IB DP Chemistry Reactivity 1.1: Measuring enthalpy changes – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – the IB holds copyright in its own papers. Use these alongside the official past papers available through your school or the IB store.

This set tests IB DP Chemistry Reactivity 1.1, “Measuring enthalpy changes”, as described in the IB Diploma Programme Chemistry guide (first assessment 2025, published February 2023), with questions on all four understandings, Reactivity 1.1.1 to 1.1.4. The guide gives this sub-topic no HL-only content, so every question suits SL and HL students sitting the exams from 2025 on.

Learn the content first from the study guide or revision notes. Other units are on the DP Chemistry hub and checklist, and a free diagnostic shows where to focus.

Data for all questions: specific heat capacity of water, c = 4.18 J g⁻¹ K⁻¹. Assume dilute solutions have density 1.00 g cm⁻³ and the same c as water. Relative atomic masses: Mg 24.31, O 16.00, C 12.01, H 1.01, N 14.01, Cl 35.45. Round final answers to 3 s.f. unless told differently.

Questions

1. Distinguish between heat and temperature. [2]

2. A sports cold pack holds solid ammonium nitrate and a sealed pouch of water. Bursting the pouch lets the solid dissolve, and the pack turns cold.

(a) Identify the system and the surroundings in this process. [2] (b) State the direction of energy transfer and the sign of ΔH when the ammonium nitrate dissolves. [2]

3. Sketch a fully labelled energy profile for an exothermic reaction, showing ΔH and the activation energy, Ea. [3]

4. On an energy profile, the reactants are at 180 kJ mol⁻¹, the transition state at 245 kJ mol⁻¹ and the products at 60 kJ mol⁻¹ (relative potential energies).

(a) Determine ΔH. [1] (b) Determine the activation energy. [1] (c) State, with a reason, whether the reactants or the products are more stable. [2]

5. The temperature of 250 g of water in a beaker goes from 18.4 °C to 23.9 °C. Calculate the heat taken in by the water, in kJ. [2]

6. In an insulated cup, 55.0 cm³ of 0.900 mol dm⁻³ hydrochloric acid is mixed with 55.0 cm³ of 0.900 mol dm⁻³ sodium hydroxide. The temperature rises by 6.1 K.

(a) Calculate the enthalpy change of neutralisation, in kJ mol⁻¹. [4] (b) State two assumptions made in your calculation. [2]

7. A student dissolves 6.00 g of ammonium chloride, NH₄Cl, in 60.0 g of water. The water temperature falls by 6.4 K.

(a) Calculate the enthalpy change, in kJ mol⁻¹, for dissolving one mole of NH₄Cl. Assume only the water changes temperature. [4] (b) Apart from the thermometer reading, state one observation showing that the process is endothermic. [1]

8. A 0.600 g cashew nut burns on a mounted needle beneath a metal can holding 100.0 g of water, which warms by 16.8 K. The packet gives cashews an energy content of 23.0 kJ g⁻¹.

(a) Calculate the energy released per gram of nut in this experiment. [3] (b) Express your answer as a percentage of the label value. [1] (c) Explain two reasons why the experimental value is lower. [2]

9. A spirit burner containing propan-1-ol, C₃H₇OH, heats 200.0 g of water in an 85.0 g copper can. The burner’s mass falls by 0.420 g, and the water and can both warm by 11.2 K. A data value for the standard enthalpy change of combustion of propan-1-ol is −2021 kJ mol⁻¹.

(a) Using only the heat absorbed by the water, determine ΔH of combustion for propan-1-ol. [4] (b) Calculate the percentage difference between your answer and the data value. [1] (c) Copper has a specific heat capacity of 0.385 J g⁻¹ K⁻¹. Recalculate the enthalpy change to include the heat absorbed by the can. [2] (d) Suggest one change to the method that would bring the result closer to the data value, and give one reason why the result would still be less exothermic. [2]

10. Magnesium powder (0.500 g) is added to 60.0 cm³ of 0.250 mol dm⁻³ zinc sulfate solution:

Mg(s) + ZnSO₄(aq) → MgSO₄(aq) + Zn(s)

Before mixing, the solution is steady at 19.6 °C. The highest reading recorded is 36.2 °C. Extending the cooling part of the temperature–time graph back to the time of mixing gives 37.7 °C.

