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school. “Paper 42” means Paper 4, variant 2.
Practice questions
Original questions written by HomiLabs in the style of the exam, with answers and mark schemes. They are not
Cambridge past paper questions and have not been reviewed by Cambridge. Try each one before opening the answer.
Q1Energy profiles & activation energy · Core · Paper 1 style · 1 mark
(a)On a reaction pathway diagram the products are at a lower energy than the reactants. What does this show?[1]
Show answer
(a) A The reaction is exothermic and ΔH is negative.
Energy is released to the surroundings, so products are lower and ΔH is negative. C has the wrong sign: exothermic means ΔH < 0.
Q2Energy profiles & activation energy · Core · Paper 1 style · 1 mark
(a)[1]
Show answer
(a) A activation energy decreases; ΔH stays the same
A catalyst provides a pathway with lower activation energy; the energies of reactants and products do not change, so ΔH is unchanged. B wrongly suggests the catalyst changes ΔH.
Q3Energy profiles & activation energy · Core · Paper 2 style · 4 marks
(a)State what is meant by activation energy.[1]
(b)The combustion of methane is exothermic. State whether ΔH is positive or negative and whether the products are above or below the reactants on its reaction pathway diagram.[2]
(c)Explain why a gas cooker needs a spark or flame to light it, even though the reaction is exothermic.[1]
Show answer
(a) The minimum energy that colliding particles must have in order to react.
B1.
(b) ΔH is negative; the products are below the reactants.
B1 each.
(c) The spark supplies the activation energy needed to start the reaction (after that, energy released keeps it going).
B1.
Q4Energy profiles & activation energy · Core · Paper 2 style · 3 marks
On a reaction pathway diagram, the reactants are at 120 kJ/mol, the top of the energy 'hump' is at 310 kJ/mol and the products are at 40 kJ/mol.
(a)Calculate the activation energy of the forward reaction.[1]
(b)Calculate ΔH for the reaction and state whether it is exothermic or endothermic.[2]
Show answer
(a) 190 kJ/mol
B1 310 − 120 = 190.
(b) ΔH = −80 kJ/mol; exothermic
M1 40 − 120 = −80; A1 exothermic (sign and word).
Q5Energy profiles & activation energy · Standard · Paper 2 style · 5 marks
For a thermal decomposition reaction, the reactants are at 0 kJ/mol, the top of the energy hump is at +250 kJ/mol and the products are at +90 kJ/mol.
(a)Calculate the activation energy and ΔH for the reaction without a catalyst.[2]
(b)With a catalyst the top of the hump is at +170 kJ/mol. Calculate the activation energy with the catalyst and state the value of ΔH with the catalyst.[2]
(c)Is the reaction exothermic or endothermic? Give a reason.[1]
Show answer
(a) activation energy 250 kJ/mol; ΔH = +90 kJ/mol
B1 each (sign needed for ΔH).
(b) 170 kJ/mol; ΔH = +90 kJ/mol (unchanged)
B1 170 − 0 = 170; B1 ΔH unchanged.
(c) Endothermic: the products have more energy than the reactants (ΔH positive).
B1.
Q6Energy profiles & activation energy · Standard · Paper 2 style · 5 marks
(a)A mixture of hydrogen and oxygen can be kept at room temperature without reacting, but it explodes when a spark is applied. Explain this using the idea of activation energy.[2]
(b)The reaction is 2H₂ + O₂ → 2H₂O and it is strongly exothermic. Describe the reaction pathway diagram for this reaction. Say where reactants and products are and how ΔH and the activation energy are shown.[3]
Show answer
(a) At room temperature too few colliding molecules have energy equal to or greater than the activation energy, so the rate is negligible. The spark gives molecules enough energy to exceed the activation energy; the energy released then supplies other molecules.
B1 particles at room temperature do not have energy ≥ Ea; B1 spark supplies the activation energy (then the reaction sustains itself).
(b) Reactants (2H₂ + O₂) on the left at a higher level; a curve rising to a peak then falling to products (2H₂O) at a lower level. Activation energy: arrow from reactants up to the peak. ΔH: arrow from reactants down to products (negative).
B1 products below reactants; B1 Ea from reactant level to top of curve; B1 ΔH arrow from reactants down to products.
