OCR A Level Chemistry B (Salters)
OCR A Level Chemistry B Developing Fuels (DF) Exam Questions
Free OCR A Level Chemistry B exam questions for Developing Fuels (DF), with a mark scheme for every pack. Covers enthalpy changes, calorimetry, Hess's law, bond enthalpies, alkanes and alkenes, isomerism, combustion, catalysts, gas volumes and pollution from fuels.
Developing Fuels (DF) 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 OCR A Level Chemistry B Developing Fuels (DF)
Developing Fuels is a Year 1 storyline of OCR A Level Chemistry B (Salters) built around the fuels we use and the search for better ones. It introduces enthalpy changes, calorimetry, Hess's law and bond enthalpies, then the chemistry of alkanes and alkenes, including naming, structural and E/Z isomerism, combustion and addition reactions. Catalysis and the problems caused by pollutants from burning fuels, along with gas volume calculations, complete the storyline. Examiners often set calorimetry experiments to evaluate, Hess cycles from combustion data, explanations of why bond enthalpies give approximate values, and questions on how catalysts work and why they can be poisoned.
What you need to know
- Enthalpy changes Standard enthalpy changes of reaction, formation, combustion and neutralisation under standard conditions. Exothermic reactions have negative delta H; enthalpy profiles show activation enthalpy.
- Calorimetry Energy transferred = mc delta T; delta H = -energy / moles. Main errors are heat loss, incomplete combustion and evaporation of fuel.
- Hess's law and bond enthalpies Hess cycles using formation or combustion data. Delta H = bonds broken - bonds formed; mean bond enthalpies give approximate answers because actual bond strengths vary between compounds.
- Alkanes and alkenes Naming, structural isomers and E/Z isomerism; branched alkanes and cyclic compounds improve fuel performance. Alkenes undergo electrophilic addition, for example with bromine and hydrogen bromide.
- Catalysts Catalysts provide an alternative route with lower activation enthalpy. Heterogeneous catalysts adsorb reactants on their surface; they can be poisoned when impurities such as lead bind strongly to active sites.
- Fuels and pollution Combustion produces CO2, CO, NOx, SO2, particulates and unburnt hydrocarbons. Catalytic converters reduce CO, NOx and hydrocarbons. Gas volumes use 24.0 dm3 mol-1 at RTP or pV = nRT.
Common mistakes in Developing Fuels (DF)
- Using the mass of fuel instead of the mass of water in mc delta T.
- Reversing the Hess cycle signs. For combustion data, delta H reaction = sum of delta cH reactants - sum of delta cH products.
- Claiming a catalyst is not involved in the reaction. It takes part but is regenerated unchanged.
- Mixing up E and Z by using size rather than atomic number priority.
- Leaving out the negative sign or the kJ mol-1 unit in final enthalpy answers.
Try three Developing Fuels (DF) exam-style questions
1. Use these enthalpies of combustion to calculate the enthalpy change of formation of ethane: C(s) -394, H2(g) -286, C2H6(g) -1560 kJ mol-1. (3 marks)
Show the mark scheme answer
2C + 3H2 gives C2H6; delta fH = [2(-394) + 3(-286)] - (-1560); = -788 - 858 + 1560 = -86 kJ mol-1.
2. Explain how a heterogeneous catalyst speeds up a reaction and how it can be poisoned. (3 marks)
Show the mark scheme answer
Reactants are adsorbed onto active sites on the catalyst surface, weakening bonds; the reaction occurs with a lower activation enthalpy and products are desorbed; poisons such as lead or sulfur compounds adsorb strongly and block active sites.
3. Suggest why the value obtained for the enthalpy of combustion of a fuel by spirit burner calorimetry is less negative than the data book value. (2 marks)
Show the mark scheme answer
Heat is lost to the surroundings (and the apparatus) rather than heating the water; incomplete combustion of the fuel (or fuel evaporating from the wick) reduces the energy measured.
Developing Fuels (DF) FAQs
What does OCR A Level Chemistry B Developing Fuels (DF) cover?
It covers enthalpy changes, calorimetry, Hess's law and bond enthalpies, alkanes and alkenes with isomerism, catalysis and the pollutants formed when fuels burn.
Which papers is Developing Fuels examined in?
In the Salters A Level, content from any storyline can be assessed in all three papers: Fundamentals of chemistry, Scientific literacy in chemistry and Practical skills in chemistry.
How do I master Hess's law for Developing Fuels?
Draw a cycle every time, label each arrow with its value and direction, and check your answer is sensible, for example that formation enthalpies of hydrocarbons are usually small compared with combustion enthalpies.
