IGCSE Chemistry Paper 4 (Theory / Extended)

Topic 5: Chemical Energetics -- Mock Exam 2

Duration1 hour 15 minutes

Total Marks80

Questions7

Time Remaining75:00

Syllabus0620

Instructions

Answer all questions in the spaces provided.
Show all working for calculations.
Use appropriate scientific terminology.
Your answers will be automatically graded when you submit.

Question Navigation

Question 1 -- Energy Changes in Reactions

Total: 12 marks

A student in Edinburgh carries out an experiment where she adds zinc powder to copper sulfate solution in a polystyrene cup.

(a) [3]

The temperature of the solution rises from 21 °C to 38 °C.

(i) State whether this is an exothermic or endothermic reaction. [1]

(ii) Explain your answer to (i). [1]

(iii) State the temperature change (ΔT) for this reaction. [1]

Model Answer -- 1(a)

Exothermic [1]

The temperature of the surroundings increased / heat energy was released to the surroundings [1]

ΔT = 38 − 21 = 17 °C [1]

⚠ If you missed marks here: For (iii), ΔT is the CHANGE, 38 − 21 = 17 °C – writing the final temperature (38 °C) or leaving out the unit loses the mark. In (ii), 'heat was absorbed, so it got hotter' reverses the idea: the temperature rose because the reaction RELEASED thermal energy into the solution around it.

1 mark -- exothermic 1 mark -- temperature surroundings increased heat energy released 1 mark -- temperature change 17 degrees

(b) [4]

(i) State the sign of ΔH for an exothermic reaction. [1]

(ii) State the sign of ΔH for an endothermic reaction. [1]

(iii) The combustion of propane has ΔH = −2220 kJ mol⁻¹. Explain what the negative sign tells you. [1]

(iv) The thermal decomposition of limestone has ΔH = +178 kJ mol⁻¹. Explain what the positive sign tells you. [1]

Model Answer -- 1(b)

ΔH is negative for an exothermic reaction [1]

ΔH is positive for an endothermic reaction [1]

The negative sign means the reaction releases 2220 kJ of energy per mole / is exothermic [1]

The positive sign means the reaction absorbs 178 kJ of energy per mole / is endothermic [1]

⚠ If you missed marks here: Think of the sign from the reacting chemicals' side: a NEGATIVE ΔH means they lose energy to the surroundings (exothermic – burning propane releases 2220 kJ per mole), and a POSITIVE ΔH means they gain it (endothermic – limestone takes in 178 kJ per mole). Answers that just say 'negative means less energy', or that swap the two signs, score nothing.

1 mark -- delta negative exothermic 1 mark -- delta positive endothermic 1 mark -- negative sign releases 2220 energy exothermic 1 mark -- positive sign absorbs 178 energy endothermic

(i) Explain why the student uses a polystyrene cup rather than a glass beaker for this experiment. [2]

(ii) State two other precautions the student should take to get accurate results. [2]

(iii) Suggest why the measured temperature change may be less than the theoretical value. [1]

Model Answer -- 1(c)

Polystyrene is a good insulator / poor conductor of heat [1]

This reduces heat loss to the surroundings, giving a more accurate temperature change [1]

Use a lid on the cup to prevent heat loss by evaporation / convection [1]

Stir the solution to ensure even heat distribution / use an accurate thermometer [1]

Some heat is always lost to the surroundings despite insulation / the cup absorbs some heat / not all reactants may have reacted [1]

⚠ If you missed marks here: 'Polystyrene is cheaper' or 'it won't break' misses (i) – it is a poor conductor of heat, so less of the energy released escapes and the measured temperature rise is closer to the true value. In (ii), safety steps such as wearing goggles do not improve ACCURACY; the credited precautions are a lid (to cut heat loss) and stirring (so the whole solution reaches the same temperature).

1 mark -- polystyrene good insulator poor conductor heat 1 mark -- reduces heat loss surroundings accurate temperature 1 mark -- lid prevent heat loss evaporation convection 1 mark -- stir solution even heat distribution accurate thermometer 1 mark -- heat lost surroundings despite insulation cup absorbs heat

Question 2 -- Energy Level Diagrams

Total: 12 marks

A teacher in Chennai uses energy level diagrams to compare two reactions: the combustion of magnesium and the thermal decomposition of copper carbonate.

(a) [5]

The combustion of magnesium is a highly exothermic reaction.

