5.2.1 Lattice Enthalpy

Lattice enthalpy

Candidates should be able to:
(a) explain and use the term lattice enthalpy (H negative, ie gaseous ions to the solid lattice) as a measure of ionic bond strength;

Born–Haber and related enthalpy cycles

Relevant Energy Terms:
enthalpy change of formation, ionisation energy,

enthalpy change of atomisation and electron affinity.
Born–Haber cycle as a model for
determination of lattice enthalpies and in testing the ionic model of bonding.
(b) use the lattice enthalpy of a simple ionic solid (ie NaCl, MgCl2) and relevant energy terms to: (i) construct Born–Haber cycles, (ii) carry out related calculations;
(c) explain and use the terms enthalpy change of solution and enthalpy change of hydration;
(d) use the enthalpy change of solution of a simple ionic solid (ie NaCl, MgCl2) and relevant energy terms (enthalpy change of
hydration, and lattice enthalpy), to: (i) construct Born–Haber cycles, (ii) carry out related calculations;
(e) explain, in qualitative terms, the effect of ionic charge and ionic radius on the exothermic value of a lattice enthalpy and enthalpy change of hydration.
5.2.2 Enthalpy and Entropy

Entropy

Candidates should be able to:
(a) explain that entropy is a measure of the ‘disorder’ of a system, and that a system becomes energetically more stable when it becomes more disordered;
(b) explain the difference in magnitude of entropy: (i) of a solid and a gas, (ii) when a solid lattice dissolves, (iii) for a reaction in which there is a change in the number of gaseous molecules;
(c) calculate the entropy change for a reaction given the entropies of the reactants and products;

Balance between entropy and enthalpy changes

(d) explain that the tendency of a process to take place depends on temperature, T, the entropy change in the system, S, and the enthalpy change, H, with the surroundings;
(e) explain that the balance between entropy and enthalpy changes is the free energy change, G, which determines the feasibility of a reaction;
(f) state and use the relationship G = H – TS;
(g) explain, in terms of enthalpy and entropy, how endothermic reactions are able to take place spontaneously.
5.2.3 Electrode Potentials and Fuel Cells

Redox

Candidates should be able to:
(a) explain, for simple redox reactions, the terms redox, oxidation number, half-reaction, oxidising agent and reducing agent
(b) construct redox equations using relevant halfequations or oxidation numbers;
(c) interpret and make predictions for reactions involving electron transfer.

Electrode potentials

E o data will be provided on examination papers.
For E o measurements, ions of the same element can have concentrations of 1 mol dm–3 or be equimolar.
(d) define the term standard electrode (redox) potential, E o ;
(e) describe how to measure, using a hydrogen electrode, standard electrode potentials of: (i) metals or non-metals in contact with their ions in aqueous solution, (ii) ions of the same element in different oxidation states;
(f) calculate a standard cell potential by combining two standard electrode potentials; Feasibility of reactions (g) predict, using standard cell potentials, the feasibility of a reaction;
(h) consider the limitations of predictions made using standard cell potentials, in terms of kinetics and concentration;

Storage and fuel cells

Recall of a specific storage cell will not be required.
Candidates will be expected to make predictions on supplied storage cells. All relevant electrode potentials and other data will be supplied.
Hydrogen-rich fuels include methanol, natural gas, or petrol, which are converted into hydrogen gas by an onboard ‘reformer’.
Development of fuel cells as an alternative to direct use of finite oil-based fuels in cars compared with logistical problems of their development and use.
Pure hydrogen emits only water whilst hydrogen-rich fuels produce only small amounts of air pollutants and CO2.
(i) apply principles of electrode potentials to modern storage cells;
(j) explain that a fuel cell uses the energy from the reaction of a fuel with oxygen to create a voltage;
(k) explain the changes that take place at each electrode in a hydrogen–oxygen fuel cell;
(l) outline that scientists in the car industry are developing fuel cell vehicles (FCVs), fuelled by: (i) hydrogen gas, (ii) hydrogen-rich fuels;
(m) state advantages of FCVs over conventional petrol or diesel-powered vehicles, in terms of: (i) less pollution and less CO2, (ii) greater efficiency;
Efficiency can be more than twice that of
similarly sized conventional vehicles; other advanced technologies can further increase efficiency.
Political and social desire to move to a hydrogen economy has many obstacles including ignorance that energy is needed to produce hydrogen and that fuel cells have a finite life.
Hydrogen is an energy carrier and not a source.
(n) understand how hydrogen might be stored in FCVs: (i) as a liquid under pressure, (ii) adsorbed on the surface of a solid material, (iii) absorbed within a solid material;
(o) consider limitations of hydrogen fuel cells, for example: (i) storing and transporting hydrogen, in terms of safety, feasibility of a pressurised liquid and a limited life cycle of a solid ‘adsorber’ or ‘absorber’, (ii) limited lifetime (requiring regular replacement and disposal) and high production costs, (iii) use of toxic chemicals in their production
(p) comment that a ‘hydrogen economy’ may contribute largely to future energy needs but limitations include: (i) public and political acceptance of hydrogen as a fuel, (ii) handling and maintenance of hydrogen systems, (iii) initial manufacture of hydrogen, requiring energy.

	Created by gallen96 about 10 years ago

A-Level Chemistry (F325: Module 2 - Energy) Note on F325: Module 2 Specification, created by gallen96 on 04/02/2014.

F325: Module 2 Specification