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Answers · Energy changes in reactions

Energy changes in reactions — final check key

Check meaning, not exact wording. These are teaching points, not official Cambridge marks.

1. NK1 · Final check

Model answer. (a) Fall = 17.5 − (−7.0) = 17.5 + 7.0 = 24.5 °C. (b) The puddle is part of the surroundings. The reaction is endothermic: it takes in thermal energy from its surroundings, through the glass, from the puddle. The puddle is left with less energy, so its temperature falls. The mixture goes below 0 °C, so the puddle can freeze and stick the beaker to the block.

Checked automatically

  • (a) Fall in temperature: 24.5 °C (exact value)
    • Common wrong answer 10.5 : The lowest reading is below zero. Count from 17.5 down to 0, then 7.0 more.
    • Common wrong answer -7.0 : That is the lowest temperature. The fall is the start minus the lowest.

A full answer includes

  • (a) I wrote 17.5 − (−7.0).
  • (a) I got a fall of 24.5 °C.
  • (b) I predicted that the puddle cools, and that it may freeze.
  • (b) I wrote that the puddle is part of the surroundings.
  • (b) I wrote that the reaction takes in thermal energy from the puddle, through the glass.
  • (b) I did not write that cold passes from the mixture into the puddle.

Also accept

  • 24.5 °C; the puddle cools and may freeze, because the reaction takes thermal energy from it

Watch for

  • “An endothermic reaction gives out cold.” — Cold does not flow. Thermal energy flows from the surroundings into the reacting chemicals, so the surroundings are left with less energy and get colder.
  • “The fall is 17.5 − 7.0 = 10.5 °C.” — The lowest reading is below zero. From 17.5 °C down to 0 °C is 17.5 degrees, and on down to −7.0 °C is 7.0 more: 24.5 °C.

2. NK2 · Final check

Model answer. (a) Rise = 27.0 − 21.5 = 5.5 °C. (b) The solution around the reacting particles is the surroundings. It gets warmer, so the reaction has transferred thermal energy to the surroundings: it is exothermic.

Checked automatically

  • (a) Temperature rise: 5.5 °C (exact value)
    • Common wrong answer 27.0 : That is the highest temperature. Subtract the start temperature.
    • Common wrong answer 48.5 : You added the two temperatures. Subtract the start from the highest.

A full answer includes

  • (a) I wrote 27.0 − 21.5.
  • (a) I got 5.5 °C.
  • (b) I wrote exothermic.
  • (b) I wrote that the solution, the surroundings, got warmer.
  • (b) I wrote that the solution gained thermal energy from the reaction.
  • (b) I did not write that the thermometer measures the reacting particles.

Also accept

  • 5.5 °C; exothermic

Watch for

  • “The thermometer measures the temperature of the reaction itself.” — The thermometer is in the surroundings: the water, the cup and the thermometer. It rises when the reaction gives thermal energy to them.

3. NK3 · Final check

Model answer. (a) The curve falls after the hump, but it ends higher than it started: the products, CuO + CO2, are on a higher level than the reactant, CuCO3, so they store more energy. Only the two levels decide, and the reaction is endothermic. (b) Both arrows start at the reactant's level. The ΔH arrow ends at the products' level, and the Ea arrow ends at the top of the hump, which is higher than the products. So Ea is bigger than ΔH.

A full answer includes

  • (a) I wrote that the products are higher than the reactant.
  • (a) I wrote that the reaction is endothermic.
  • (a) I wrote that only the two levels decide, not the shape of the curve.
  • (a) I did not use the fall after the hump to decide.
  • (b) I chose Ea.
  • (b) I wrote that both arrows start at the reactant's level.
  • (b) I wrote that the top of the hump is above the products' level.
  • (b) I did not compare the top of the hump with the bottom of the graph.

Also accept

  • (a) The products end higher than the reactant: endothermic; (b) Ea, because the hump is above the products

Watch for

  • “The curve comes down at the end, so the reaction gives out energy.” — The curve falls from the top of the hump, but it ends higher than it started. Compare only the products' level with the reactant's level.
  • “ΔH is bigger, because the products are high up.” — Both arrows start at the reactant's level. The Ea arrow goes on past the products' level, up to the top of the hump, so it is longer.