2Mg(s) + O₂(g) → 2MgO(s) ΔH = −1204 kJ mol⁻¹

(i) Draw a labelled energy level diagram for this reaction. Include the activation energy, ΔH, and labels on both axes. [4]

(ii) Explain why magnesium must be ignited with a match before it will burn in air. [1]

Model Answer -- 2(a)

Reactants (2Mg + O₂) at higher energy level [1]

Products (2MgO) at lower energy level (large gap showing −1204 kJ) [1]

Activation energy (Eₐ) shown from reactants to top of hump [1]

ΔH labelled as negative, axes labelled (Energy vs Progress of reaction) [1]

The match provides the activation energy needed to start the reaction / overcome the energy barrier [1]

⚠ If you missed marks here: For an exothermic reaction the ΔH arrow points DOWN from the 2Mg + O₂ line to the 2MgO line and is labelled negative (−1204 kJ mol⁻¹), and E_a must be measured from the REACTANTS up to the peak. In (ii), 'the reaction is endothermic until it starts' is wrong – the match simply supplies the activation energy, after which the energy released keeps the magnesium burning.

1 mark -- reactants higher energy level 1 mark -- products lower energy level large gap 1 mark -- activation energy shown reactants top hump 1 mark -- delta labelled negative axes labelled 1 mark -- match provides activation energy start reaction overcome energy barrier

(b) [4]

The thermal decomposition of copper carbonate is endothermic.

CuCO₃(s) → CuO(s) + CO₂(g)

(i) Draw a labelled energy level diagram for this reaction, showing Eₐ and ΔH. [3]

(ii) Explain why continuous heating is required for this reaction to occur. [1]

Model Answer -- 2(b)

Reactants (CuCO₃) at lower energy level than products (CuO + CO₂) [1]

ΔH shown as positive (upward arrow from reactants to products) [1]

Activation energy shown as a hump above reactant level [1]

Continuous heating provides the energy needed because the reaction is endothermic -- it constantly absorbs energy / the products are at a higher energy than the reactants [1]

⚠ If you missed marks here: In (ii), 'the heat provides the activation energy' only explains how it STARTS – it needs CONTINUOUS heating because it is endothermic and keeps taking in energy, so it stops when the heating stops (unlike the burning magnesium in (a)). On the diagram the hump must rise ABOVE the CuO + CO₂ line, with E_a measured from the CuCO₃ line to the top of the hump and ΔH as an upward arrow labelled positive.

1 mark -- reactants lower energy level products higher 1 mark -- delta shown positive upward 1 mark -- activation energy hump above reactant level 1 mark -- continuous heating provides energy endothermic constantly absorbs energy

An energy level diagram shows the following values:
-- Energy of reactants = 400 kJ
-- Energy at the top of the curve = 520 kJ
-- Energy of products = 250 kJ

Calculate:
(i) The activation energy (Eₐ) [1]
(ii) The enthalpy change (ΔH) [1]
(iii) State whether the reaction is exothermic or endothermic. [1]

Model Answer -- 2(c)

Eₐ = 520 − 400 = 120 kJ [1]

ΔH = 250 − 400 = −150 kJ [1]

Exothermic (because ΔH is negative / products are at a lower energy level) [1]

⚠ If you missed marks here: Both values are measured FROM THE REACTANTS: E_a = 520 − 400 = 120 kJ (270 means you measured from the products), and ΔH = products − reactants = 250 − 400 = −150 kJ. Writing +150 (reactants − products) gives the wrong sign – and the negative sign is exactly what tells you in (iii) that the reaction is exothermic.

1 mark -- activation energy 120 1 mark -- delta negative 150 1 mark -- exothermic negative products lower

Question 3 -- Bond Energies: Concepts

Total: 10 marks

A teacher in Hyderabad introduces the concept of bond energies and their use in predicting enthalpy changes.

(a) [2]

Define the term bond energy.

Model Answer -- 3(a)

Bond energy is the energy needed / required to break one mole of a particular covalent bond [1]

in a gaseous molecule / measured in kJ mol⁻¹ [1]

⚠ If you missed marks here: 'The energy released when a bond breaks' reverses it – bond energy is the energy NEEDED to break a bond, and the definition must say ONE MOLE of that particular covalent bond (in gaseous molecules). 'How strong a bond is' describes it but does not define it, so it scores nothing.

