Edexcel A Level Chemistry
Edexcel A Level Chemistry Topic 8: Energetics I Exam Questions
Free Edexcel A Level Chemistry exam questions for Topic 8: Energetics I, with a mark scheme for every pack. Covers enthalpy changes, standard conditions, calorimetry, Hess’s law cycles, enthalpy of formation and combustion data, and mean bond enthalpies.
Topic 8: Energetics I 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 8: Energetics I
Topic 8 of Edexcel A Level Chemistry, Energetics I, deals with the heat energy transferred in chemical reactions. You define enthalpy change and standard enthalpy changes of reaction, formation, combustion and neutralisation, draw enthalpy level diagrams, and measure enthalpy changes by calorimetry using Q = mc delta T. Hess's law lets you calculate enthalpy changes that cannot be measured directly, using formation or combustion data, and mean bond enthalpies give estimates for gas-phase reactions. Core practicals include finding enthalpy changes by calorimetry and Hess's law. Topic 8 is assessed in Paper 1 and leads into Energetics II in Topic 13. Examiners test definitions, signs, cycles and sources of error.
What you need to know
- Enthalpy change delta H is the heat energy change measured at constant pressure. Exothermic reactions have negative delta H and endothermic reactions positive delta H. Standard conditions are 100 kPa and a stated temperature, usually 298 K.
- Standard enthalpy definitions Learn standard enthalpy changes of formation, combustion, neutralisation and reaction, including one mole of the stated substance and substances in their standard states.
- Calorimetry Q = mc delta T, with m the mass of the solution or water heated and c = 4.18 J g-1 K-1. Divide by moles of the limiting reactant and add the sign.
- Hess's law The total enthalpy change is independent of the route. Use formation data (products minus reactants) or combustion data (reactants minus products), or build cycles where direct measurement is not possible.
- Mean bond enthalpies delta H = bonds broken minus bonds made. Mean values are averages over many compounds, so results differ from values calculated with formation data.
- Errors in calorimetry Heat loss to the surroundings is the main error. Use an insulated cup with a lid and extrapolate a temperature-time graph back to the moment of mixing.
Common mistakes in Topic 8: Energetics I
- Using only the mass of a solid reactant in Q = mc delta T; m is the mass of the liquid being heated.
- Forgetting to convert J to kJ before dividing by moles.
- Writing a definition of standard enthalpy of formation that forms the compound from other compounds rather than its elements.
- Mixing up formation and combustion cycle directions, giving the wrong sign.
- Using bond enthalpies for substances that are liquids in the equation without noting that they apply to gases.
Try three Topic 8: Energetics I exam-style questions
1. Use mean bond enthalpies to calculate delta H for CH4 + 2O2 → CO2 + 2H2O. C-H 413, O=O 498, C=O 805, O-H 464 kJ mol-1. (3 marks)
Show the mark scheme answer
Bonds broken = 4(413) + 2(498) = 2648; bonds made = 2(805) + 4(464) = 3466; delta H = 2648 - 3466 = -818 kJ mol-1.
2. Define standard enthalpy change of combustion. (2 marks)
Show the mark scheme answer
Enthalpy change when one mole of a substance is completely burned in oxygen; under standard conditions (100 kPa and 298 K) with all substances in standard states.
3. Explain why the calculated value from mean bond enthalpies differs from the data book value. (1 mark)
Show the mark scheme answer
Mean bond enthalpies are averages taken from many different compounds, not the specific bonds in these molecules (and they apply only to gases).
Topic 8: Energetics I FAQs
What does Edexcel A Level Chemistry Topic 8 Energetics I cover?
It covers enthalpy changes, standard definitions, calorimetry, Hess's law and mean bond enthalpies.
Which paper is Energetics I examined in?
Topic 8 is assessed in Paper 1, and the calorimetry practical skills can also be tested in Paper 3.
How do I avoid sign errors in energetics?
Decide first whether the reaction is exothermic or endothermic from the temperature change, and always write the sign in front of your final delta H.