4. NK4 · Final check

Model answer. The reactants are at 90 kJ/mol, the products at 215 and the top of the hump at 330. (a) Ea is measured from the reactants: 330 − 90 = 240 kJ/mol. (b) ΔH = products − reactants = 215 − 90 = +125 kJ/mol: positive, so the reaction is endothermic. (c) Activation energy is the minimum energy that colliding particles must have to react.

Checked automatically

  • (a) Activation energy: 240 kJ/mol (exact value)
    • Common wrong answer 330 : That is the top of the hump read from zero. Subtract the reactants' energy.
    • Common wrong answer 115 : That is measured from the products. Ea starts at the reactants' level.
  • (b) Enthalpy change: 125 kJ/mol (exact value)
    • Common wrong answer -125 : The products are higher than the reactants, so the chemicals gain energy: ΔH is positive.

A full answer includes

  • (a) I wrote Ea = 330 − 90.
  • (a) I got Ea = 240 kJ/mol.
  • (b) I wrote ΔH = 215 − 90.
  • (b) I got ΔH = +125 kJ/mol, with a plus sign.
  • (c) I wrote: the minimum energy that colliding particles must have to react.
  • (c) I used the word minimum.
  • I did not give Ea as 330 kJ/mol, the top of the hump read from zero.
  • I did not measure Ea from the products.

Also accept

  • 240 kJ/mol; +125 kJ/mol; the minimum energy colliding particles must have to react

Watch for

  • “Ea is the height of the top of the hump read from the bottom of the graph.” — Measure Ea from the level of the reactants up to the top of the hump.
  • “Ea is measured from the products.” — Ea is the energy the reactants need, so it goes from the reactants' level to the top of the hump.

5. NK5 · Final check

Model answer. ΔH is positive, so the reaction is endothermic and the products are 69 kJ/mol above the reactant. (a) Reactant = 194 − 69 = 125 kJ/mol. (b) The top of the hump is Ea above the reactant: 125 + 140 = 265 kJ/mol, higher than the products. (c) The ΔH arrow points up from the reactant's level, 125 kJ/mol, to the products' level, 194 kJ/mol, and is labelled ΔH = +69 kJ/mol. The Ea arrow points up from the reactant's level to the top of the hump, 265 kJ/mol, and is labelled Ea = 140 kJ/mol.

Checked automatically

  • (a) Reactant's level: 125 kJ/mol (exact value)
    • Common wrong answer 263 : ΔH is positive, so the reactant is below the products: 194 − 69, not 194 + 69.
  • (b) Top of the hump: 265 kJ/mol (exact value)
    • Common wrong answer 334 : That is Ea above the products. Ea is measured from the reactant's level: 125 + 140.
    • Common wrong answer 140 : That is Ea itself. The top of the hump is Ea above the reactant's level.
    • Common wrong answer 403 : Find the reactant's level first: 194 − 69, not 194 + 69. Then add Ea.

A full answer includes

  • (a) I wrote 194 − 69.
  • (a) I got 125 kJ/mol.
  • (b) I wrote 125 + 140.
  • (b) I got 265 kJ/mol.
  • (c) I described the ΔH arrow pointing up, from the reactant's level to the products' level.
  • (c) I labelled the ΔH arrow ΔH = +69 kJ/mol.
  • (c) I described the Ea arrow pointing up, from the reactant's level to the top of the hump.
  • (c) I labelled the Ea arrow Ea = 140 kJ/mol.
  • I did not add 69 to the products' level.
  • I did not add Ea to the products' level.

Also accept

  • 125 kJ/mol; 265 kJ/mol; both arrows start at the reactant's level

Watch for

  • “Ea is measured from the products.” — Ea is the energy the reactants need, so it goes from the reactants' level to the top of the hump.
  • “To find the reactant's level, add ΔH to the products' level.” — A positive ΔH means the products are above the reactant, so the reactant is lower: 194 − 69.