1 mark -- energy needed required break one mole particular covalent bond 1 mark -- gaseous molecule measured kJ

(b) [4]

(i) Explain why energy is needed to break a bond. [1]

(ii) Explain why energy is released when a bond is formed. [1]

(iii) State the formula used to calculate the overall enthalpy change of a reaction using bond energies. [1]

(iv) If a reaction has ΔH = −350 kJ mol⁻¹, state whether more energy was absorbed breaking bonds or released making bonds. [1]

Model Answer -- 3(b)

Energy is needed to overcome the attractive forces between the shared electrons and the nuclei of the bonded atoms [1]

When a bond is formed, the atoms reach a more stable / lower energy state, so energy is released [1]

ΔH = Σ bond energies broken − Σ bond energies formed [1]

More energy was released making bonds than was absorbed breaking bonds (because ΔH is negative) [1]

⚠ If you missed marks here: In (iii) the order matters: ΔH = Σ(bonds broken) − Σ(bonds formed); 'products − reactants', which works for energy levels, gives the wrong sign when used with bond energies. For (i), 'because bonds are strong' is not an explanation – energy is needed to overcome the attraction between the nuclei and the shared pair of electrons.

1 mark -- overcome attractive forces shared electrons nuclei bonded atoms 1 mark -- bond formed atoms more stable lower energy state energy released 1 mark -- formula bond energies broken minus bond energies formed 1 mark -- more energy released making bonds absorbed breaking bonds negative

The table shows the bond energies of some bonds.

Bond	Bond energy / kJ mol⁻¹
C–C	347
C=C	614
C≡C	839
N–N	163
N=N	410
N≡N	945

(i) Describe the trend in bond energy as the number of shared electron pairs increases from single to double to triple bonds. [1]

(ii) Explain this trend. [1]

(iii) Use the data to explain why nitrogen gas (N₂) is very unreactive. [1]

(iv) Suggest why the N≡N bond energy is not simply three times the N–N bond energy. [1]

Model Answer -- 3(c)

Bond energy increases as the number of shared electron pairs increases / double bonds are stronger than single, triple bonds are strongest [1]

More shared electrons means a greater attractive force between the nuclei and the shared electrons, making the bond harder to break [1]

N₂ has a triple bond with a very high bond energy (945 kJ mol⁻¹), so a very large amount of energy is needed to break the bond and start a reaction [1]

Bond energies are averages / the electron pairs in a triple bond are not identical -- the pi bonds are weaker than the sigma bond [1]

⚠ If you missed marks here: 'Nitrogen has a full outer shell' does not answer (iii) – the question says use the DATA: the N≡N triple bond needs 945 kJ mol⁻¹ to break, so a huge amount of energy is needed before N₂ can react. For (ii), link the trend to electrons: more shared pairs means a stronger attraction between the shared electrons and the two nuclei, so the bond is harder to break.

1 mark -- bond energy increases shared electron pairs double stronger single triple strongest 1 mark -- more shared electrons greater attractive force nuclei harder break 1 mark -- nitrogen triple bond very high bond energy 945 large amount energy needed break 1 mark -- bond energies averages pi bonds weaker sigma bond not identical

Question 4 -- Bond Energy Calculation: Formation of HCl

Total: 12 marks

A student in Manchester investigates the reaction between hydrogen and chlorine to form hydrogen chloride gas.

(a) [3]

(i) Write the balanced equation for the reaction of hydrogen with chlorine. [1]

(ii) List the bonds that are broken in the reactants. [1]

(iii) List the bonds that are formed in the products. [1]

Model Answer -- 4(a)

H₂(g) + Cl₂(g) → 2HCl(g) [1]

Bonds broken: 1 × H–H and 1 × Cl–Cl [1]

Bonds formed: 2 × H–Cl [1]

⚠ If you missed marks here: H + Cl → HCl loses (i) because hydrogen and chlorine are diatomic, and H₂ + Cl₂ → HCl is unbalanced – it must be H₂ + Cl₂ → 2HCl. That 2 matters in (iii): TWO H–Cl bonds are formed, and writing just one carries the error into your calculation in (b).

1 mark -- balanced equation hydrogen chlorine two HCl 1 mark -- bonds broken one H-H one Cl-Cl 1 mark -- bonds formed two H-Cl

(b) [4]

Use the bond energy data below to calculate the enthalpy change (ΔH) for the reaction.

Bond	Bond energy / kJ mol⁻¹
H–H	436
Cl–Cl	242
H–Cl	431

Show all your working.