Q7Energy profiles & activation energy · Standard · Paper 2 style · 4 marks
Hydrogen peroxide decomposes: 2H₂O₂ → 2H₂O + O₂. ΔH = −98 kJ per mole of H₂O₂. The activation energy is 75 kJ/mol without a catalyst and 58 kJ/mol with manganese(IV) oxide.
(a)Calculate the decrease in activation energy caused by the catalyst.[1]
(b)State the value of ΔH for the catalysed reaction.[1]
(c)Explain, in terms of particles and energy, why the catalysed reaction is faster.[2]
Show answer
(a) 17 kJ/mol
B1 75 − 58 = 17.
(b) −98 kJ/mol
B1 a catalyst does not change ΔH.
(c) With a lower activation energy, a larger fraction of colliding particles have energy equal to or above the activation energy, so there are more successful collisions per second.
B1 more particles have energy ≥ Ea; B1 more successful collisions per unit time.
Q8Energy profiles & activation energy · Standard · Paper 2 style · 5 marks
Lime kilns near Rohri in Sindh heat limestone to make quicklime.
(a)Calcium carbonate is decomposed in a lime kiln: CaCO₃ → CaO + CO₂, ΔH = +178 kJ/mol. Describe the reaction pathway diagram you would draw for this reaction, including how each label is placed.[4]
(b)Explain why the kiln must be heated strongly all the time, not just to start the reaction.[1]
Show answer
(a) Energy axis upward; reactant level CaCO₃ lower, product level CaO + CO₂ higher; curve from reactants rising to a peak above the product level, then down to products; activation energy shown by an arrow from reactants to the peak; ΔH (+178 kJ/mol) shown by an upward arrow from reactants to products.
B1 products higher than reactants; B1 curve with a peak above both levels; B1 Ea arrow reactants → peak; B1 ΔH arrow reactants → products, labelled positive / +178.
(b) The reaction is endothermic: it takes in energy continuously, so heat must be supplied throughout.
B1.
Q9Energy profiles & activation energy · Challenge · Paper 2 style · 4 marks
A reversible reaction has a forward activation energy of 160 kJ/mol and ΔH = −45 kJ/mol for the forward reaction.
(a)Calculate the activation energy for the reverse reaction.[2]
(b)State ΔH for the reverse reaction.[1]
(c)A catalyst lowers the forward activation energy to 110 kJ/mol. Calculate the new activation energy for the reverse reaction.[1]
Show answer
(a) 205 kJ/mol
M1 the peak is 160 kJ/mol above reactants and products are 45 kJ/mol below reactants; A1 160 + 45 = 205.
(b) +45 kJ/mol
B1 same size, opposite sign.
(c) 155 kJ/mol
B1 110 + 45 = 155 (a catalyst lowers both by the same amount).
Q10Energy profiles & activation energy · Challenge · Paper 2 style · 5 marks
The energy profile of a reaction shows: reactants at 50 kJ/mol, the peak at 300 kJ/mol and products at 230 kJ/mol.
(a)Calculate ΔH for the forward reaction.[1]
(b)Calculate the activation energy of the forward reaction and of the reverse reaction.[2]
(c)In this reaction, is more energy taken in when bonds break or given out when bonds form? Explain.[2]
Show answer
(a) +180 kJ/mol
B1 230 − 50.
(b) forward 250 kJ/mol; reverse 70 kJ/mol
B1 300 − 50 = 250; B1 300 − 230 = 70.
(c) More energy is taken in to break bonds than is released when new bonds form, because ΔH is positive (endothermic).
B1 bond breaking > bond making; B1 linked to positive ΔH / endothermic.
Q11Energy profiles & activation energy · Challenge · Paper 2 style · 6 marks
Data for two reactions:Reaction A: activation energy 30 kJ/mol, ΔH = −20 kJ/molReaction B: activation energy 180 kJ/mol, ΔH = −400 kJ/mol
(a)Which reaction would you expect to be faster at room temperature? Explain.[2]
(b)Which reaction releases more energy per mole? Calculate how many times more.[2]
(c)A student says: 'Reaction B has a very negative ΔH, so it must be very fast.' Evaluate this statement.[2]
Show answer
(a) Reaction A: its activation energy (30 kJ/mol) is much lower, so many more colliding particles have enough energy to react.
B1 A; B1 lower Ea → greater proportion of collisions successful.
(b) Reaction B; 20 times more
B1 B; B1 400 ÷ 20 = 20.
(c) The statement is wrong. ΔH only tells how much energy is released overall; rate depends on the activation energy. B has a high activation energy, so it is slow at room temperature even though it is very exothermic.
