Mark Scheme

Section A — Structured Questions

Question 1

(a) Calculate the mass of calcium chloride produced

Mark scheme: • M(CaCO₃) = 40 + 12 + (16 × 3) = 100 ✓ • moles CaCO₃ = 5.00/100 = 0.0500 mol (1:1 ratio so moles CaCl₂ = 0.0500) • M(CaCl₂) = 40 + (35.5 × 2) = 111 ✓ • mass = 0.0500 × 111 = 5.55 g ✓

Accept: Any correct method leading to 5.55 g Reject: Answers without working

(3 marks)

(b)(i) Determine the rate of reaction

Mark scheme: • Volume at 60s = 60 cm³, volume at 30s = 40 cm³ (read from graph) ✓ • Rate = (60 - 40)/(60 - 30) = 20/30 = 0.67 cm³/s (or 0.667 cm³/s) ✓

Accept: 0.66-0.67 cm³/s depending on graph reading accuracy Reject: Incorrect unit or no unit

(2 marks)

(b)(ii) Explain why rate decreases

Mark scheme: • The concentration of HCl/reactants decreases ✓ • So there are fewer collisions (per unit time) / collisions are less frequent ✓

Accept: "Acid is used up" for first mark Accept: "Fewer particles to collide" for second mark Reject: "Acid runs out" (implies complete consumption)

(2 marks)

(c) Difference with powdered calcium carbonate

Mark scheme: • The reaction would be faster / take less time to produce 100 cm³ ✓ • Because the surface area is greater / more surface area exposed ✓ • (So) there are more frequent (successful) collisions ✓

Accept: "Powder has smaller particles" for second mark Reject: "More collisions" without reference to frequency unless stated elsewhere in the answer

(3 marks)

Question 2

(a)(i) State the trend in reactivity

Mark scheme: • Reactivity increases (going down the group) ✓

Accept: "Gets more reactive" Reject: Decreases

(1 mark)

(a)(ii) Explain the trend

Mark scheme: • (Going down the group) atoms have more electron shells / shells are further from the nucleus ✓ • (So) the outer electron is less strongly attracted to the nucleus / easier to lose ✓ • (Because of) increased shielding / inner shells shield the outer electron ✓

Accept: "More energy levels" for first mark Accept: "Weaker attraction" or "lower ionisation energy" for second mark Reject: "Bigger atoms" without reference to electron shells

(3 marks)

(b)(i) Describe observations

Mark scheme: Two from: • Sodium floats / moves around (on the surface) ✓ • Fizzing / effervescence / gas produced ✓ • Sodium melts (into a ball) ✓ • (Eventually) sodium disappears / dissolves / gets smaller ✓

Accept: "Bubbles" for fizzing Reject: "Explodes" or "catches fire" (these are observations for potassium, not sodium)

(2 marks)

(b)(ii) Universal indicator colour and explanation

Mark scheme: • Purple / dark blue / blue ✓ • Because sodium hydroxide is (a strong) alkali / the solution is alkaline ✓

Accept: pH 11-14 for first mark Reject: Just "alkaline" for first mark without colour

(2 marks)

(c) Explain why potassium is stored under oil

Mark scheme: • To prevent contact with oxygen / air / moisture / water (in the air) ✓ • To prevent (a vigorous) reaction / oxidation / to keep it safe ✓

Accept: "Stop it reacting" for second mark Reject: First mark only if just "air" without making clear the component being prevented from reacting

(2 marks)

Question 3

(a) Define hydrocarbon

Mark scheme: • A compound containing only hydrogen and carbon (atoms / elements) ✓

Accept: "Made from" or "consists of" instead of "containing" Reject: "Mixture" instead of "compound"

(1 mark)

(b)(i) Explain why fractions separate

Mark scheme: • Different fractions have different boiling points ✓ • The column is cooler at the top / has a temperature gradient / gets hotter towards the bottom ✓ • (Each fraction) condenses when it reaches its boiling point / the temperature at which it condenses ✓