Model Answer -- 4(b)

Bonds broken: (1 × 436) + (1 × 242) = 678 kJ [1]

Bonds formed: 2 × 431 = 862 kJ [1]

ΔH = 678 − 862 = −184 kJ mol⁻¹ [1]

The reaction is exothermic [1]

⚠ If you missed marks here: If you got +247 you formed only ONE H–Cl bond – the equation makes 2HCl, so 2 × 431 = 862 kJ is released. If you got +184 you did formed − broken; it is always broken − formed = 678 − 862 = −184 kJ mol⁻¹, and the negative sign is what makes it exothermic.

1 mark -- bonds broken 678 1 mark -- bonds formed 862 1 mark -- delta negative 184 1 mark -- reaction exothermic

(i) Using your calculated values, draw an energy level diagram for the reaction H₂ + Cl₂ → 2HCl. Show numerical values for Eₐ and ΔH. [3]

(ii) The activation energy for this reaction is 17 kJ mol⁻¹. Calculate the energy at the top of the activation energy curve if the reactants start at 678 kJ. [1]

(iii) Suggest why this reaction is dangerous and can be explosive when hydrogen and chlorine are mixed in the presence of UV light. [1]

Model Answer -- 4(c)

Reactants (H₂ + Cl₂) shown at higher energy level, products (2HCl) at lower energy level [1]

ΔH = −184 kJ mol⁻¹ labelled correctly [1]

Activation energy hump labelled (Eₐ = 17 kJ mol⁻¹) [1]

Energy at the top = 678 + 17 = 695 kJ [1]

UV light provides the activation energy / the reaction is very exothermic with a very low activation energy, so once started it releases a large amount of energy rapidly, causing an explosion [1]

⚠ If you missed marks here: In (ii) the top of the hump is ABOVE the reactants by E_a, so it is 678 + 17 = 695 kJ – subtracting (661 kJ) puts the peak below the reactants, which is impossible. For (iii), 'HCl is a dangerous acid' misses the point: UV light supplies the small activation energy, and because the reaction is very exothermic the energy released sets off more molecules so fast that it explodes.

1 mark -- reactants higher products lower energy level diagram 1 mark -- delta negative 184 labelled correctly 1 mark -- activation energy hump labelled 17 1 mark -- energy top 695 1 mark -- UV provides activation energy exothermic low activation energy large energy rapidly explosion

Question 5 -- Bond Energy Calculation: Combustion of Propane

Total: 12 marks

Propane (C₃H₈) is widely used as cooking gas (LPG) across India and in camping stoves in the UK. A student calculates the energy released when propane burns.

(a) [3]

The structural formula of propane is CH₃CH₂CH₃.

(i) List the types of bonds present in one molecule of propane. [1]

(ii) State the number of each type of bond. [2]

Model Answer -- 5(a)

Bond types: C–H and C–C [1]

8 × C–H bonds [1]

2 × C–C bonds [1]

⚠ If you missed marks here: Three carbons in a chain are joined by only TWO C–C bonds (count the links, not the atoms), so '3 C–C' loses the mark. All eight hydrogens are bonded to carbon (3 + 2 + 3 in CH₃CH₂CH₃), giving 8 C–H; propane is an alkane, so every bond is single.

1 mark -- bond types C-H C-C 1 mark -- eight C-H bonds 1 mark -- two C-C bonds

(b) [6]

The equation for the complete combustion of propane is:

C₃H₈(g) + 5O₂(g) → 3CO₂(g) + 4H₂O(g)

Use the bond energy data to calculate ΔH.

Bond	Bond energy / kJ mol⁻¹
C–H	413
C–C	347
O=O	498
C=O	805
O–H	464

Show all working clearly.

Model Answer -- 5(b)

Bonds broken: 8(C–H) + 2(C–C) + 5(O=O) [1]

= (8×413) + (2×347) + (5×498) = 3304 + 694 + 2490 = 6488 kJ [1]

Bonds formed: 6(C=O) + 8(O–H) [1]

= (6×805) + (8×464) = 4830 + 3712 = 8542 kJ [1]

ΔH = 6488 − 8542 = −2054 kJ mol⁻¹ [1]

The large negative value shows this is a highly exothermic reaction, explaining why propane is an effective fuel [1]

⚠ If you missed marks here: Each CO₂ has TWO C=O bonds and each H₂O has TWO O–H bonds, so 3CO₂ + 4H₂O forms 6 C=O and 8 O–H – +361 means you used only 3 C=O, and −198 means you used only 4 O–H. With 6488 kJ broken and 8542 kJ formed, ΔH = 6488 − 8542 = −2054 kJ mol⁻¹.