B1 ΔH does not determine rate; B1 rate depends on Ea, which is high for B.
Q12Exothermic & endothermic · Core · Paper 1 style · 1 mark
(a)Which process is endothermic?[1]
Show answer
(a) B dissolving ammonium nitrate in water
Dissolving ammonium nitrate takes in energy from the surroundings, so the temperature falls (used in cold packs). Neutralisation (C) gives out heat.
Q13Exothermic & endothermic · Core · Paper 2 style · 6 marks
(a)State what is meant by an exothermic reaction and by an endothermic reaction.[2]
(b)Classify each process as exothermic or endothermic:(i) burning natural gas in a stove(ii) photosynthesis in leaves(iii) thermal decomposition of calcium carbonate(iv) magnesium reacting with dilute hydrochloric acid[4]
Show answer
(a) Exothermic: transfers thermal energy to the surroundings (temperature of surroundings rises). Endothermic: takes in thermal energy from the surroundings (temperature falls).
B1 each.
(b) (i) exothermic (ii) endothermic (iii) endothermic (iv) exothermic
B1 each.
Q14Exothermic & endothermic · Core · Paper 4 style · 5 marks
A student mixes 25 cm³ of four different pairs of solutions, A–D, in a polystyrene cup and records the temperatures.reaction: initial temperature / final temperature (°C)A: 22.0 / 31.5B: 22.5 / 16.0C: 21.0 / 21.5D: 23.0 / 35.5
(a)Calculate the temperature change for each reaction. Use + for a rise and − for a fall.[2]
(b)Which reaction is endothermic? Give a reason.[1]
(c)Which reaction is the most exothermic? Give a reason.[1]
(d)Suggest why the student used the same volume of solution each time.[1]
Show answer
(a) A +9.5 °C; B −6.5 °C; C +0.5 °C; D +12.5 °C
2 marks all four correct; 1 mark for two or three.
(b) B, because the temperature fell.
B1.
(c) D, because it has the largest temperature rise.
B1.
(d) To make it a fair test: the same volume means temperature changes can be compared.
B1.
Q15Exothermic & endothermic · Core · Paper 2 style · 4 marks
(a)A cold pack for sports injuries contains solid ammonium nitrate and a separate bag of water. When the pack is squeezed the solid dissolves and the pack gets cold. Explain why it gets cold.[2]
(b)A self-heating hand warmer works by the slow oxidation of iron powder. State whether this reaction is exothermic or endothermic and state the sign of ΔH.[2]
Show answer
(a) Dissolving ammonium nitrate is endothermic; it takes in thermal energy from the surroundings (the pack and the skin), so the temperature falls.
B1 endothermic; B1 energy taken in from surroundings.
(b) Exothermic; ΔH is negative.
B1 each.
Q16Exothermic & endothermic · Standard · Paper 4 style · 5 marks
A student adds aqueous sodium hydroxide, 5 cm³ at a time, to 25 cm³ of dilute hydrochloric acid in a polystyrene cup with a lid and records the highest temperature after each addition.volume of NaOH added / cm³: 0, 5, 10, 15, 20, 25, 30, 35temperature / °C: 24.0, 27.1, 30.0, 32.8, 35.2, 35.0, 34.1, 33.3
(a)State the maximum temperature rise.[1]
(b)Use the results to estimate the volume of sodium hydroxide that exactly neutralises the acid.[1]
(c)Explain why the temperature falls after 20 cm³ of alkali has been added.[2]
(d)Suggest why a polystyrene cup with a lid is used rather than a glass beaker.[1]
Show answer
(a) 11.2 °C
B1 35.2 − 24.0.
(b) about 20 cm³ (where the temperature is highest)
B1 20 cm³ (accept 20–22 cm³ from the highest temperature).
(c) The acid has all reacted, so no more heat is released; adding more (cooler) alkali cools the mixture, and heat is lost to the surroundings.
B1 reaction complete / no more energy released; B1 cooler solution added / heat loss.
(d) Polystyrene is a good insulator and the lid reduces heat loss, so the temperature rise measured is more accurate.
B1.
Q17Exothermic & endothermic · Standard · Paper 2 style · 4 marks
(a)State whether bond breaking and bond making are exothermic or endothermic.[2]
(b)The burning of hydrogen is exothermic. Explain this in terms of the energy changes when bonds are broken and made.[2]
Show answer
(a) Bond breaking is endothermic; bond making is exothermic.
