Question 1
(a)(i)-(ii): Subtopic: B9.1 Circulatory systems
(b)(i)-(ii): Subtopic: B9.3 Blood vessels
(c)(i)-(ii): Subtopic: B9.4 Blood
(a) Mammals have a double circulatory system. Fish have a single circulatory system.
Fig. 1.1 shows the circulatory system of a fish and the circulatory system of a mammal.
(i) Use Fig. 1.1 to describe two ways the circulatory system of a fish is different from the circulatory system of a mammal.
▶️Answer/Explanation
1. Fish have a single circulatory system where blood passes through the heart once per complete circuit, while mammals have a double circulatory system where blood passes through the heart twice per complete circuit.
2. Fish hearts have only two chambers (one atrium and one ventricle), while mammalian hearts have four chambers (two atria and two ventricles).
(ii) Explain one advantage of having a double circulatory system.
▶️Answer/Explanation
A double circulatory system maintains higher blood pressure to body tissues, allowing for more efficient delivery of oxygen and removal of waste products. It also prevents delicate lung tissues from being damaged by high pressure blood flow.
(b) Table 1.1 shows the diameter and the thickness of the wall of two different blood vessels in humans.
(i) Calculate the difference in thickness of the wall of the aorta and the vena cava.
▶️Answer/Explanation
2.0 mm – 1.5 mm = 0.5 mm
(ii) Explain why the walls of the aorta and the vena cava need to be of different thicknesses.
▶️Answer/Explanation
The aorta has thicker walls because it needs to withstand higher blood pressure as it carries oxygenated blood directly from the heart. The vena cava carries deoxygenated blood at lower pressure back to the heart, so its walls can be thinner.
(c) Red blood cells transport oxygen around the body.
(i) Describe two ways red blood cells are adapted for their function.
▶️Answer/Explanation
1. Biconcave shape increases surface area for oxygen absorption
2. Contain hemoglobin which binds to oxygen molecules
3. Lack a nucleus to maximize space for hemoglobin (any two)
(ii) State two other major components of blood.
▶️Answer/Explanation
1. White blood cells
2. Platelets
3. Plasma (any two)
Question 2
(a)-(e): Subtopic: C12.3 Chromatography
Paper chromatography is used to find out whether a fruit drink, D, contains a harmful food colouring, X.
Spots of substances D and X, and spots of three non-harmful food colourings, A, B, and C, are placed on chromatography paper.
Fig. 2.1 shows the results of the chromatography experiment.
(a) State why the start line is drawn using pencil, rather than using ink.
▶️Answer/Explanation
Pencil is used because it doesn’t dissolve in the solvent, unlike ink which would run and interfere with the chromatogram.
(b) State whether the fruit drink D is a pure substance or a mixture. Explain your answer.
▶️Answer/Explanation
D is a mixture because it contains more than one substance (evidenced by multiple spots on the chromatogram), whereas a pure substance would show only one spot.
(c) State whether the fruit drink contains the harmful food colouring X. Explain your answer.
▶️Answer/Explanation
The fruit drink does contain food colouring X because it has a spot that travels the same distance (has the same Rf value) as the known sample of X.
(d) Calculate the Rf value of the spot from food colouring A.
▶️Answer/Explanation
Rf = distance moved by spot ÷ distance moved by solvent front
(Example calculation if distances were: spot moved 3.5 cm, solvent moved 4.0 cm)
Rf = 3.5 ÷ 4.0 = 0.875
(e) Tartrazine is a synthetic orange-yellow food colouring. Tartrazine has a relative molecular mass, Mr, of 534. A tartrazine dye solution has a concentration of 84 g/dm3. Calculate the concentration of the tartrazine dye in mol/dm3.
▶️Answer/Explanation
Concentration (mol/dm3) = concentration (g/dm3) ÷ Mr
= 84 ÷ 534
= 0.157 mol/dm3 (to 3 significant figures)
Question 3
(a): Subtopic: P1.2 Motion
(b): Subtopic: P2.2.2 Melting, boiling and evaporation
(c): Subtopic: P4.2.3 Voltage (electromotive force and potential difference)
(d)(i)-(ii): Subtopic: P4.3.2 Series and parallel circuits
(e)(i)-(ii): Subtopic: P3.2.2 Refraction of light
(a) A car travels along a road at 8 m/s. Describe the difference between the terms speed and velocity.