(a) Explain why the extrapolated temperature is used rather than the highest recorded one. [2] (b) Show that zinc sulfate is the limiting reactant. [2] (c) Calculate the enthalpy change for the reaction, in kJ mol⁻¹. [3]

11. At very high temperatures, nitrogen and oxygen combine: N₂(g) + O₂(g) → 2NO(g). The reaction is endothermic.

(a) State what happens to the temperature of the surroundings as the reaction proceeds. [1] (b) Explain, in terms of the relative stability of reactants and products, why the reaction is endothermic. [2] (c) Outline what the symbol ⦵ in ΔH⦵ tells you about the conditions. [2]

Answers

1. Temperature indicates the average kinetic energy of the particles, measured in K [1]. Heat is energy transferred because of a temperature difference, measured in J, and it depends on the amount of substance [1]. [2] Examiner insight: Make two separate points; calling temperature “the amount of heat an object holds” contradicts your own definition of heat.

2. (a) System: the ammonium nitrate and the water it dissolves in [1]. Surroundings: everything else, such as the casing, the air and the skin [1]. (b) Energy moves from surroundings to system [1]; ΔH is positive [1]. Examiner insight: The water takes part in the change here, so state what you count as the system and stay consistent.

3. Axes labelled reaction coordinate (horizontal) and potential energy (vertical) [1]. Product level drawn below the reactant level, with a ΔH arrow pointing down from reactants to products [1]. One peak between them, with Ea shown as an arrow rising from the reactant line to the summit [1]. Examiner insight: Label reactants, products, ΔH and Ea on the sketch itself; unlabelled lines are hard to credit.

4. (a) ΔH = 60 − 180 = −120 kJ mol⁻¹ [1] (b) Ea = 245 − 180 = 65 kJ mol⁻¹ [1] (c) The products [1], because their potential energy is lower than that of the reactants [1]. Examiner insight: Show the sign in (a); 120 on its own does not tell the reader that energy is released.

5. ΔT = 23.9 − 18.4 = 5.5 K; Q = 250 × 4.18 × 5.5 = 5747.5 J [1] = 5.75 kJ [1] [2] Examiner insight: The question asks for kJ, so 5750 J is the right energy in the wrong unit; convert before the final line.

6. (a) m = 55.0 + 55.0 = 110.0 g; Q = 110.0 × 4.18 × 6.1 = 2805 J = 2.805 kJ [1] n(H₂O formed) = 0.0550 × 0.900 = 0.0495 mol (equal amounts, so neither is in excess) [1] ΔH = −2.805 ÷ 0.0495 [1] = −56.7 kJ mol⁻¹ [1] (b) Any two of: density of the mixture is 1.00 g cm⁻³ [1]; its specific heat capacity equals water’s [1]; no energy is lost or taken up by the cup. Examiner insight: Taking m as 55.0 g (one solution) halves Q; show the combined mass in your working.

7. (a) Q = 60.0 × 4.18 × 6.4 = 1605 J = 1.605 kJ [1] M(NH₄Cl) = 14.01 + (4 × 1.01) + 35.45 = 53.50 g mol⁻¹; n = 6.00 ÷ 53.50 = 0.1121 mol [1] ΔH = +1.605 ÷ 0.1121 [1] = +14.3 kJ mol⁻¹ [1] (b) The beaker feels cold, or moisture condenses on its outside surface [1]. Examiner insight: The water cooled, so the answer must be positive; check your sign against what the thermometer did, not against habit.