Accept: "Evaporate at different temperatures" for first mark Accept: Any correct description of temperature variation in column for second mark Reject: "Different densities" as only reason

(3 marks)

(b)(ii) State which fraction has highest boiling point

Mark scheme: • Bitumen ✓

Accept: No alternative Reject: Any other answer

(1 mark)

(c)(i) Balanced equation for combustion

Mark scheme: • Correct formulae: C₈H₁₈ + O₂ → CO₂ + H₂O ✓ • Correctly balanced: 2C₈H₁₈ + 25O₂ → 16CO₂ + 18H₂O (or C₈H₁₈ + 12.5O₂ → 8CO₂ + 9H₂O) ✓

Accept: Fractional coefficients or whole number multiples Reject: Incorrect formulae (no marks)

(2 marks)

(c)(ii) Explain why carbon monoxide is dangerous

Mark scheme: • It is toxic / poisonous ✓ • It combines / bonds with haemoglobin (in red blood cells) / prevents oxygen being transported (around the body) / reduces oxygen capacity of blood ✓

Accept: "Stops blood carrying oxygen" Reject: Just "toxic" without explanation for full marks

(2 marks)

(d)(i) Type of reaction

Mark scheme: • Thermal decomposition ✓

Accept: "Decomposition" Reject: "Cracking" (this is the name of the process, not the type of reaction)

(1 mark)

(d)(ii) Use for ethene

Mark scheme: One from: • (To make) polymers / plastics / poly(ethene) / polythene ✓ • (To make) ethanol ✓

Accept: Specific named polymers or products Reject: Just "making chemicals" without specification

(1 mark)

Question 4

(a) Explain why carbon cannot reduce aluminium oxide

Mark scheme: • Aluminium is more reactive than carbon / aluminium is higher in the reactivity series ✓ • (So) carbon cannot displace aluminium / aluminium has a greater affinity for oxygen ✓

Accept: "Carbon is not reactive enough" Reject: "Aluminium is a metal" (insufficient)

(2 marks)

(b)(i) Half equation at cathode

Mark scheme: • Al³⁺ + 3e⁻ → Al ✓✓

Award: • 2 marks for completely correct equation with state symbols or without • 1 mark for correct species but incorrect balancing (e.g., Al³⁺ + e⁻ → Al)

Reject: 0 marks if oxidation equation given

(2 marks)

(b)(ii) Explain why anodes need replacing

Mark scheme: • Oxygen is produced at the anode / oxide ions lose electrons (to form oxygen) ✓ • Oxygen reacts with the (carbon / graphite) anodes ✓ • (To form) carbon dioxide / CO₂ / (or carbon monoxide) / so the anode is worn away / burns away ✓

Accept: "Anodes are oxidised" for first two marks combined Accept: Equation: C + O₂ → CO₂ as part of explanation

(3 marks)

(c) Properties making aluminium suitable for aircraft

Mark scheme: Two from: • Low density / lightweight ✓ • Strong / high strength-to-weight ratio ✓ • Resistant to corrosion / does not rust / forms a protective oxide layer ✓ • Good conductor (of heat / electricity) ✓ • Malleable / ductile ✓

Accept: "Light but strong" for both first and second marks Reject: Just "strong" or just "light" without context (only award one mark)

(2 marks)

(d) Calculate cost

Mark scheme: • Cost = 15.0 × 12.5 ✓ • = 187.5 p (or £1.88 / £1.875) ✓

Accept: Any correct numerical answer with appropriate unit Reject: Incorrect or missing unit (lose final mark only)

(2 marks)

Question 5

(a) Calculate X

Mark scheme: • X = 1/61 ✓ • = 0.01639 or 0.0164 s⁻¹ ✓

Accept: 0.0164 (4 sf as required) Reject: Answers not to 4 sf (lose final mark only if correct value obtained)

(2 marks)

(b) Describe the pattern

Mark scheme: • As concentration increases, the rate increases ✓ • The relationship is (directly) proportional / linear / rate doubles when concentration doubles ✓

Accept: "Time decreases as concentration increases" for first mark Accept: Reference to specific data points for second mark