▶️Answer/Explanation
Speed is a scalar quantity (magnitude only) measuring how fast an object moves, while velocity is a vector quantity (magnitude and direction) measuring both speed and direction of motion.
(b) Some puddles of water have formed on the road. Explain, in terms of water molecules, how the rate of evaporation of water from a puddle is affected by the strength of the wind blowing across the puddle.
▶️Answer/Explanation
Stronger wind increases evaporation rate because:
1. It removes water vapor molecules near the surface, reducing humidity
2. This creates a greater concentration gradient, allowing more molecules to escape the liquid
3. Wind provides energy to help molecules overcome surface tension
(c) The car battery has an electromotive force (e.m.f.) of 12 V. State what is meant by electromotive force.
▶️Answer/Explanation
Electromotive force (e.m.f.) is:
1. The electrical energy supplied by a source per unit charge
2. Measured in volts (V), it represents the energy converted from chemical to electrical form per coulomb of charge
(d) Fig. 3.1 shows part of the lighting circuit for the car. Two lamps, L1 and L2, each have a resistance of 16 Ω.
(i) When the switch is closed the current in the fuse is 1.5A. Determine the current in L1.
▶️Answer/Explanation
In parallel circuits, current splits equally when resistances are equal:
Current through each lamp = Total current ÷ 2 = 1.5A ÷ 2 = 0.75A
(ii) State one reason why the lamps are connected as shown in Fig. 3.1 and not in series. Explain your answer.
▶️Answer/Explanation
Reason: If one lamp fails, the other remains lit
Explanation: In parallel circuits, current can still flow through the other branch when one component fails, unlike series circuits where a single break stops all current flow.
(e) Modern cars use optical fibres to transfer information using visible light rays.
Fig. 3.2 shows a ray of light entering an optical fibre.
(i) Explain why the ray of light is able to stay inside the optical fibre. You may draw on Fig. 3.2 if it helps your answer.
▶️Answer/Explanation
Light stays inside due to total internal reflection:
1. Light enters at angle greater than critical angle
2. It reflects completely at core-cladding boundary
3. This repeated reflection guides light along fibre
(ii) Visible light rays are transverse waves. Draw labelled diagrams to show the difference between a transverse wave and a longitudinal wave.
▶️Answer/Explanation
Transverse wave:
– Diagram showing oscillations perpendicular to wave direction
– Labeled with crests, troughs, and wavelength
Longitudinal wave:
– Diagram showing compressions and rarefactions parallel to wave direction
– Labeled with regions of high and low pressure
Question 4
(a)-(b): Subtopic: B3.2 Osmosis
(c): Subtopic: B2.1 Cell structure
(d): Subtopic: B2.1 Cell structure
(a) A student investigates what effect immersing grapes in distilled water and concentrated salt solution has on the mass of the grapes. The student measures the mass of a grape before and after immersion.
The results are shown in Table 4.1.
| mass before immersion/g | mass after immersion/g | percentage change in mass | |
|---|---|---|---|
| distilled water | 5.0 | 5.1 | +2.0 |
| concentrated salt solution | 5.1 | 4.8 | -5.9 |
Explain why the grape placed in distilled water increased in mass.
▶️Answer/Explanation
1. The grape gained mass due to osmosis
2. Distilled water has higher water potential than grape cells
3. Water molecules moved into grape cells through partially permeable membrane
4. This occurred down the water potential gradient
5. Resulting in net gain of water and increased mass
(b) The student repeated the investigation by immersing a grape in 100% pure grape juice. This time there was no change in mass. Suggest why there was no change in mass.
▶️Answer/Explanation
There was no change because:
The grape juice had the same water potential as the grape cells, so no net movement of water occurred (dynamic equilibrium).
(c) Fig. 4.1 shows diagrams of plant cells that have been immersed in different concentrations of solutions.
Each cell can be described using one of these words:
- flaccid
- plasmolysed
- turgid
Write on the lines underneath each cell in Fig. 4.1, the correct word to describe the cell.
▶️Answer/Explanation
From left to right:
1. Turgid (cell membrane pressed against cell wall)
2. Flaccid (cell membrane slightly pulled away)
3. Plasmolysed (cell membrane completely shrunk from cell wall)
(d) State two structures in plant cells that are not found in animal cells.
