Entropy and Energetics

EntropySign up

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 the random dispersal of molecules and of energy quanta between molecules · understand that the entropy of a substance increases with temperature, that entropy increases as solid -> liquid -> gas and that perfect crystals at zero kelvin have zero entropy · be able to interpret the natural direction of change as being in the direction of increasing total entropy (positive entropy change), including gases spread spontaneously through a room · understand why entropy changes occur during: (i) changes of state; (ii) dissolving of a solid ionic lattice; (iii) reactions in which there is a change in the number of moles from reactants to products · understand that the total entropy change of any reaction is the sum of the entropy change of the system and the entropy change of the surroundings, summarised by the expression: delta S_total = delta S_system + delta S_surroundings · be able to calculate the entropy change of the system for a reaction, delta S_system, given the entropies of the reactants and products · be able to calculate the entropy change in the surroundings, and hence delta S_total, using the expression delta S_surroundings = -delta H / T · understand that the feasibility of a reaction depends on: (i) the balance between delta S_system and delta S_surroundings, so that even endothermic reactions can occur spontaneously at room temperature; (ii) temperature, as higher temperatures decrease the magnitude of delta S_surroundings so its contribution to delta S_total is less. Students should be able to calculate the temperature at which a reaction is feasible. Students may also use delta G = delta H - T delta S_system in answers, although this approach is not a requirement of the specification. · understand that reactions can occur as long as delta S_total is positive even if one of the other entropy changes is negative · understand and distinguish between the concepts of thermodynamic stability and kinetic stability

Lattice Energy and Born-Haber CyclesSign up

be able to define the terms: (i) standard enthalpy change of atomisation, delta_atH; (ii) electron affinity; (iii) 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 and carry out related calculations · 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 terms 'enthalpy change of solution, delta_solH' and 'enthalpy change of hydration, delta_hydH of an ion' · be able to use energy cycles and energy level diagrams to calculate the enthalpy change of solution of an ionic compound, using enthalpy change of hydration and lattice energy · understand the effect of ionic charge and ionic radius on the values of enthalpy change of hydration and the lattice energy of an ionic compound · be able to use entropy and enthalpy changes of solution values to predict the solubility of ionic compounds and discuss trends in the solubility of ionic compounds covered in Unit 2