1 mark -- correct identification bonds broken eight C-H two C-C five O=O 1 mark -- bonds broken total 6488 1 mark -- correct identification bonds formed six C=O eight O-H 1 mark -- bonds formed total 8542 1 mark -- answer negative 2054 1 mark -- highly exothermic effective fuel

(i) State what is meant by incomplete combustion. [1]

(ii) Name two products of incomplete combustion of propane. [1]

(iii) Suggest, in terms of bond energies, why incomplete combustion releases less energy than complete combustion. [1]

Model Answer -- 5(c)

Incomplete combustion occurs when there is insufficient oxygen for complete combustion [1]

Products include carbon monoxide (CO) and/or carbon (soot) along with water [1]

Each carbon ends up in CO with a single C≡O bond (about 1077 kJ mol⁻¹) instead of in CO₂ with two C=O bonds (2 × 805 = 1610 kJ mol⁻¹). The individual C≡O bond in CO is in fact the stronger of the two, but only ONE bond is formed per carbon atom instead of two, so less energy is released overall and the reaction is less exothermic [1]

⚠ If you missed marks here: 'The fuel does not burn completely' just repeats the term – (i) needs the cause, an insufficient supply of oxygen, and (ii) needs carbon monoxide and/or carbon (soot), not carbon dioxide. In (iii), 'the bonds in CO are weaker' is false (the C≡O bond, about 1077 kJ mol⁻¹, is stronger than one C=O bond); less energy is released because each carbon forms only ONE bond to oxygen instead of two.

1 mark -- incomplete combustion insufficient oxygen 1 mark -- carbon monoxide carbon soot water products 1 mark -- only one bond formed per carbon in CO instead of two C=O bonds in CO2 so less energy released

Question 6 -- Catalysts and Enzymes

Total: 12 marks

A biochemistry student in Cambridge studies how catalysts and enzymes affect the energy profiles of reactions.

(a) [4]

The diagram below shows the energy profile for a reaction with and without a catalyst.

(i) Which curve (A or B) represents the catalysed reaction? Explain your answer. [2]

(ii) Explain why the ΔH is the same for both curves. [1]

(iii) State what happens to the catalyst at the end of the reaction. [1]

Model Answer -- 6(a)

Curve B represents the catalysed reaction [1]

Because it has a lower activation energy / the peak of the curve is lower [1]

ΔH is the same because the catalyst only affects the activation energy, not the energy levels of the reactants or products [1]

The catalyst is chemically unchanged / not used up at the end of the reaction [1]

⚠ If you missed marks here: Choosing Curve A because 'it has more energy, so it is faster' is backwards – the catalysed route is the one with the LOWER peak, Curve B, because a catalyst lowers the activation energy. For (ii), 'the catalyst is not used up' answers (iii), not (ii): ΔH is the same because both curves start at the same reactants and end at the same products, so the gap between those levels is unchanged.

1 mark -- curve B catalysed reaction 1 mark -- lower activation energy peak curve lower 1 mark -- delta same catalyst affects activation energy not energy levels reactants products 1 mark -- catalyst chemically unchanged not used up end reaction

(b) [4]

Enzymes are biological catalysts found in living organisms.

(i) State one similarity between enzymes and inorganic catalysts. [1]

(ii) State two differences between enzymes and inorganic catalysts. [2]

(iii) Name the enzyme that catalyses the decomposition of hydrogen peroxide in the liver. [1]

Model Answer -- 6(b)

Both speed up reactions / lower activation energy without being used up [1]

Enzymes are specific to one reaction / substrate, inorganic catalysts can catalyse many reactions [1]

Enzymes are denatured by high temperatures / work best at a specific pH, inorganic catalysts work over a wide range of temperatures and pressures [1]

Catalase [1]

⚠ If you missed marks here: 'Enzymes are killed by high temperatures' loses the mark – enzymes are proteins, not living things, so they are DENATURED, not killed; and 'both are proteins' is wrong for (i), because catalysts such as iron or platinum are not proteins. The enzyme in (iii) is catalase; amylase breaks down starch, not hydrogen peroxide.

1 mark -- both speed reactions lower activation energy without used up 1 mark -- enzymes specific one reaction substrate inorganic many reactions 1 mark -- enzymes denatured high temperatures specific pH inorganic wide range 1 mark -- catalase

Cars in both India and the UK are fitted with catalytic converters to reduce harmful emissions.

