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ACSEE (Form 6) Energetics — practice questions

Explore 5 free Energetics questions in ACSEE (Form 6) Chemistry. Open each question or exercise to read its content, reference answers and available explanation.

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Energetics questions and exercises with answers

Showing questions 1–5 of 5.

For C₂H₄ + H₂ → CH₃CH₃ with bond enthalpies C–H 416, C=C 612, C–C 348 and H–H 436 kJ, the enthalpy of hydrogenation is:Read the answer and worked solution →A Born-Haber cycle differs from an enthalpy of formation in that the cycle:Read the answer and worked solution →Balance the ammonia synthesisRead the answer and worked solution →Work out the enthalpyRead the answer and worked solution →Spot the false claimRead the answer and worked solution →

Worked practice questions

For C₂H₄ + H₂ → CH₃CH₃ with bond enthalpies C–H 416, C=C 612, C–C 348 and H–H 436 kJ, the enthalpy of hydrogenation is:

  1. +132 kJ
  2. +264 kJ
  3. −264 kJ
  4. −132 kJ
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Correct answer: D. −132 kJ

1. Use $\Delta H=\sum(\text{bonds in reactants})-\sum(\text{bonds in products})$. 2. Reactants: one C=C at 612, four C–H at 416, one H–H at 436, giving $612+1664+436=2712$ kJ. 3. Products: one C–C at 348 and six C–H at 416, giving $348+2496=2844$ kJ. 4. So $\Delta H=2712-2844=-132$ kJ. 5. The negative sign means hydrogenation releases heat, so it is exothermic; reversing the subtraction gives $+132$ and the wrong direction. _Source: NECTA ACSEE 2023 Chemistry 132/1, Question 6: Energetics_

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A Born-Haber cycle differs from an enthalpy of formation in that the cycle:

  1. measures only the heat released on melting
  2. breaks the formation into separate energy steps
  3. applies solely to organic combustion reactions
  4. gives the heat change per litre of solution
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Correct answer: B. breaks the formation into separate energy steps

1. The enthalpy of formation is a single value: the heat change when one mole of a compound forms from its elements. 2. A Born-Haber cycle is not a single value but a route. 3. It breaks that overall formation into separate steps — atomisation, ionisation, electron affinity, lattice energy. 4. Because energy is a state function, the steps must sum to the enthalpy of formation. 5. That is what makes the cycle useful: it lets an unmeasurable step such as lattice energy be found from the measurable ones. _Source: NECTA ACSEE 2023 Chemistry 132/1, Question 6: Energetics_

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Balance the ammonia synthesis

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Left
  1. Coefficient
    1
    Formula
    N2
  2. Coefficient
    3
    Formula
    H2
Right
  1. Coefficient
    2
    Formula
    NH3
Instructions
Set the coefficients until nitrogen and hydrogen balance.

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