CIE A Level Chemistry Chemical Energetics Exam Questions

About CIE A Level Chemistry Chemical Energetics

Chemical energetics in Cambridge International 9701 is split between AS and A Level. At AS you define standard enthalpy changes, calculate enthalpy changes from calorimetry using q = mc delta T, construct Hess's law cycles and use average bond energies. The A Level extension introduces lattice energy, Born-Haber cycles, enthalpies of hydration and solution, entropy and Gibbs free energy, letting you predict whether a reaction is feasible at a given temperature. The topic links to Group 2 solubility trends, equilibria and electrochemistry. Examiners commonly test precise definitions, energy cycles with careful signs, calorimetry with sources of error, and the use of delta G = delta H - T delta S.

What you need to know

• Standard enthalpy changes Learn definitions for enthalpy change of reaction, formation, combustion, neutralisation and atomisation under standard conditions (298 K, 101 kPa). Exothermic changes have negative delta H.
• Calorimetry q = mc delta T, using the mass of the solution or water, c = 4.18 J g-1 K-1. Divide q by the moles of the limiting reagent and add the sign to obtain delta H in kJ mol-1.
• Hess's law The enthalpy change for a reaction is independent of the route taken. Use formation data (products minus reactants) or combustion data (reactants minus products) in cycles.
• Bond energies delta H = sum of bonds broken minus sum of bonds formed. Values from average bond energies differ from experimental values because bond energies vary between compounds and must refer to gaseous species.
• Lattice energy and Born-Haber cycles (A Level) Lattice energy is the enthalpy change when one mole of an ionic compound is formed from its gaseous ions. It becomes more exothermic with smaller ions and higher charges. Born-Haber cycles combine atomisation, ionisation energy, electron affinity and formation enthalpy.
• Hydration and solution (A Level) Enthalpy change of solution = enthalpy change of hydration of the ions minus lattice energy. Hydration enthalpy is more exothermic for small, highly charged ions.
• Entropy and Gibbs free energy (A Level) Entropy, S, measures the number of ways particles and energy can be arranged and increases from solid to liquid to gas. delta G = delta H - T delta S, with delta S converted to kJ K-1 mol-1; a reaction is feasible when delta G is zero or negative.

Common mistakes in Chemical Energetics

• Using the volume of acid only for m in q = mc delta T; use the total mass of solution, assuming 1 cm3 has mass 1 g.
• Leaving the sign off a calorimetry answer; a temperature rise means delta H is negative.
• Forgetting to convert entropy from J K-1 mol-1 to kJ K-1 mol-1 before using delta G = delta H - T delta S.
• Defining lattice energy from the solid separating into ions; in 9701 it is formed from gaseous ions, so it is exothermic.
• Forgetting to multiply bond energies by the number of bonds, or using bond energies for liquids such as water.

Try three Chemical Energetics exam-style questions