O Level Chemistry (5070) — MCQs & Practice Questions

Particles are closely packed in a regular, fixed arrangement and can only vibrate about fixed positions.

Gas particles move rapidly in all directions and collide with the walls; the force of these collisions per unit area is the pressure.

Heating gives particles more kinetic energy so they vibrate more; at the melting point the forces holding the lattice break and particles move past each other (liquid); at the boiling point particles gain enough energy to escape completely (gas).

1. Diffusion is the movement of particles from a region of higher concentration to lower concentration[1]
2. ...until they are evenly spread / down a concentration gradient[1]
3. Bromine molecules are heavier / have greater mass than hydrogen molecules[1]
4. At the same temperature heavier particles move more slowly, so diffuse more slowly[1]
5. Raising temperature increases the rate of diffusion[1]
6. Because particles gain kinetic energy and move faster[1]

Diffusion = spreading down a concentration gradient; lighter particles diffuse faster; higher temperature → faster diffusion.

Before melting the temperature rises as particles gain kinetic energy. During melting it stays constant (a plateau) because the energy overcomes the forces holding the lattice. After melting it rises again as the liquid warms.

1. Particles move rapidly and randomly, colliding with the walls[1]
2. the force of these collisions per unit area is the pressure[1]
3. Heating gives particles more kinetic energy so they move faster[1]
4. collisions are harder and more frequent, so pressure rises[1]
5. Halving the volume means the same particles are in a smaller space[1]
6. so collisions with the walls are twice as frequent and the pressure doubles[1]

Pressure = wall collisions per area; heating raises it (faster, more frequent collisions); halving the volume doubles it (more frequent collisions).

Proton: charge +1, mass 1. Neutron: charge 0, mass 1. Electron: charge −1, mass negligible (about 1/1840).

Atoms of the same element (same number of protons / same proton number) that have different numbers of neutrons, and so different mass numbers.

Protons = 17, electrons = 17 (neutral atom), neutrons = 35 − 17 = 18.

1. Both isotopes have the same number of electrons / the same electronic configuration (2,8,7)[1]
2. Chemical properties depend on the (outer) electrons, so they react identically[1]
3. Ar = (35 × 75 + 37 × 25) ÷ 100sets up weighted mean[1]
4. Ar = (2625 + 925)/100 = 35.5[1]
5. 7 electrons in the outer shell[1]
6. Gains 1 electron, so the chloride ion is Cl⁻ (charge −1)[1]

Same electron arrangement → identical chemistry; Ar = 35.5; 7 outer electrons, so Cl⁻.

Aᵣ = (10 × 20 + 11 × 80) ÷ 100 = (200 + 880) ÷ 100 = 1080 ÷ 100 = 10.8.

1. 1s² 2s² 2p⁶ 3s² 3p⁶ 4s²[1]
2. electron count totals 20[1]
3. Ca²⁺: 1s² 2s² 2p⁶ 3s² 3p⁶ (loses the two 4s electrons)[1]
4. The first two electrons are removed from the outer 4s sub-shell[1]
5. The third is removed from the 3p, a shell closer to the nucleus[1]
6. which is held much more strongly, so far more energy is needed — showing two electrons in the outer shell[1]

Ca = 1s²2s²2p⁶3s²3p⁶4s²; Ca²⁺ = 1s²2s²2p⁶3s²3p⁶; the jump after 2 electrons shows only two electrons in the outer shell.

Sodium loses one electron to chlorine; sodium becomes Na⁺ and chlorine becomes Cl⁻, and the oppositely charged ions attract by strong electrostatic forces.

It has a giant ionic lattice; the many strong electrostatic forces between oppositely charged ions need a large amount of energy to overcome.

Methane has weak intermolecular forces between molecules, which need little energy to overcome. Diamond is a giant covalent structure with strong covalent bonds throughout, needing very large energy to break.

1. Magnesium loses 2 electrons to oxygen, forming Mg²⁺ and O²⁻[1]
2. The oppositely charged ions attract by strong electrostatic forces[1]
3. When molten, the ions are free to move and carry charge[1]
4. When solid, the ions are fixed in the lattice and cannot move[1]
5. CO₂ is simple molecular with weak intermolecular forces[1]
6. Little energy is needed to separate the molecules, so it is a gas[1]
7. MgO is a giant ionic lattice with strong forces needing much energy, so it is a high-melting solid[1]

Mg→Mg²⁺ + O→O²⁻ (electrostatic attraction); MgO conducts molten (mobile ions) not solid (fixed ions); CO₂ gas (weak intermolecular forces) vs MgO solid (giant lattice, strong forces).

