Edexcel A Level Chemistry
Edexcel A Level Chemistry Topic 13: Energetics II Exam Questions
Free Edexcel A Level Chemistry exam questions for Topic 13: Energetics II, with a mark scheme for every pack. Covers lattice energy, Born-Haber cycles, polarisation and covalent character, enthalpies of solution and hydration, entropy and Gibbs energy.
Topic 13: Energetics II exam questions
1 question pack of real past exam questions, each with its mark scheme. Do not stop after a few: answer as many as you can without notes, then mark every answer with the mark scheme. The more questions you do, the more quickly you will spot how examiners ask about each idea.
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About Edexcel A Level Chemistry Topic 13: Energetics II
Topic 13 of Edexcel A Level Chemistry, Energetics II, extends Energetics I to ionic compounds and feasibility. You define lattice energy, construct Born-Haber cycles and compare experimental and theoretical lattice energies to judge the degree of covalent character caused by polarisation. Enthalpy changes of solution and hydration explain solubility. The second half covers entropy: system, surroundings and total entropy changes, with delta S surroundings = -delta H/T, and Gibbs energy, delta G = delta H - T delta S system, to decide whether a reaction is thermodynamically feasible. It is assessed in Paper 1 and links to equilibria and electrode potentials. Examiners test cycles, entropy units and feasibility.
What you need to know
- Lattice energy The enthalpy change when one mole of an ionic solid is formed from its gaseous ions under standard conditions. It is more exothermic for smaller ions with higher charges.
- Born-Haber cycles Combine enthalpy of formation, atomisation, ionisation energies, electron affinities and lattice energy. Use the cycle to calculate any one unknown value.
- Covalent character If the experimental lattice energy (from a Born-Haber cycle) is more exothermic than the theoretical value from a purely ionic model, the bonding has some covalent character, caused by a small, highly charged cation polarising a large anion.
- Solution and hydration Enthalpy change of solution = - lattice energy + sum of hydration enthalpies. Hydration enthalpy is more exothermic for smaller ions with higher charge density.
- Entropy Entropy measures the disorder or number of ways particles and energy can be arranged. It increases from solid to liquid to gas and when the number of gas molecules increases.
- Total entropy change delta S total = delta S system + delta S surroundings, where delta S surroundings = -delta H/T. A reaction is feasible when delta S total is positive.
- Gibbs energy delta G = delta H - T delta S system, and a reaction is feasible when delta G is zero or negative. A feasible reaction may still not occur if its activation energy is high (kinetic inertness).
Common mistakes in Topic 13: Energetics II
- Mixing units by using delta H in kJ and delta S in J K-1 without converting.
- Forgetting to double values in Born-Haber cycles, such as two electron affinities for chlorine in MgCl2.
- Saying a negative delta G means the reaction happens quickly; it is about feasibility, not rate.
- Getting the sign of the second electron affinity of oxygen wrong; it is endothermic (positive).
- Saying a larger difference between experimental and theoretical lattice energy means more ionic; it shows more covalent character.
Try three Topic 13: Energetics II exam-style questions
1. For CaCO3(s) → CaO(s) + CO2(g), delta H = +178 kJ mol-1 and delta S system = +161 J K-1 mol-1. Calculate the temperature above which the reaction becomes feasible. (2 marks)
Show the mark scheme answer
At the limit delta G = 0, so T = delta H/delta S; T = 178000/161 = 1106 K.
2. Explain why the experimental lattice energy of silver iodide is significantly more exothermic than the theoretical value. (2 marks)
Show the mark scheme answer
The large iodide ion is easily polarised by Ag+; so the bonding has some covalent character, giving extra attraction not included in the ionic model.
3. Explain why the entropy of the system increases when ammonium nitrate dissolves in water. (2 marks)
Show the mark scheme answer
Solid lattice of ions breaks up; ions become free to move in solution, so more ways of arranging particles and energy (more disorder).
Topic 13: Energetics II FAQs
What does Edexcel A Level Chemistry Topic 13 Energetics II cover?
It covers lattice energy, Born-Haber cycles, polarisation, enthalpies of solution and hydration, entropy changes and Gibbs energy.
Which paper is Energetics II examined in?
Topic 13 is assessed in Paper 1, Advanced Inorganic and Physical Chemistry, and in Paper 3.
Which equations do I need for Energetics II?
delta S surroundings = -delta H/T, delta S total = delta S system + delta S surroundings, delta G = delta H - T delta S system and delta S total = R ln K.
