

understand the effect of ionic charge on enthalpy change of hydration

understand the effect of ionic radius on enthalpy change of hydration

understand the effect of ionic charge on lattice energy

understand the effect of ionic radius on lattice energy

be able to use entropy changes of solution to predict solubility of ionic compounds

be able to use enthalpy changes of solution to predict solubility of ionic compounds

be able to discuss trends in solubility of ionic compounds covered in Unit 2