Key Takeaways

1. Electrolysis

Key Rules & Examples:

  • Aluminium Extraction:
    • Aluminium oxide (Al2O3Al2​O3​) is dissolved in molten cryolite to lower the melting point (from 2040°C to 950°C), reducing energy costs.
    • Half-equations:
      • Reduction (negative electrode): Al3++3e−→AlAl3++3e−→Al.
      • Oxidation (positive electrode): 2O2−→O2+4e−2O2−→O2​+4e−.
    • Substances formed:
      • Positive electrode: Oxygen (reacts with carbon electrodes to form CO2CO2​).
      • Negative electrode: Aluminium metal.
    Faraday Calculations:
    • 1 Faraday = 96,500 C (charge of 1 mole of electrons).
    • To produce 1 mole of AlAl, 3 moles of electrons are needed.
      Faradays=Moles of electrons1=3 FFaradays=1Moles of electrons​=3F

Common Mistakes & Tips:

  • Always check ion charges (e.g., Al3+Al3+, not Al2+Al2+).
  • Balance half-equations for atoms and charges.

2. Metal Extraction

Iron & Zinc Extraction:

  • Iron: Reduced using carbon (blast furnace):
    Fe2O3+3C→2Fe+3COFe2​O3​+3C→2Fe+3CO
    • Why not electrolysis? Iron is less reactive than aluminium; carbon reduction is cheaper.
  • Zinc: Zinc sulfide (ZnSZnS) is roasted:
    2ZnS+3O2→2ZnO+2SO22ZnS+3O2​→2ZnO+2SO2​
    Then reduced with carbon:
    ZnO+C→Zn+COZnO+C→Zn+CO

Environmental Issues:

  • SO2SO2​ causes acid rain; solved by scrubbing gases with calcium oxide.

3. Chemical Equations & Calculations

Key Rules:

  • Molar Mass: Sum of atomic masses.
    • E.g., CaCO3CaCO3​: 40+12+(16×3)=100 g/mol40+12+(16×3)=100g/mol.
  • Reacting Masses: Use molar ratios.
    • Example: Heating 2 moles of CaCO3CaCO3​:
      Moles of CO2=2→Volume=2×24=48 dm3 (at r.t.p.)Moles of CO2​=2→Volume=2×24=48dm3(at r.t.p.)
  • Percentage Yield:
    Yield=ActualTheoretical×100Yield=TheoreticalActual​×100

Tips:

  • Always show working to earn partial marks.
  • For gases, 1 mole = 24 dm³ at r.t.p.

4. Fractional Distillation & Cracking

Fractional Distillation:

  • Separates crude oil into fractions by boiling point.
  • Larger hydrocarbons (e.g., bitumen) have higher boiling points and are more viscous.

Cracking:

  • Breaks long-chain hydrocarbons into shorter ones (e.g., C20H42→C16H34+C2H4C20​H42​→C16​H34​+C2​H4​).
  • Uses heat + catalyst (e.g., zeolite).

5. The Haber Process

Conditions:

  • Temperature: 450°C (compromise between rate and yield).
  • Pressure: 200 atm (high pressure favours NH3NH3​).
  • Catalyst: Iron.

Equilibrium:

  • Exothermic reaction: higher temps reduce yield but increase rate.
  • Unreacted N2N2​ and H2H2​ are recycled.

6. Polymerisation

Addition Polymers:

  • Formed from monomers with double bonds (e.g., ethene → poly(ethene)).
  • Disposal Issues: Non-biodegradable; recycling/incineration required.

Condensation Polymers:

  • Formed with two monomers (e.g., nylon), releasing water.

7. Chemical Tests

  • Chlorine: Bleaches damp litmus paper.
  • Hydrogen: “Squeaky pop” with a lit splint.
  • Oxygen: Relights a glowing splint.

8. Exam Tips

  • Read questions carefully: Underline key terms (e.g., “explain”, “calculate”).
  • Graphs: Draw construction lines to read values accurately.
  • Ionic Equations: Always balance charges and atoms.

Example Mistake:

  • Incorrectly writing Al2+Al2+ instead of Al3+Al3+.