(2 marks)

(c) Explain pattern using collision theory

Mark scheme: • Higher concentration means more (thiosulfate) particles / particles closer together / particles per unit volume ✓ • (So there are) more collisions per second / more frequent collisions ✓ • (So) more successful collisions / more particles with activation energy ✓

Accept: "More likely to collide" for second mark Reject: Just "more collisions" without reference to frequency/time (maximum 2 marks)

(3 marks)

(d) Predict and explain effect of higher temperature

Mark scheme: • The rate would increase / reaction would be faster ✓ • Particles have more (kinetic) energy / move faster ✓ • (So) collisions are more frequent AND more collisions exceed the activation energy / more successful collisions ✓

Accept: "Particles collide harder" for third mark Reject: Only mentioning frequency OR energy (not both) - maximum 2 marks

(3 marks)

Question 6

(a) Name type of organism

Mark scheme: • Yeast ✓

Accept: "Microorganisms" or "fungi" or "microbes" Reject: "Bacteria" or "enzymes"

(1 mark)

(b) Calculate atom economy

Mark scheme: • M(products) = 2(C₂H₅OH) = 2[(2 × 12) + (6 × 1) + 16] = 2 × 46 = 92 ✓ • M(all products) = 92 + 2[12 + (2 × 16)] = 92 + 88 = 180 ✓ • Atom economy = (92/180) × 100 = 51.1% (3 sf) ✓

Accept: 51% or 51.11% Reject: Using only reactant mass in denominator (must use all products)

(3 marks)

(c) Compare advantages and disadvantages

Mark scheme:

Level 3 (5-6 marks): A detailed comparison covering at least three different factors (e.g., rate, raw materials, conditions, sustainability, purity). Both methods are evaluated with clear advantages and disadvantages for each. Uses correct scientific terminology throughout.

Level 2 (3-4 marks): A comparison covering at least two factors with some evaluation. Some correct use of scientific terminology. May be unbalanced (focus on one method more than the other).

Level 1 (1-2 marks): Limited comparison, listing one or two basic points. May only describe the methods rather than compare them. Limited scientific terminology.

0 marks: No relevant content.

Indicative content: • Fermentation uses renewable resources (glucose from plants) vs hydration uses finite resources (crude oil) ✓ • Fermentation is slower vs hydration is faster/continuous ✓ • Fermentation uses mild conditions (30-40°C) / lower energy costs vs hydration requires high temperature and pressure / more expensive ✓ • Fermentation produces dilute ethanol requiring distillation vs hydration can produce pure ethanol ✓ • Fermentation is a batch process vs hydration is continuous ✓ • Fermentation produces CO₂ (waste/carbon neutral) vs hydration no waste product ✓

(6 marks)

Section B — Extended Response

Question 7

Mark scheme:

Level 3 (9-12 marks): A comprehensive and coherent discussion that covers all four pollutants. Explains formation of each pollutant clearly. Describes multiple methods to reduce emissions for each pollutant. Evaluates effectiveness of methods with reference to limitations and/or disadvantages. Uses scientific terminology accurately throughout. Answer is well-structured and detailed.

Level 2 (5-8 marks): A reasonable discussion covering at least three pollutants. Some explanation of formation. Describes methods to reduce emissions. Some attempt at evaluation but may lack detail or balance. Generally accurate use of terminology. Answer has some structure.

Level 1 (1-4 marks): A basic discussion mentioning some pollutants. Limited explanation of formation or methods. Little or no evaluation. Limited scientific terminology. Answer may lack structure or coherence.

0 marks: No relevant content.