8. (a) Q = 100.0 × 4.18 × 16.8 = 7022 J = 7.022 kJ [1] Energy per gram = 7.022 ÷ 0.600 [1] = 11.7 kJ g⁻¹ [1] (b) 11.7 ÷ 23.0 × 100 = 50.9 % [1] (c) Any two of: much of the flame’s energy heats the air, not the water [1]; the can absorbs some energy [1]; the nut burns incompletely; energy escapes while the lit nut is moved under the can. Examiner insight: “Heat loss” is one vague idea; for two marks, give two distinct routes by which energy misses the water.

9. (a) M(C₃H₇OH) = (3 × 12.01) + (8 × 1.01) + 16.00 = 60.11 g mol⁻¹; n = 0.420 ÷ 60.11 = 6.987 × 10⁻³ mol [1] Q(water) = 200.0 × 4.18 × 11.2 = 9363 J = 9.363 kJ [1] ΔH = −9.363 ÷ (6.987 × 10⁻³) [1] = −1340 kJ mol⁻¹ [1] (b) (2021 − 1340) ÷ 2021 × 100 = 33.7 % [1] (c) Q(can) = 85.0 × 0.385 × 11.2 = 367 J; total Q = 9363 + 367 = 9730 J [1] ΔH = −9.730 ÷ (6.987 × 10⁻³) = −1390 kJ mol⁻¹ [1] (d) Change: e.g. a draught shield, or a smaller gap between wick and can [1]. Remaining reason: e.g. incomplete combustion, or energy still lost to the air [1]. Examiner insight: In (a), m is the 200.0 g of water, not the 0.420 g of fuel; label each mass in your working.

10. (a) Energy leaks away while the reaction is still releasing it, so the recorded peak is too low [1]. Extending the cooling line back to the mixing time estimates the loss-free maximum [1]. (b) n(ZnSO₄) = 0.0600 × 0.250 = 0.0150 mol [1]; n(Mg) = 0.500 ÷ 24.31 = 0.02057 mol, more than the 0.0150 mol needed for 1 : 1 reaction, so ZnSO₄ is limiting [1]. (c) ΔT = 37.7 − 19.6 = 18.1 K; Q = 60.0 × 4.18 × 18.1 = 4539 J = 4.539 kJ [1] ΔH = −4.539 ÷ 0.0150 [1] = −303 kJ mol⁻¹ [1] Examiner insight: Basing ΔT on 36.2 °C gives −278 kJ mol⁻¹ and ignores the correction the question sets up; use a supplied extrapolated value.

11. (a) The temperature of the surroundings falls [1]. (b) The product, NO, is less stable (higher in potential energy) than the reactants [1]; N₂ is especially stable because of its strong triple bond, so overall energy has to be absorbed [1]. (c) Standard conditions: a pressure of 100 kPa, with values usually quoted at 298 K [1]; every substance in its standard state [1]. Examiner insight: In (b), “it needs energy” just restates endothermic; compare the stability of reactants and products.

Where marks are usually lost

• Choosing the wrong m: it is the liquid that warmed or cooled, never the fuel or solid.
• Dividing Q by the amount of a reagent that was left over at the end.
• Dividing a value in joules by moles and then quoting kJ mol⁻¹.
• Missing signs: an exothermic ΔH needs a minus; a falling thermometer means ΔH is positive.
• Counting only one solution’s volume after mixing two.
• Ignoring an extrapolated temperature that the question supplies.
• Energy profile axes labelled “time” or “energy”, or Ea measured from the product level.

Next steps

• Go over the key facts in the measuring enthalpy changes revision notes.
• Return to the study guide for weak spots.
• Find gaps across the course with all our free 10-minute diagnostics.
• Explore other units on the IB DP Chemistry hub.
• Book a free trial class.
• Track progress with the DP Chemistry checklist.

Official syllabus

International Baccalaureate Organization, Diploma Programme Chemistry guide, first assessment 2025 (published February 2023). Material on this page: Reactivity 1.1 (Measuring enthalpy changes).