B1 each.
(b) The energy released when the new O–H bonds form is greater than the energy taken in to break the H–H and O=O bonds.
B1 energy released forming bonds; B1 greater than energy absorbed breaking bonds.
Q18Exothermic & endothermic · Standard · Paper 4 style · 5 marks
A student adds 1.0 g of a powdered metal to 25 cm³ of aqueous copper(II) sulfate in an insulated cup. The starting temperature is 20.0 °C each time.metal: highest temperature / °Cmagnesium: 58.5zinc: 31.0iron: 26.5silver: 20.0
(a)Calculate the temperature rise for each metal.[2]
(b)Use the results to put the four metals in order of reactivity, most reactive first.[1]
(c)Explain why there was no temperature change with silver.[1]
(d)State one variable, other than the volume of solution, that must be kept the same.[1]
Show answer
(a) magnesium 38.5 °C; zinc 11.0 °C; iron 6.5 °C; silver 0.0 °C
2 marks all correct; 1 mark for two or three.
(b) magnesium > zinc > iron > silver
B1.
(c) Silver is less reactive than copper, so it does not displace copper and no reaction occurs.
B1.
(d) Mass of metal / particle size (powder) / concentration of copper(II) sulfate / starting temperature.
B1 any one.
Q19Exothermic & endothermic · Challenge · Paper 4 style · 4 marks
A student dissolves 5.0 g of potassium nitrate in 50 cm³ of water in a polystyrene cup. The temperature falls from 21.5 °C to 15.3 °C.
(a)Calculate the temperature change with 5.0 g of potassium nitrate.[1]
(b)Assuming the temperature change is proportional to the mass dissolved in the same volume of water, predict the final temperature when 10.0 g is dissolved, starting at 21.5 °C.[2]
(c)In practice the measured fall with 10.0 g was smaller than predicted. Suggest one reason.[1]
Show answer
(a) −6.2 °C (a fall of 6.2 °C)
B1 15.3 − 21.5.
(b) 9.1 °C
M1 fall = 2 × 6.2 = 12.4 °C; A1 21.5 − 12.4 = 9.1 °C.
(c) Heat is absorbed from the surroundings during the experiment (more time to dissolve), or not all the solid dissolved.
B1 either.
Q20Exothermic & endothermic · Challenge · Paper 4 style · 6 marks
A student compares two liquid fuels by heating 200 cm³ of water in a copper can with a spirit burner.fuel: mass burned / g, temperature rise / °CX: 0.92, 18.4Y: 1.25, 30.0
(a)Calculate the temperature rise per gram of fuel burned for each fuel.[2]
(b)Which fuel releases more energy per gram? Explain.[1]
(c)Give two reasons why the results are lower than the true values, and suggest one improvement.[3]
Show answer
(a) fuel X: 20.0 °C per g; fuel Y: 24.0 °C per g
M1 rise ÷ mass; A1 both values.
(b) Fuel Y, because it gives the larger temperature rise per gram for the same mass of water.
B1.
(c) Heat lost to the air and to the can; incomplete combustion (soot on the can); some heat warms the can not the water. Improvement: shield the flame from draughts / put a lid on the can / keep the flame close to the can.
1 mark each for any two sources of error; 1 mark for a matching improvement.
Q21Exothermic & endothermic · Challenge · Paper 2 style · 5 marks
(a)The complete combustion of methane has ΔH = −890 kJ/mol. Calculate the energy released when 4.0 g of methane is burned. (Ar: C = 12, H = 1)[2]
(b)2NH₃ → N₂ + 3H₂, ΔH = +92 kJ (for 2 mol of ammonia). Calculate the energy needed to decompose 3.4 g of ammonia. (Ar: N = 14, H = 1)[3]
(c) Bond energies are average values from many compounds (and the data assume all substances are gases).
B1.
Q30Bond energy calculations · Challenge · Paper 2 style · 5 marks
Bond energies in kJ/mol: O–H 464, O–O 146, O=O 498
(a)Hydrogen peroxide decomposes: 2H₂O₂ → 2H₂O + O₂. Each H₂O₂ molecule has the structure H–O–O–H. Calculate ΔH for the equation as written.[3]
(b)Show that, in this reaction, the only bonds that really change are two O–O single bonds broken and one O=O double bond made, and that this gives the same answer.[2]