Indicative content:

Formation: • CO₂: Complete combustion of fossil fuels containing carbon / C + O₂ → CO₂ • SO₂: Combustion of fossil fuels containing sulfur impurities / S + O₂ → SO₂ • Nitrogen oxides: High temperatures in engines cause nitrogen and oxygen from air to react / N₂ + O₂ → 2NO • Particulates: Incomplete combustion (of hydrocarbons) due to insufficient oxygen

Methods to reduce emissions: • CO₂: Use alternative energy sources (renewable/nuclear) / improve energy efficiency / carbon capture and storage / reduce fossil fuel consumption / use biofuels • SO₂: Remove sulfur from fuels before combustion (desulfurisation) / use low-sulfur fuels / flue gas desulfurisation with calcium carbonate/oxide • Nitrogen oxides: Catalytic converters in vehicles (convert to N₂ and CO₂) / lower combustion temperatures / recirculate exhaust gases • Particulates: Particulate filters in diesel engines / cleaner combustion / regular engine maintenance / switch to cleaner fuels

Evaluation: • Renewable energy: effective at reducing CO₂ but expensive to implement / weather-dependent / requires infrastructure changes • Catalytic converters: very effective when working properly but require warm-up time / precious metal catalysts are expensive / don't work on already-emitted pollutants • Desulfurisation: effective at reducing SO₂ but adds cost to fuel / requires additional processing • Carbon capture: promising technology but currently expensive / energy-intensive / limited deployment • Particulate filters: very effective (>95% reduction) but require maintenance / can affect engine performance • Alternative fuels: biofuels are carbon neutral but require land for crops / food vs fuel debate • International cooperation needed but difficult to enforce / economic implications

(12 marks)

Question 8

Mark scheme:

Level 3 (9-12 marks): A comprehensive evaluation using data from the table to support explanations. Clear application of Le Chatelier's principle to explain effects of temperature and pressure. Detailed explanation of compromise conditions with reference to both equilibrium position and rate. Explains role of catalyst accurately. Considers economic factors thoroughly (e.g., equipment costs, energy costs, yield vs rate). Uses scientific terminology accurately. Answer is well-structured and logically developed.

Level 2 (5-8 marks): A reasonable evaluation with some use of data. Some application of Le Chatelier's principle. Explains compromise with some detail. Mentions catalyst and some economic factors. Generally accurate terminology. Answer has reasonable structure.

Level 1 (1-4 marks): Basic evaluation with limited use of data. Limited application of Le Chatelier's principle. May describe rather than evaluate conditions. Limited mention of economic factors. Basic terminology. Answer may lack structure.

0 marks: No relevant content.

Indicative content:

Effect of pressure (using data): • Increasing pressure increases yield (at all temperatures) ✓ • E.g., at 450°C: 18% at 100 atm, 30% at 200 atm, 40% at 300 atm ✓ • Le Chatelier: increasing pressure favours the side with fewer moles of gas ✓ • Forward reaction produces 2 moles from 4 moles, so equilibrium shifts right ✓ • Higher pressure would give higher yield but very high pressures are expensive / require stronger equipment / higher energy costs / safety concerns ✓

Effect of temperature (using data): • Increasing temperature decreases yield (at all pressures) ✓ • E.g., at 200 atm: 55% at 350°C, 30% at 450°C, 15% at 550°C ✓ • Le Chatelier: increasing temperature favours the endothermic direction ✓ • Forward reaction is exothermic (ΔH = -92 kJ/mol) so equilibrium shifts left at higher temperatures ✓ • Lower temperature gives higher yield but rate would be too slow / economically unviable ✓ • Higher temperature gives lower yield but much faster rate / reaches equilibrium more quickly ✓

Compromise conditions: • 450°C is a compromise between yield (better at lower temperature) and rate (better at higher temperature) ✓ • At 450°C the rate is fast enough to be economical while maintaining reasonable yield ✓ • 200 atm is a compromise between yield (better at higher pressure) and cost/safety ✓ • Higher pressures are very expensive (equipment, energy) and dangerous ✓

Role of catalyst: • Iron catalyst increases the rate of reaction / speeds up both forward and reverse reactions ✓ • Does not affect the equilibrium position / final yield ✓ • Allows equilibrium to be reached more quickly ✓ • Allows reaction to occur at lower temperature (than would otherwise be needed) ✓

Economic factors: • Need to maximize profit: consider both yield and rate (amount produced per time) ✓ • Operating costs: high temperature and pressure require energy ✓ • Capital costs: high-pressure equipment is expensive ✓ • Unreacted gases are recycled to improve overall efficiency / atom economy ✓ • Must balance production rate against operating costs ✓

(12 marks)

Question 9

(a) Purification methods for three samples

Mark scheme:

Level 3 (9-10 marks): Comprehensive description of appropriate methods for all three samples. Clear explanation of how each method works with correct scientific principles. Clear explanation of why each method is suitable for its specific purpose. Uses accurate scientific terminology throughout. Well-structured answer with logical progression.