▶️Answer/Explanation
1. Cell wall (made of cellulose)
2. Chloroplasts (for photosynthesis)
3. Permanent vacuole (with cell sap)
4. Plasmodesmata (any two of these)
Question 5
(a)(i)-(iii): Subtopic: C6.2 Rate of reaction
(b): Subtopic: C6.2 Rate of reaction
(c): Subtopic: C10.2 Air quality and climate
A scientist investigates the reaction between calcium carbonate and dilute hydrochloric acid. Carbon dioxide gas is given off during the reaction. The scientist measures the mass of the flask and its contents every 30 seconds during the experiment. Fig. 5.1 shows the apparatus the scientist uses.
After every reading, the scientist calculates the mass of carbon dioxide gas given off. The scientist repeats the experiment using the same amount of calcium carbonate and dilute hydrochloric acid. This time he uses warm dilute hydrochloric acid instead of cold dilute hydrochloric acid. Fig. 5.2 shows the scientist’s results.
(a) (i) Use Fig. 5.2 to state the mass of carbon dioxide made after 2.5 minutes using cold acid.
▶️Answer/Explanation
0.9 g (read from graph at 2.5 minutes on cold acid curve)
(ii) Warm acid reacts faster with calcium carbonate than cold acid. Explain why both reactions make 1.2g of carbon dioxide gas.
▶️Answer/Explanation
Both reactions use the same limiting reactant quantities (equal amounts of calcium carbonate and acid), so they produce the same maximum yield of CO2. Temperature only affects reaction rate, not total product amount.
(iii) Calculate the volume occupied by 1.2g of carbon dioxide gas at 25°C. The molar gas volume at 25°C is 24 dm3. [Ar; C, 12; O, 16]
▶️Answer/Explanation
1. Calculate moles of CO2:
Mr = 12 + (16×2) = 44
moles = mass ÷ Mr = 1.2 ÷ 44 = 0.0273 mol
2. Calculate volume:
volume = moles × molar volume = 0.0273 × 24 = 0.655 dm3
(b) The reaction with warm acid is faster than the reaction with cold acid. Explain why reactions are faster at higher temperatures. Explain your answer in terms of collisions between particles.
▶️Answer/Explanation
1. Particles gain more kinetic energy at higher temperatures
2. They move faster and collide more frequently
3. More particles have energy ≥ activation energy
4. This increases the successful collision rate, speeding up reaction
(c) State two other processes that produce carbon dioxide.
▶️Answer/Explanation
1. Respiration in living organisms
2. Combustion of carbon-based fuels
3. Thermal decomposition of carbonates
4. Fermentation (any two of these)
Question 6
(a)(i)-(ii): Subtopic: P1.5 Force
(b): Subtopic: P1.2 Motion
(c): Subtopic: P1.7 Pressure
(d): Subtopic: P1.6.2 Work
(a) A farmer drives his tractor at a constant speed. Fig. 6.1 shows four forces P, Q, R and S acting on the tractor.
(i) State the letter corresponding to the gravitational force acting on the tractor.
▶️Answer/Explanation
Q (gravitational force always acts downward toward Earth’s center)
(ii) Force P is 1500 N. State the value of force R. Explain your answer.
▶️Answer/Explanation
force R = 1500 N
explanation: At constant speed, forces are balanced (Newton’s First Law). Therefore, forward force P equals backward force R (friction/air resistance).
(b) The tractor accelerates. The force causing this acceleration is 4200 N. The weight of the tractor is 35 000 N. The gravitational field strength g is 10 N/kg. Calculate the acceleration of the tractor.
▶️Answer/Explanation
1. Calculate mass:
mass = weight ÷ g = 35,000 ÷ 10 = 3,500 kg
2. Apply F=ma:
a = F/m = 4,200 ÷ 3,500 = 1.2 m/s2
(c) The tractor has very wide tyres as shown in Fig. 6.2.
The tractor sinks into the soil if the pressure acting on the ground is too large. Explain why having wider tyres reduces the pressure of the tractor on the ground.
▶️Answer/Explanation
1. Pressure = Force ÷ Area (P=F/A)
2. Wider tyres increase contact area with ground
3. With constant weight (force), increased area reduces pressure
4. Prevents sinking by distributing weight over larger surface
(d) The farmer lifts a bucket of water from a well. The bucket of water has a weight of 120 N and is lifted through a vertical distance of 18 m. Calculate the work done.
▶️Answer/Explanation
Work done = force × distance
= 120 N × 18 m
= 2,160 J
Question 7
(a)(i)-(iv): Subtopic: B18.2 Food chains and food webs
(b)(i)-(ii): Subtopic: B17.2 Selection
(a) Fig. 7.1 shows a food web in a desert.