Down the group the atoms have more electrons, so the van der Waals (London) forces between them are stronger and need more energy to overcome, raising the boiling point.

1. Methane is tetrahedral[1]
2. with a bond angle of 109.5°[1]
3. because four bonding pairs repel to get as far apart as possible[1]
4. Water molecules form hydrogen bonds (O–H, H bonded to O)[1]
5. methane has only weak van der Waals forces[1]
6. hydrogen bonds are much stronger, so water needs far more energy to boil[1]

Methane: tetrahedral, 109.5°, four repelling bonding pairs; water boils higher because of strong hydrogen bonding vs methane's weak van der Waals forces.

Mr of CH₄ = 12 + 4 = 16. Moles = mass ÷ Mr = 8.0 ÷ 16 = 0.50 mol.

The sum of the relative atomic masses of all the atoms shown in the molecular formula of a substance.

4 Al + 3 O₂ → 2 Al₂O₃.

1. Mr(CaCO₃) = 40 + 12 + 48 = 100[1]
2. Moles CaCO₃ = 25 ÷ 100 = 0.25 mol[1]
3. Moles CaO = 0.25 (1:1 ratio); Mr(CaO) = 56[1]
4. Mass CaO = 0.25 × 56 = 14 g[1]
5. Moles CO₂ = 0.25 (1:1 ratio)[1]
6. Volume = moles × 24 = 0.25 × 24[1]
7. Volume CO₂ = 6.0 dm³[1]

Mr = 100; 14 g CaO; 6.0 dm³ CO₂.

V = 500 ÷ 1000 = 0.50 dm³. c = n ÷ V = 0.20 ÷ 0.50 = 0.40 mol/dm³.

1. Mr(Fe₂O₃) = 112 + 48 = 160[1]
2. Moles Fe₂O₃ = 320 ÷ 160 = 2 (tonne-moles)[1]
3. Moles Fe = 2 × 2 = 4; Ar(Fe) = 56[1]
4. Mass Fe = 4 × 56 = 224 tonnes[1]
5. Desired product mass in equation = 2 × 56 = 112; total product mass = 112 + 3 × 44 = 244[1]
6. Atom economy = (112 ÷ 244) × 100[1]
7. = 45.9% (≈ 46%)[1]

Mr = 160; 224 tonnes of iron; atom economy ≈ 46%.

Oxidation is loss of electrons; reduction is gain of electrons (OIL RIG).

The breakdown (decomposition) of an ionic compound, when molten or in aqueous solution, by the passage of an electric current.

Cathode (−): Pb²⁺ + 2e⁻ → Pb (lead metal). Anode (+): 2Br⁻ → Br₂ + 2e⁻ (bromine gas).

1. Melting frees the ions[1]
2. so they can move to the electrodes and carry the current[1]
3. Al³⁺ + 3e⁻ → Al[1]
4. balanced with electrons[1]
5. Oxygen is produced at the anode[1]
6. The hot oxygen reacts with the carbon anodes forming carbon dioxide[1]
7. so the anodes burn away and must be replaced[1]

Molten = mobile ions to carry current; cathode Al³⁺ + 3e⁻ → Al; anode makes O₂, which burns the carbon anodes to CO₂ so they wear away.

Oxygen is −2, so 4 × (−2) = −8. The whole ion has a charge of −1, so Mn + (−8) = −1, giving Mn = +7.

1. The anode (positive electrode) is the impure copper[1]
2. Cu → Cu²⁺ + 2e⁻ (copper dissolves)[1]
3. Pure copper deposits on the cathode[1]
4. Cu²⁺ + 2e⁻ → Cu[1]
5. Impurities do not dissolve / are not deposited[1]
6. they collect below the anode as anode sludge[1]

Anode = impure copper (Cu → Cu²⁺ + 2e⁻); cathode gains pure copper (Cu²⁺ + 2e⁻ → Cu); impurities drop as anode sludge.

A reaction that transfers energy to the surroundings (temperature of surroundings rises), e.g. combustion / neutralisation.