Level 2 (5-8 marks): Reasonable description of methods for at least two samples. Some explanation of how methods work. Some explanation of suitability. Generally accurate terminology. Answer has some structure.

Level 1 (1-4 marks): Basic description of some methods. Limited explanation of how they work or why suitable. Limited scientific terminology. May lack structure.

0 marks: No relevant content.

Indicative content:

Sample A (drinking water from seawater): • Desalination by distillation: seawater is heated to evaporate water / water vapour is condensed ✓ • Or reverse osmosis: water forced through semi-permeable membrane under high pressure / membrane allows water through but not dissolved salts ✓ • Removes dissolved salts / ions to make water safe to drink ✓ • Followed by sterilisation (chlorination / UV treatment / ozone) to kill microorganisms ✓ • Suitable because seawater has very high dissolved salt content that must be removed for drinking ✓

Sample B (deionised water for laboratory): • Pass through ion exchange columns / resins ✓ • Positive ions exchanged for H⁺ ions / negative ions exchanged for OH⁻ ions ✓ • H⁺ and OH⁻ combine to form water ✓ • Or distillation: heating water to evaporate, condensing the vapour ✓ • Removes all dissolved ions / produces very pure water with very low conductivity ✓ • Suitable because laboratory work requires water without any dissolved ions that might interfere with experiments / react with chemicals ✓

Sample C (irrigation water from contaminated river): • Screening: remove large debris / solid objects ✓ • Sedimentation: allow solid particles / sewage to settle out ✓ • Biological treatment / sewage treatment: aerobic bacteria break down organic matter ✓ • Filtration: pass through sand/gravel beds to remove remaining particles ✓ • May add sterilisation (chlorination) to kill pathogenic bacteria ✓ • Suitable because irrigation water doesn't need to be as pure as drinking water / main concern is removing sewage and pathogens / organic matter from sewage would actually provide nutrients for plants ✓

(10 marks)

(b) Evaluate desalination vs treating fresh water

Mark scheme:

Level 2 (5-6 marks): Balanced evaluation covering both environmental and economic impacts of both methods. Clear comparison made. Uses appropriate terminology. Well-structured answer.

Level 1 (2-4 marks): Some evaluation of impacts. May be unbalanced (only one method or only environmental or only economic). Limited comparison. Basic terminology.

(1 mark): One or two basic relevant points.

0 marks: No relevant content.

Indicative content:

Economic impacts: • Desalination requires much more energy / significantly higher operating costs ✓ • Desalination requires expensive equipment (especially for reverse osmosis) / higher capital costs ✓ • Treating fresh water is much cheaper / more economical ✓ • But in areas with limited fresh water sources, desalination may be only option / cost may be justified ✓

Environmental impacts: • Desalination uses much more energy (often from fossil fuels) / higher CO₂ emissions / larger carbon footprint ✓ • Desalination produces concentrated brine / salt waste that must be disposed of / can harm marine ecosystems if returned to sea ✓ • Treating fresh water has lower energy requirements / lower environmental impact ✓ • But extracting water from rivers/reservoirs can affect aquatic ecosystems / reduce water flow / affect wildlife ✓ • Treating fresh water requires less chemical use ✓ • Desalination may reduce pressure on limited fresh water resources in dry regions / allows fresh water to be conserved ✓

(6 marks)

Sample Answers with Examiner Commentary

Question 7 — Sample Answers

Grade 9 answer

Carbon dioxide is produced by the complete combustion of any carbon-containing fossil fuel. When hydrocarbons burn in sufficient oxygen, all the carbon is oxidised to CO₂. This is a major contributor to the greenhouse effect because CO₂ absorbs infrared radiation, trapping heat in the atmosphere and causing global warming.