(i) Construct a food chain containing scorpions.
▶️Answer/Explanation
Example food chain:
cacti → insects → scorpions
or
cacti → kangaroo rats → scorpions
(Must show correct direction of energy flow with arrows)
(ii) State the number of trophic levels in the food chain in your answer to (a)(i).
▶️Answer/Explanation
3 trophic levels
(Producer → Primary consumer → Secondary consumer)
(iii) Explain why desert foxes can be described as feeding at the third and fourth trophic level.
▶️Answer/Explanation
1. Third trophic level: When eating herbivores (kangaroo rats) which are primary consumers
2. Fourth trophic level: When eating carnivores (snakes/lizards) which are secondary consumers
3. Foxes are omnivores feeding at multiple levels
(iv) Suggest reasons why the population of desert foxes is much smaller than the population of kangaroo rats.
▶️Answer/Explanation
1. Energy loss at each trophic level (only ~10% transferred)
2. Foxes need larger territories to find enough food
3. Fewer foxes can be supported by the available energy
4. Kangaroo rats have more abundant plant food sources
(b) Fig. 7.2 is a photograph of a falcon and its owner.
Falcons have developed several adaptations that have been useful for their survival. More recently falcons have been selectively bred for their speed and hunting ability.
(i) Complete the definition of adaptation.
Adaptation is the process, resulting from natural selection, by which populations become more suited to their …… over many …… …… .
▶️Answer/Explanation
…become more suited to their environment over many generations.
(ii) Describe three ways in which selective breeding is different from natural selection.
▶️Answer/Explanation
1. Human-directed vs nature-directed selection
2. Faster process (years vs millennia)
3. Focuses on specific desired traits rather than overall fitness
4. Can reduce genetic diversity
5. May prioritize traits not beneficial in wild
Question 8
(a)(i)-(iii): Subtopic: C5.1 Exothermic and endothermic reactions
(b): Subtopic: C2.5 Simple molecules and covalent bonds
(c): Subtopic: C11.1 Formulas and terminology
Methanol, CH3OH, is made by reacting carbon dioxide with hydrogen. Water is also made in this reaction.
(a) (i) Write the balanced symbol equation for this reaction.
▶️Answer/Explanation
CO2 + 3H2 → CH3OH + H2O
1. Correct formulas for all compounds
2. Proper balancing (1:3:1:1 ratio)
(ii) This reaction is an exothermic reaction. State the meaning of the word exothermic.
▶️Answer/Explanation
Exothermic means:
1. A reaction that releases energy to surroundings
2. Typically as heat
3. Products have less energy than reactants
(iii) On Fig. 8.1, complete the energy level diagram for an exothermic reaction. Label the activation energy and the energy change on your diagram.
▶️Answer/Explanation
1. Reactants higher than products
2. Downward curve showing energy release
3. Activation energy labeled as hump from reactants to peak
4. Energy change labeled as vertical drop from reactants to products
(b) Methanol, CH3OH, is a covalent molecule. Draw a dot-and-cross diagram to show the bonding in methanol. Show only the outer shell electrons.
▶️Answer/Explanation
1. Carbon with 4 outer electrons (dots/crosses)
2. Three hydrogens each sharing 1 electron with carbon
3. Oxygen with 6 outer electrons sharing 1 with carbon and 1 with hydrogen
4. Two lone pairs on oxygen
5. Correct overlapping electron pairs for all bonds
(c) Methanol is not a hydrocarbon. Explain why.
▶️Answer/Explanation
Methanol contains an oxygen atom (OH group), while hydrocarbons contain only hydrogen and carbon atoms.
Question 9
(a)(i)-(ii): Subtopic: P1.2 Motion
(b): Subtopic: P1.1 Physical quantities and measurement techniques
(c)(i)-(ii): Subtopic: P5.2.2 The three types of nuclear emission
(a) A student cycles to school.
Fig. 9.1 shows a speed-time graph for the journey.
(i) Draw an X on Fig. 9.1 to identify the part of the journey where there is maximum acceleration.
▶️Answer/Explanation
X should be placed on the steepest positive slope (typically the initial acceleration phase 0-5s or the final acceleration after stopping)
(ii) Calculate the acceleration of the student and bicycle at time = 5 s.