Bond breaking absorbs energy and bond making releases energy; in an exothermic reaction more energy is released making bonds than is absorbed breaking them.

Reactants at a higher energy than products; a 'hump' above the reactants marks the activation energy; ΔH is the (negative) drop from reactants to products.

1. Bonds broken = 2(H–H) + 1(O=O) = 2(436) + 498[1]
2. = 872 + 498 = 1370 kJ[1]
3. Bonds made = 4 O–H = 4 × 464[1]
4. = 1856 kJ[1]
5. ΔH = energy in − energy out = 1370 − 1856 = −486 kJ/mol[1]
6. Negative ΔH, so the reaction is exothermic[1]

Broken = 1370 kJ; made = 1856 kJ; ΔH = −486 kJ/mol → exothermic.

q = mcΔT = 200 × 4.2 × 25 = 21 000 J = 21 kJ.

1. q = mcΔT = 100 × 4.2 × 24[1]
2. = 10 080 J = 10.08 kJ[1]
3. moles methanol = 0.32 ÷ 32 = 0.010 mol[1]
4. ΔH = 10.08 ÷ 0.010 = 1008 kJ/mol (magnitude)[1]
5. ΔH ≈ −1010 kJ/mol (exothermic, so negative)[1]
6. Heat lost to the surroundings / incomplete combustion / evaporation[1]

q = 10.08 kJ; ΔH ≈ −1010 kJ/mol; less negative than the true value because heat escapes / combustion is incomplete.

Particles must collide, and they must collide with enough energy (at least the activation energy) and correct orientation.

Higher concentration means more particles per unit volume, so collisions are more frequent, increasing the rate.

A catalyst provides an alternative reaction pathway with a lower activation energy, so a greater proportion of collisions are successful, increasing the rate. It is not used up.

1. At higher temperature particles have more kinetic energy and move faster[1]
2. Collisions are more frequent[1]
3. and a greater proportion have energy ≥ activation energy, so more are successful[1]
4. Powder has a larger surface area (area to volume)[1]
5. so more frequent collisions with acid particles / more sites for reaction[1]
6. The rate is the gradient (slope) of the graph[1]

Higher temperature: faster, more energetic collisions; powder: greater surface area → more frequent collisions; rate = gradient of the volume–time graph.

Collect the gas in a gas syringe and record the volume at fixed times, or stand the flask on a balance and record the loss in mass as the gas escapes.

1. Particles gain kinetic energy and move faster, so collisions are more frequent[1]
2. a greater proportion of collisions have energy above the activation energy[1]
3. so a much larger fraction are successful — the rate rises sharply[1]
4. A catalyst provides an alternative pathway with a lower activation energy[1]
5. so a greater fraction of the existing molecules have enough energy to react[1]
6. more successful collisions per second; the catalyst is not used up[1]

Higher temperature: faster, more energetic collisions, more exceed Ea. Catalyst: lower Ea, so more molecules already exceed it; recovered unchanged.

In a closed system, the forward and backward reactions occur at equal rates, so the concentrations of reactants and products stay constant while both reactions continue.

If the conditions of a system at equilibrium are changed, the equilibrium shifts in the direction that opposes (partly cancels) the change.

Increasing temperature shifts the equilibrium in the endothermic (backward) direction, so the yield of ammonia decreases (though the reaction is faster).

1. There are 4 gas molecules on the left and 2 on the right[1]
2. Higher pressure shifts the equilibrium to the side with fewer gas molecules[1]
3. so it shifts right, increasing the yield of ammonia[1]
4. The Contact process uses a vanadium(V) oxide (V₂O₅) catalyst[1]
5. 2SO₂ + O₂ ⇌ 2SO₃[1]
6. balanced equation with correct formulae[1]

High pressure shifts to fewer gas molecules (right) → more NH₃; Contact process uses V₂O₅ for 2SO₂ + O₂ ⇌ 2SO₃.

Acids produce hydrogen ions, H⁺, in aqueous solution; their pH is below 7.

A strong acid is fully ionised in water; a weak acid is only partly ionised. It is about degree of ionisation, not concentration.

Hydrochloric acid + sodium hydroxide → sodium chloride + water. HCl + NaOH → NaCl + H₂O.