Revision Strategy:

  • Practice balancing equations and Faraday calculations.
  • Memorise key industrial processes (Haber, electrolysis, cracking).

50 GCSE Chemistry Questions (Edexcel International GCSE)

Electrolysis

  1. What substance is formed at the positive electrode during aluminium extraction? Explain why.
  2. Write the ionic half-equation for the reduction of aluminium ions.
  3. How many faradays are needed to produce 1 mole of aluminium?
  4. Why is aluminium oxide dissolved in cryolite?
  5. Name the ion attracted to the negative electrode in aluminium extraction.

Metal Extraction

  1. Why is iron extracted using carbon instead of electrolysis?
  2. Complete the equation:
    Fe2O3+C→____+____Fe2​O3​+C→____+____
  3. What is the purpose of limestone in a blast furnace?
  4. Write the equation for the reaction of zinc sulfide with oxygen.
  5. Why can zinc oxide be reduced by carbon?

Chemical Equations & Calculations

  1. Calculate the molar mass of calcium carbonate (CaCO3CaCO3​).
  2. If 2 moles of CaCO3CaCO3​ are heated, what volume of CO2CO2​ is produced at r.t.p.?
  3. A farmer heated 125 tonnes of CaCO3CaCO3​. Calculate the theoretical mass of CaOCaO.
  4. If 55 tonnes of CaOCaO were produced, calculate the percentage yield.
  5. Balance the equation:
    C20H42→C16H34+C2H4C20​H42​→C16​H34​+C2​H4​

Fractional Distillation & Cracking

  1. Why does diesel have a higher boiling point than gasoline?
  2. Explain why bitumen is the hardest fraction to ignite.
  3. What is the purpose of cracking?
  4. Name the catalyst used in cracking.
  5. Describe the relationship between carbon chain length and viscosity.

Haber Process

  1. Write the equation for the Haber process.
  2. Why is a temperature of 450°C used?
  3. Explain why the reaction is exothermic.
  4. What catalyst is used in the Haber process?
  5. Why is the ammonia yield only 15–20%?

Polymers

  1. Draw the repeat unit of poly(ethene).
  2. What is the difference between addition and condensation polymers?
  3. Name a use of poly(chloroethene).
  4. Why is Teflon (PTFE) used for non-stick pans?
  5. Why is polymer disposal difficult?

Chemical Tests

  1. Describe a test for chlorine gas.
  2. How would you confirm hydrogen gas?
  3. What test identifies oxygen gas?
  4. A solution turns blue around the negative electrode during electrolysis. What does this indicate?
  5. How can you prove a liquid is pure water?

Electrolysis of Solutions

  1. Name the gas produced at the positive electrode during brine electrolysis.
  2. Why is sodium hydroxide formed at the negative electrode?
  3. Write the half-equation for chlorine production.
  4. What safety precaution is needed when electrolysing sodium chloride?
  5. Why is hydrogen produced instead of sodium during brine electrolysis?

Graphs & Data Analysis

  1. Using the fractional distillation table, explain why refinery gases ignite instantly.
  2. From the sulfur trioxide graph, estimate the % conversion at 500°C.
  3. How does pressure affect ammonia yield at 450°C?
  4. Describe the trend between carbon atoms and ignition time.
  5. From the ethanol fermentation graph, which temperature produces the most ethanol?

Stoichiometry

  1. Calculate the moles of nitrogen needed to produce 1 mole of ammonia.
  2. What is the mass of 1 mole of NH3NH3​?
  3. Calculate the volume of 51 g of NH3NH3​ at r.t.p.
  4. If 8 g of Fe2O3Fe2​O3​ reacts with carbon, what mass of CO2CO2​ is produced?
  5. A current of 2.5 A flows for 1 hour. Calculate the moles of copper deposited.

