OxfordAQA International A Level Chemistry
OxfordAQA International A Level Chemistry 3.1.4 Energetics Exam Questions
Free OxfordAQA International A Level Chemistry revision for 3.1.4 Energetics: what you need to know, common mistakes and exam-style questions with answers. Covers enthalpy changes, standard conditions, enthalpies of combustion and formation, calorimetry with q = mcΔT, Hess's law and mean bond enthalpies.
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About OxfordAQA International A Level Chemistry 3.1.4 Energetics
Energetics is a Unit 1 topic about the heat energy changes in reactions. You need to define enthalpy change, standard conditions and the standard enthalpies of combustion and formation, and use q = mcΔT to calculate enthalpy changes from experimental data. You need to apply Hess's law using combustion or formation data, and use mean bond enthalpies to estimate enthalpy changes for gas-phase reactions, explaining why these values differ from Hess's law values. Required practical 2, measuring an enthalpy change, belongs here. This topic leads to Born-Haber cycles and Gibbs free energy in 3.1.8 Thermodynamics. Calculations and sign errors are tested every year.
What you need to know
- Enthalpy change Enthalpy change, ΔH, is the heat energy change measured at constant pressure. Standard conditions are 100 kPa and a stated temperature, usually 298 K.
- Standard enthalpies Standard enthalpy of combustion is the enthalpy change when one mole of a substance is completely burned in oxygen under standard conditions, with all substances in standard states. Standard enthalpy of formation is the enthalpy change when one mole of a compound is formed from its elements in their standard states.
- Calorimetry q = mcΔT, where m is the mass of the solution or water heated (not the reactants), c is the specific heat capacity (given) and ΔT is the temperature change. ΔH = -q / moles, converted to kJ mol-1.
- Hess's law The enthalpy change for a reaction is independent of the route taken. Using formation data, ΔH = sum of ΔfH products - sum of ΔfH reactants; using combustion data, ΔH = sum of ΔcH reactants - sum of ΔcH products.
- Mean bond enthalpies ΔH = sum of bonds broken - sum of bonds made. Values differ from Hess's law values because mean bond enthalpies are averages over many compounds and only apply to gases.
- Required practical 2 Measure the temperature change for a reaction in an insulated cup. Plot temperature against time and extrapolate back to correct for heat loss.
Common mistakes in 3.1.4 Energetics
- Forgetting the negative sign for an exothermic reaction. If the temperature rises, ΔH is negative.
- Using the mass of a solid reactant in q = mcΔT. Use the mass of the water or solution being heated.
- Leaving the answer in J when kJ mol-1 is asked for. Divide by 1000 and by the number of moles.
- Using mean bond enthalpies for liquids. They apply to gaseous species only.
Try three 3.1.4 Energetics exam-style questions
1. 0.0100 mol of a fuel heated 100 g of water by 15.0 K. c = 4.18 J g-1 K-1. Calculate the enthalpy of combustion. (3 marks)
Show the mark scheme answer
q = 100 x 4.18 x 15.0 = 6270 J; 6.27 kJ / 0.0100 mol = 627 kJ mol-1; ΔH = -627 kJ mol-1.
2. Use the bond enthalpies H-H 436, Cl-Cl 243 and H-Cl 432 kJ mol-1 to calculate ΔH for H2 + Cl2 → 2HCl. (2 marks)
Show the mark scheme answer
Bonds broken = 436 + 243 = 679; bonds made = 2 x 432 = 864; ΔH = 679 - 864 = -185 kJ mol-1.
3. Explain why the value from mean bond enthalpies differs from the value found from enthalpies of formation. (1 mark)
Show the mark scheme answer
Mean bond enthalpies are averages from a range of compounds, so they are not exact for the bonds in this reaction.
3.1.4 Energetics FAQs
What does OxfordAQA International A Level Chemistry 3.1.4 Energetics cover?
It covers enthalpy changes, standard conditions, enthalpies of combustion and formation, calorimetry with q = mcΔT, Hess's law and mean bond enthalpies.
Which unit is 3.1.4 Energetics examined in?
It is examined in Unit 1, the first International AS paper, which covers inorganic and physical chemistry. The Unit 5 practical and synoptic paper at A2 can also assess it.
How do I avoid sign errors in Hess's law?
Draw the cycle, write the direction of each arrow and reverse the sign whenever you go against an arrow. Check whether you are given formation or combustion data, because the formulae differ.