(i) Name the metals used as catalysts in catalytic converters. [1]

(ii) Name two harmful gases that are converted in the catalytic converter. [1]

(iii) Write one equation for a reaction that takes place in the catalytic converter. [1]

(iv) Explain why a catalytic converter must reach a high temperature before it works effectively. [1]

Model Answer -- 6(c)

Platinum, palladium and/or rhodium [1]

Carbon monoxide (CO) and nitrogen oxides (NO_x) [1]

2CO + 2NO → 2CO₂ + N₂ (or other valid equation) [1]

The catalyst still requires the activation energy to be reached / the gases must have enough energy to react on the catalyst surface / the catalyst lowers Eₐ but does not eliminate it [1]

⚠ If you missed marks here: Carbon dioxide is a PRODUCT of the converter, not a gas it removes – the two harmful gases converted are carbon monoxide and nitrogen oxides, for example 2CO + 2NO → 2CO₂ + N₂ (check the N atoms balance). For (iv), a catalyst LOWERS the activation energy but does not remove it, so the gases still need to be hot enough for their collisions to react.

1 mark -- platinum palladium rhodium 1 mark -- carbon monoxide nitrogen oxides 1 mark -- valid equation CO NO converted 1 mark -- activation energy still needed gases enough energy react catalyst surface

Question 7 -- Photosynthesis and Respiration Energy Changes

Total: 10 marks

A biology teacher in Bangalore links chemistry to biology by explaining the energy changes in photosynthesis and respiration.

(a) [4]

(i) Write the word equation for photosynthesis. [1]

(ii) State whether photosynthesis is exothermic or endothermic. [1]

(iii) Explain your answer to (ii) in terms of energy. [1]

(iv) State the source of energy for photosynthesis. [1]

Model Answer -- 7(a)

Carbon dioxide + water → glucose + oxygen [1]

Endothermic [1]

Energy is absorbed from the surroundings (sunlight) and stored in the bonds of glucose [1]

Sunlight / light energy / solar energy [1]

⚠ If you missed marks here: Writing 'light' or 'chlorophyll' as a reactant, or putting glucose + oxygen on the left, loses (i): the equation is carbon dioxide + water → glucose + oxygen, with light as the energy source, not a substance. Photosynthesis is endothermic because it TAKES IN light energy and stores it in glucose; 'it gives out oxygen, so it is exothermic' confuses a product with an energy change.

1 mark -- carbon dioxide water glucose oxygen equation 1 mark -- endothermic 1 mark -- energy absorbed surroundings stored bonds glucose 1 mark -- sunlight light energy solar energy

(b) [3]

(i) Write the word equation for aerobic respiration. [1]

(ii) State whether respiration is exothermic or endothermic. [1]

(iii) Explain how the energy from respiration is used by living organisms. [1]

Model Answer -- 7(b)

Glucose + oxygen → carbon dioxide + water [1]

Exothermic [1]

The energy released is used for muscle contraction / growth / maintaining body temperature / active transport / nerve impulses [1]

⚠ If you missed marks here: Respiration is a chemical reaction in cells, not breathing – 'oxygen in, carbon dioxide out' is not the word equation, which is glucose + oxygen → carbon dioxide + water. For (iii), name a specific use of the energy released, such as muscle contraction, growth or keeping the body warm; 'to stay alive' is too vague.

1 mark -- glucose oxygen carbon dioxide water equation 1 mark -- exothermic 1 mark -- energy released muscle contraction growth body temperature

(i) Explain why photosynthesis and respiration can be considered as reverse reactions. [1]

(ii) If the enthalpy change for respiration is −2803 kJ mol⁻¹, state the enthalpy change for photosynthesis. [1]

(iii) Draw a simple energy level diagram showing both processes on the same diagram. Label clearly which arrow represents photosynthesis and which represents respiration. [1]

Model Answer -- 7(c)

The reactants of one process are the products of the other / they are the reverse of each other [1]

ΔH for photosynthesis = +2803 kJ mol⁻¹ (same magnitude but opposite sign) [1]

Diagram showing glucose + O₂ at higher energy, CO₂ + H₂O at lower energy, with respiration arrow going down and photosynthesis arrow going up [1]

⚠ If you missed marks here: Reversing a reaction keeps the SIZE of ΔH but flips its SIGN, so photosynthesis is +2803 kJ mol⁻¹ – copying −2803 loses (ii). On the diagram glucose + oxygen must sit ABOVE carbon dioxide + water, because photosynthesis stores energy: the photosynthesis arrow points UP and the respiration arrow points DOWN.

1 mark -- reactants one process products other reverse each other 1 mark -- delta photosynthesis positive 2803 same magnitude opposite sign 1 mark -- diagram glucose higher CO2 lower respiration down photosynthesis up

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