To reduce CO₂ emissions, we can use renewable energy sources like wind, solar, or hydroelectric power instead of burning fossil fuels. This is effective because these sources don't produce CO₂ during operation, though there are emissions during manufacture of equipment. Carbon capture and storage can also be used at power stations to capture CO₂ before it enters the atmosphere and store it underground. This is effective when properly implemented, capturing up to 90% of emissions, but it's very expensive and requires suitable geological storage sites. Improving energy efficiency in homes and vehicles reduces the amount of fuel burned and is cost-effective, but requires investment and behavior change.

Sulfur dioxide is formed when fossil fuels containing sulfur impurities are burned - the sulfur reacts with oxygen to form SO₂. This dissolves in water in the atmosphere to form acid rain, which damages buildings, trees, and aquatic ecosystems. Sulfur can be removed from fuels before combustion through desulfurisation processes, which is very effective at reducing emissions but adds to fuel costs. Alternatively, flue gas desulfurisation uses calcium carbonate or calcium oxide to neutralise SO₂ in waste gases at power stations. This is highly effective (95%+ removal) but requires additional equipment and produces calcium sulfate waste which must be disposed of, though this can be used to make plasterboard.

Nitrogen oxides form when nitrogen and oxygen from the air react together at the high temperatures in vehicle engines or furnaces. They contribute to acid rain and also cause photochemical smog in cities. Catalytic converters in vehicle exhaust systems are very effective at reducing NOₓ emissions, converting them to harmless nitrogen gas. The reaction is: 2NO → N₂ + O₂ (catalysed by platinum/rhodium). However, catalytic converters are expensive due to precious metal catalysts, only work when hot, and don't address emissions from existing old vehicles. Reducing combustion temperatures through exhaust gas recirculation also helps but slightly reduces engine efficiency.

Particulates are produced by incomplete combustion of hydrocarbons when there's insufficient oxygen. Diesel engines are a major source. These particles cause respiratory problems like asthma and contribute to global dimming by reflecting sunlight. Diesel particulate filters can remove over 95% of particles and are now mandatory on new vehicles in the UK, making them very effective. However, they require periodic cleaning and can affect engine performance. Switching to cleaner fuels like natural gas or electric vehicles eliminates particulate emissions but requires new infrastructure and vehicle replacement.

Overall, the most effective approaches combine multiple methods and require both technological solutions and policy changes. While many reduction methods are highly effective technically, economic costs and the need for international cooperation present challenges to implementation.

Mark: 12/12

Examiner commentary: This is an exemplary response that demonstrates comprehensive understanding. The candidate explains the formation of all four pollutants with appropriate chemical detail and links them clearly to their environmental effects. Multiple reduction methods are described for each pollutant with clear explanations of how they work. The evaluation is sophisticated, considering effectiveness quantitatively (e.g., "95%+ removal", "up to 90%"), discussing limitations and disadvantages for each method (cost, infrastructure, waste products, efficiency trade-offs), and recognising the complexity of implementation. Scientific terminology is used accurately throughout (desulfurisation, flue gas, catalytic converters, precious metals) and the answer is well-structured with clear paragraphs for each pollutant. The conclusion appropriately recognises that multiple approaches are needed. This response fully meets all Level 3 criteria.

Grade 6 answer

Carbon dioxide causes global warming and climate change. It is made when fossil fuels burn. To reduce it we can use renewable energy like solar panels and wind turbines which don't produce CO₂. This is effective but solar and wind are not always available. We can also use carbon capture to trap CO₂ at power stations but this is expensive.

Sulfur dioxide makes acid rain which damages stone buildings and kills fish in lakes. It is made when coal with sulfur in it burns. We can remove sulfur from fuels before burning them or use scrubbers in power stations to remove SO₂ from the smoke. These methods work well at reducing SO₂ but they cost money.