▶️Answer/Explanation
1. Identify speed change (e.g., from 0 to 8 m/s in 5s)
2. acceleration = Δspeed ÷ time = (8-0)/5 = 1.6 m/s2
(Note: Values may vary based on graph scale)
(b) At school, the student is asked how she would accurately measure the width of one of the brake cables on her bicycle. Name a measuring device suitable for measuring very small distances accurately.
▶️Answer/Explanation
Micrometer screw gauge (can measure to 0.01mm precision)
or
Digital calipers (for slightly less precision)
(c) The student watches her teacher set up an experiment to detect the β-radiation emitted by a radioactive source, strontium-90 (Sr). When strontium-90 decays it produces an isotope of yttrium (Y).
(i) Use the correct nuclide notation to complete the symbol equation for this decay process.
▶️Answer/Explanation
9038Sr → 9039Y + 0-1β
1. Yttrium has atomic number 39 (one more than Sr)
2. Mass number remains 90
3. Beta particle correctly shown as electron
(ii) State one difference between the behavior of β-particles and γ-rays in an electric field.
▶️Answer/Explanation
β-particles are deflected (due to charge) while γ-rays are not deflected (being neutral electromagnetic waves)
Question 10
(a)-(e): Subtopic: B15.3 Sexual reproduction in plants
(a) Fig. 10.1 is a photograph of wind-pollinated flowers.
Identify the parts labelled A and B in Fig. 10.1.
▶️Answer/Explanation
A: Anther (pollen-producing male part)
B: Stigma (pollen-receiving female part)
(Note: Identification depends on exact labels in the image)
(b) Table 10.1 compares the features of pollen from an insect-pollinated flower and a wind-pollinated flower. Complete Table 10.1 to show the features of pollen from an insect-pollinated flower and a wind-pollinated flower.
| feature | insect-pollinated | wind-pollinated |
|---|---|---|
| relative size | …… | …… |
| relative mass | …… | …… |
| appearance of surface | …… | …… |
▶️Answer/Explanation
| relative size | larger | smaller |
| relative mass | heavier | lighter |
| appearance of surface | spiky/sticky | smooth |
(c) Describe two ways the petals of an insect-pollinated flower are different from the petals of a wind-pollinated flower.
▶️Answer/Explanation
1. Insect-pollinated: Brightly colored vs wind-pollinated: Dull/green
2. Insect-pollinated: Scented vs wind-pollinated: No scent
3. Insect-pollinated: Nectar guides present vs absent
4. Insect-pollinated: Larger showy petals vs small/inconspicuous
(d) Flowers are the reproductive structures in plants. Plants and human females both contain ovaries.
(i) State the function of the ovary in plants.
▶️Answer/Explanation
Produces and contains ovules (which develop into seeds after fertilization)
(ii) State the function of the ovary in humans.
▶️Answer/Explanation
Produces egg cells (ova) and female sex hormones (estrogen/progesterone)
(e) Plants can reproduce asexually or sexually. A cell is formed by fusion of the nuclei of two gametes during sexual reproduction. State the name of this cell.
▶️Answer/Explanation
Zygote (diploid cell formed by fusion of haploid gametes)
Question 11
(a)(i)-(iii): Subtopic: C4.1 Electrolysis
(b): Subtopic: C4.1 Electrolysis
(c): Subtopic: C2.4 Ions and ionic bonds
(a) Concentrated aqueous sodium chloride can be electrolysed using inert electrodes.
Fig. 11.1 shows the electrolysis.
Chlorine gas is made at the anode.
(i) Explain why chlorine gas is made at the anode.
▶️Answer/Explanation
1. Chloride ions (Cl–) are attracted to the anode (positive electrode)
2. Chloride ions lose electrons (oxidation): 2Cl– → Cl2 + 2e–
3. Chlorine atoms combine to form Cl2 gas
4. Water molecules are harder to oxidize than chloride ions
(ii) State the name of the product at the cathode.
▶️Answer/Explanation
Hydrogen gas (H2)
(Formed by reduction of water: 2H2O + 2e– → H2 + 2OH–)
(iii) Explain why the electrodes used must be inert electrodes.
▶️Answer/Explanation
Inert electrodes (e.g., platinum/graphite) don’t react with the products or electrolyte, allowing only the desired electrolysis reactions to occur.
(b) Molten sodium chloride can also be electrolysed. Sodium is made at the cathode. Write the balanced ionic half-equation for the reaction at the cathode. Include state symbols.