Detailed Answers

  1. Oxygen forms at the positive electrode. It is formed because oxide ions (O2−O2−) lose electrons (oxidation):
    2O2−→O2+4e−2O2−→O2​+4e−
    The carbon electrode reacts with oxygen to form CO2CO2​.
  2. Reduction of Al3+Al3+:
    Al3++3e−→AlAl3++3e−→Al
  3. 3 faradays (1 Faraday = 1 mole of electrons; 3e−3e− needed per AlAl atom).
  4. Cryolite lowers the melting point of Al2O3Al2​O3​ from 2040°C to 950°C, saving energy.
  5. Al3+Al3+ is attracted to the negative electrode.
  6. Iron is less reactive than carbon. Carbon reduction is cheaper than electrolysis.
  7. Fe2O3+3C→2Fe+3COFe2​O3​+3C→2Fe+3CO
  8. Limestone removes impurities (e.g., silicon dioxide) as slag.
  9. 2ZnS+3O2→2ZnO+2SO22ZnS+3O2​→2ZnO+2SO2​
  10. Zinc is below carbon in the reactivity series, so carbon can reduce zinc oxide.
  11. CaCO3=40+12+(16×3)=100 g/molCaCO3​=40+12+(16×3)=100g/mol
  12. Volume=2×24=48 dm3Volume=2×24=48dm3
  13. Mass of CaO=125100×56=70 tonnesMass of CaO=100125​×56=70tonnes
  14. Yield=5570×100=78.6%Yield=7055​×100=78.6%
  15. C20H42→C18H38+C2H4C20​H42​→C18​H38​+C2​H4​
  16. Diesel has longer hydrocarbon chains with stronger intermolecular forces, requiring more energy to boil.
  17. Bitumen has the most carbon atoms, leading to slow combustion (longest ignition time).
  18. Cracking converts long-chain hydrocarbons into shorter, more useful ones (e.g., petrol).
  19. Zeolite (aluminosilicate catalyst).
  20. Longer chains → higher viscosity and higher boiling points.
  21. N2+3H2⇌2NH3N2​+3H2​⇌2NH3​
  22. Compromise: Higher temps speed up the reaction but reduce yield (exothermic).
  23. The graph shows lower yields at higher temps, indicating heat is released (exothermic).
  24. Iron catalyst.
  25. Equilibrium: Unreacted gases are recycled, so low yield per cycle is acceptable.
  26. −CH2−CH2−−CH2​−CH2​−
  27. Addition polymers form from one monomer; condensation polymers form from two, releasing water.
  28. PVC (pipes, insulation).
  29. Non-reactive and heat-resistant.
  30. Polymers are non-biodegradable and release toxins when burned.
  31. Bleaches damp litmus paper (turns white).
  32. Lit splint produces a squeaky pop.
  33. Relights a glowing splint.
  34. Alkaline solution (e.g., NaOHNaOH) forms, turning universal indicator blue.
  35. Boiling point test (pure water boils at 100°C at 1 atm).
  36. Chlorine (Cl2Cl2​).
  37. Na+Na+ ions gain electrons, but H2OH2​O is reduced instead:
    2H2O+2e−→H2+2OH−2H2​O+2e−→H2​+2OH−
  38. 2Cl−→Cl2+2e−2Cl−→Cl2​+2e−
  39. Fume cupboard to avoid inhaling chlorine gas.
  40. H2OH2​O is preferentially reduced over Na+Na+.
  41. Refinery gases have short chains, so volatile and ignite instantly.
  42. ~90% (from graph interpolation).
  43. Higher pressure increases yield (favours fewer gas moles).
  44. More carbon atoms → longer ignition time (harder to combust).
  45. 30°C (yeast enzymes denature at higher temps).
  46. 0.5 moles (ratio 1N2:2NH31N2​:2NH3​).
  47. NH3=14+(1×3)=17 g/molNH3​=14+(1×3)=17g/mol
  48. Moles=5117=3 mol→Volume=3×24=72 dm3Moles=1751​=3mol→Volume=3×24=72dm3
  49. Moles of Fe2O3=8160=0.05 mol→Mass of CO2=0.05×44=2.2 gMoles of Fe2​O3​=1608​=0.05mol→Mass of CO2​=0.05×44=2.2g
  50. Charge=2.5×3600=9000 CCharge=2.5×3600=9000C
    Moles of e−=900096500=0.0933 molMoles of e−=965009000​=0.0933mol
    Moles of Cu=0.09332=0.0467 molMoles of Cu=20.0933​=0.0467mol