Nitrogen oxides are made in car engines at high temperatures. They cause acid rain and smog. Catalytic converters in cars reduce nitrogen oxides to nitrogen gas which is harmless. These are effective and are used in all modern cars, but old cars don't have them.

Particulates come from incomplete combustion especially in diesel engines. They cause breathing problems. Particulate filters remove most of them and are now required on new diesel cars. This is effective but the filters need cleaning.

In conclusion, there are several ways to reduce atmospheric pollution and most of them are effective but they often cost money which is a disadvantage.

Mark: 7/12

Examiner commentary: This response demonstrates sound understanding and achieves a mid-Level 2 mark. The candidate addresses all four pollutants with correct identification of formation and effects, and describes appropriate reduction methods. However, explanations lack detail - for example, no chemical equations, no mention of specific processes like desulfurisation or flue gas treatment by name, and limited explanation of how methods work (e.g., how catalytic converters actually function). The evaluation is present but superficial - effectiveness is stated but not quantified or analysed in depth, and "cost money" is mentioned repeatedly but without considering different types of costs or trade-offs. To reach Level 3, the answer needs more scientific depth (mechanisms, chemical principles), more detailed evaluation (specific advantages/disadvantages for each method, quantitative data), and better use of scientific terminology. The structure is adequate but paragraphs are shorter and less developed than the Grade 9 response.

Grade 3 answer

Carbon dioxide is a greenhouse gas that causes global warming. It is made from burning fossil fuels. To reduce it we should use less fossil fuels and use more renewable energy. This is good for the environment.

Sulfur dioxide causes acid rain. To reduce it we can use filters to stop it getting into the air. This works quite well.

Nitrogen oxides are from cars and cause pollution. Cars have catalytic converters which remove the pollution. This is effective because new cars have to have them by law.

Particulates are small bits of soot from diesel cars. They are bad for breathing. We can use filters to remove them.

All of these methods are effective at reducing pollution but they are expensive so some people might not use them.

Mark: 3/12

Examiner commentary: This response shows basic awareness but lacks the depth and detail required for higher marks, placing it in Level 1. The candidate identifies the pollutants and mentions some reduction methods, but explanations of formation are minimal or absent (no mention of sulfur impurities, high temperatures for NOₓ, or insufficient oxygen for particulates). The descriptions of reduction methods are vague ("use filters", "use less fossil fuels") without naming specific technologies or explaining mechanisms. There is very limited evaluation - effectiveness is stated but not justified or explained. A common misconception is evident: the candidate seems to think all reduction methods are optional based on cost ("some people might not use them"), not recognizing that many are regulatory requirements. The answer also lacks appropriate scientific terminology and contains no chemical equations or specific examples. To improve, the candidate should: explain formation mechanisms with reference to combustion conditions, name specific reduction technologies (desulfurisation, scrubbers, catalytic converters, DPFs), explain how these work using scientific principles, and provide balanced evaluation with specific advantages and limitations. The structure is too brief and underdeveloped for this 12-mark extended response question.

Question 8 — Sample Answers

Grade 9 answer

Looking at Table 3, increasing the pressure increases the percentage yield of ammonia at all temperatures. For example, at 450°C, the yield increases from 18% at 100 atm to 30% at 200 atm and 40% at 300 atm. This can be explained using Le Chatelier's principle. The forward reaction produces 2 moles of ammonia from 4 moles of reactants (1 N₂ + 3 H₂), so increasing the pressure favours the side with fewer moles of gas. The equilibrium position shifts to the right to oppose the increase in pressure, producing more ammonia.

The table also shows that increasing temperature decreases the yield of ammonia. At 200 atm, the yield is 55% at 350°C but falls to 30% at 450°C and only 15% at 550°C. This is because the forward reaction is exothermic (ΔH = -92 kJ/mol). According to Le Chatelier's principle, increasing temperature favours the endothermic direction. Since the forward reaction releases heat, the reverse reaction absorbs heat (endothermic), so higher temperatures shift the equilibrium to the left, decomposing ammonia back to nitrogen and hydrogen.