▶️Answer/Explanation
Na+(l) + e– → Na(l)
1. Correct ions and electrons
2. Proper state symbol (l) for molten conditions
3. Balanced charges
(c) Sodium chloride has a lattice structure. Draw a labelled diagram of the lattice structure of sodium chloride.
▶️Answer/Explanation
1. Alternating Na+ and Cl– ions in 3D grid
2. Each Na+ surrounded by 6 Cl– and vice versa
3. Correct ratio 1:1
4. Labels for both ion types
5. Example diagram:
| | |
[Cl–]–[Na+]–[Cl–]
| | |
[Na+]–[Cl–]–[Na+]
Question 12
(a)(i)-(ii): Subtopic: P4.5.6 The transformer
(b): Subtopic: P4.2.5 Electrical energy and electrical power
(c): Subtopic: P1.5.2 Turning effect of forces
(d): Subtopic: P2.3.3 Radiation
(a) Fig. 12.1 shows a laptop computer and charger.
The charger contains a transformer. The input voltage across the primary coil is 250V. The primary coil has 5000 turns. The output voltage from the secondary coil is 19V.
(i) Explain why this transformer is called a step-down transformer.
▶️Answer/Explanation
It is called step-down because the output voltage (19V) is lower than the input voltage (250V), reducing the voltage for safe laptop operation.
(ii) Calculate the number of turns on the secondary coil.
▶️Answer/Explanation
Using the transformer equation: Vp/Vs = Np/Ns
250/19 = 5000/Ns
Ns = (19 × 5000) ÷ 250 = 380 turns
(b) The laptop computer has a rechargeable battery. The battery takes 2 hours to charge fully when a voltage of 19V is used with a current of 1.1A. Calculate the energy transferred during the 2 hours.
▶️Answer/Explanation
1. Convert time to seconds: 2 hours = 7200s
2. Energy = V × I × t = 19 × 1.1 × 7200
3. = 150,480 J (or 150.48 kJ)
(c) Fig. 12.2 shows the laptop computer being closed by a force of 12N.
Calculate the moment of the force about the pivot.
▶️Answer/Explanation
1. Convert distance: 24cm = 0.24m
2. Moment = force × perpendicular distance = 12 × 0.24
3. = 2.88 Nm
(d) The microprocessor in the laptop generates large quantities of thermal energy. The thermal energy must be removed so that the microprocessor does not overheat.
Fig. 12.3 shows a heat sink placed in contact with the microprocessor.
Thermal energy is conducted from the microprocessor into the metal fins of the heat sink.
Suggest and explain two ways in which the design of the heat sink allows thermal energy to be removed efficiently from the heat sink.
▶️Answer/Explanation
1. Black surface: Enhances thermal radiation as black bodies are good emitters
2. Multiple fins: Increase surface area for better convection cooling
3. Metal construction: High thermal conductivity quickly transfers heat from microprocessor
4. Air gaps between fins: Allow for effective air circulation (any two)
Question 13
(a)-(b)(i)-(iii): Subtopic: C6.2 Rate of reaction
Ammonia, NH3, is used in the manufacture of nitrogen-containing fertilisers.
(a) Explain why nitrogen-containing fertilisers are important.
▶️Answer/Explanation
Nitrogen is essential for plant growth as it’s needed to make:
1. Amino acids for protein synthesis
2. Chlorophyll for photosynthesis
3. DNA and RNA molecules
Natural soils often lack sufficient nitrogen for optimal crop yields
(b) Ammonia is made in the Haber process.
Fig. 13.1 shows stages in the Haber process.
(i) Write a balanced symbol equation for the formation of ammonia in the Haber process.
▶️Answer/Explanation
N2 + 3H2 ⇌ 2NH3
1. Correct formulas for all substances
2. Proper balancing (1:3:2 ratio)
3. Reversible reaction arrow (⇌) essential
(ii) Suggest a suitable pressure to use in the reaction chamber.
▶️Answer/Explanation
200 atmospheres (standard industrial pressure)
High pressure favors the forward reaction (4 moles → 2 moles) but must balance safety and cost
(iii) Explain why
- an iron catalyst
- a temperature of 450°C
are used in the reaction chamber.
▶️Answer/Explanation
Iron catalyst:
1. Speeds up the reaction without being consumed
2. Provides alternative reaction pathway with lower activation energy
450°C temperature:
1. Compromise between rate and yield
2. Higher temperatures increase rate but decrease yield (exothermic reaction)
3. 450°C gives reasonable yield (15-20%) with good reaction rate