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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

Fewer C=O double bonds are formed (C=O in CO₂ has higher bond energy than C≡O in CO), so less energy is released when bonds form / the difference between bonds broken and formed is smaller [1]

1 mark -- incomplete combustion insufficient oxygen 1 mark -- carbon monoxide carbon soot water products 1 mark -- fewer C=O bonds formed less energy released difference smaller

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]

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]

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]

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]

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]

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]

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

Self-Assessment

Tick marks earned, then click Calculate Grade.

Your Score

Out Of

Percentage