The industrial conditions of 450°C and 200 atm represent a compromise between opposing factors. While lower temperatures would give higher yields (55% at 350°C vs 30% at 450°C), the rate of reaction would be too slow to be economically viable. At 350°C, the reaction would take much longer to reach equilibrium, reducing the amount of ammonia produced per day. Using 450°C sacrifices some yield but ensures the reaction proceeds at a commercially useful rate. Similarly, while very high pressures (e.g., 300 atm or more) would increase yield further, the costs become prohibitive. High-pressure equipment is extremely expensive to build and maintain, requiring very strong vessels and pipes. The energy costs of compressing gases to very high pressures are also substantial. 200 atm provides a reasonable yield while keeping capital and operating costs manageable. There are also safety considerations - the higher the pressure, the greater the risk of dangerous leaks or explosions.

The iron catalyst is crucial to the process. It provides an alternative reaction pathway with lower activation energy, which increases the rate of both the forward and reverse reactions. This means equilibrium is reached much more quickly. Importantly, the catalyst does not affect the position of equilibrium or the final percentage yield - it only affects how fast that equilibrium is established. Without the catalyst, even higher temperatures would be needed to achieve an economically viable rate, which would further decrease the yield. The catalyst therefore allows the process to operate at a lower temperature than would otherwise be necessary, giving a better yield while maintaining an acceptable rate.

Economic factors are central to choosing these conditions. The Haber process operates continuously in large-scale plants, so the rate of production (kg per hour) is as important as the percentage yield. Unreacted nitrogen and hydrogen are recycled, so the lower percentage yield is partially offset - the overall efficiency is better than the 30% might suggest. The compromise conditions maximize profit by balancing the value of ammonia produced against the costs of energy, equipment, and raw materials. Operating at slightly lower than optimal yield is economically rational if it significantly reduces costs or increases production rate.

Mark: 12/12

Examiner commentary: This is an outstanding response demonstrating comprehensive understanding at the highest level. The candidate systematically uses data from the table to support explanations (citing specific values), applies Le Chatelier's principle accurately with clear reference to the number of moles and the enthalpy change, and provides detailed evaluation of the compromise conditions. The economic analysis is sophisticated, considering multiple factors: equipment costs, energy costs, production rate vs. yield, safety, and the role of recycling. The explanation of the catalyst is precise, correctly noting it doesn't affect equilibrium position but speeds up the rate. The answer is exceptionally well-structured, using data to support theoretical explanations and then synthesizing this into evaluation of industrial practice. Scientific terminology is used accurately throughout (activation energy, endothermic, equilibrium position, capital costs). This fully meets all Level 3 descriptors and demonstrates the analytical thinking expected of top-grade candidates.

Grade 6 answer

Table 3 shows that higher pressure gives higher yield. At 450°C, the yield goes from 18% at 100 atm to 30% at 200 atm to 40% at 300 atm. This is because Le Chatelier's principle says that if you increase pressure, the equilibrium moves to the side with fewer molecules. The left side has 4 molecules (N₂ + 3H₂) and the right side has 2 molecules (2NH₃), so increasing pressure moves equilibrium to the right and makes more ammonia.

Higher temperature gives lower yield. At 200 atm, the yield is 55% at 350°C but only 30% at 450°C. This is because the forward reaction is exothermic (releases heat). Le Chatelier says if you increase temperature, the equilibrium moves in the endothermic direction to absorb the heat. The backward reaction is endothermic, so higher temperature gives less ammonia.

The industrial conditions are a compromise. They don't use 350°C even though it gives better yield because the reaction would be too slow. They use 450°C so the reaction is faster even though the yield is lower. They don't use 300 atm even though it gives better yield because very high pressure is expensive and dangerous. 200 atm is high enough to give good yield but not too expensive.

The iron catalyst speeds up the reaction so it reaches equilibrium faster. It doesn't change the yield, only the rate. This means they can get a reasonable amount of ammonia in a reasonable time.

The economic reason for these conditions is that they need to make ammonia quickly and cheaply. If