1. CuO + H₂SO₄ → CuSO₄ + H₂O (correct formulae)[1]
2. Equation balanced[1]
3. Warm the acid and add excess copper(II) oxide, then filter off the unreacted solid[1]
4. Heat the filtrate to evaporate some water / to the point of crystallisation[1]
5. Leave to cool so crystals form[1]
6. Filter and dry the crystals (e.g. between filter paper)[1]
7. Excess ensures all the acid reacts, and the excess solid can simply be filtered off[1]

CuO + H₂SO₄ → CuSO₄ + H₂O; react with excess base, filter, evaporate to crystallisation, cool, filter, dry; excess guarantees all acid used and is easily removed.

pH = −log₁₀(1 × 10⁻³) = 3. pH 3 is below 7, so the solution is acidic.

1. CuO + H₂SO₄ → CuSO₄ + H₂O (correct formulae)[1]
2. balanced[1]
3. Warm the acid and add copper(II) oxide in excess until no more reacts[1]
4. Filter to remove the excess copper(II) oxide[1]
5. Heat the filtrate to evaporate water to the point of crystallisation, then leave to cool[1]
6. Filter off the crystals and dry them (e.g. between filter paper)[1]

CuO + H₂SO₄ → CuSO₄ + H₂O; react with excess base, filter, evaporate to crystallisation, cool, filter, dry.

In order of increasing proton (atomic) number, arranged in periods (rows) and groups (columns) of similar properties.

Reactivity increases down Group I because the outer electron is further from the nucleus and more easily lost.

Group VII atoms all have 7 outer electrons, so they react similarly (gaining/sharing one electron). Noble gases have a full outer shell, so they have no tendency to gain, lose or share electrons and are unreactive.

1. Reactivity decreases down the group[1]
2. The outer shell is further from the nucleus / more shielded[1]
3. So it is harder to attract/gain an electron[1]
4. Chlorine is more reactive than bromine, so it displaces bromine[1]
5. The solution turns orange (bromine formed)[1]
6. Cl₂ + 2KBr → 2KCl + Br₂[1]

Reactivity falls down the group (outer shell further/shielded, harder to gain e⁻); chlorine displaces bromine: Cl₂ + 2KBr → 2KCl + Br₂.

Reactivity increases down the group because the outer electron is further from the nucleus and more shielded, so it is lost more easily.

1. Melting point rises from Na to a peak at Si, then falls sharply to Ar[1]
2. Silicon is a giant covalent (macromolecular) structure[1]
3. so a huge number of strong covalent bonds must be broken — very high melting point[1]
4. First ionisation energy generally increases across the period[1]
5. because nuclear charge increases while shielding stays similar[1]
6. so the outer electron is held more tightly and needs more energy to remove[1]

Melting point rises to a giant-covalent peak at Si then falls to molecular Ar; first ionisation energy rises (more nuclear charge, similar shielding).

A list of metals arranged in order of their reactivity, from most reactive to least reactive.

Magnesium fizzes, giving off hydrogen gas: Mg + 2HCl → MgCl₂ + H₂.

Zinc is more reactive than copper, so it loses electrons more readily and displaces copper from solution: Zn + Cu²⁺ → Zn²⁺ + Cu.

1. Iron is less reactive than carbon, so carbon can displace/reduce it[1]
2. Aluminium is more reactive than carbon, so carbon cannot reduce it (electrolysis is used)[1]
3. Fe₂O₃ + 3CO → 2Fe + 3CO₂ (correct formulae)[1]
4. Equation balanced[1]
5. Iron oxide loses oxygen[1]
6. Loss of oxygen is reduction (it also gains electrons)[1]

Carbon reduces metals below it (iron) but not those above it (aluminium); Fe₂O₃ + 3CO → 2Fe + 3CO₂; iron is reduced because it loses oxygen / gains electrons.

Set up three test tubes with iron nails: one with water and air (rusts), one with boiled water and a layer of oil to exclude air (no rust), and one with dry air and a drying agent like calcium chloride (no rust). Only the tube with both water and air rusts.

1. Galvanising coats the iron with a layer of zinc, forming a barrier to water and air[1]
2. Zinc is more reactive than iron, so it corrodes in preference (sacrificial protection)[1]
3. so the iron is protected even if the coating is scratched[1]
4. Zinc displaces iron from the solution (zinc is more reactive)[1]
5. grey iron is deposited / the solution's colour fades[1]
6. Zn + Fe²⁺ → Zn²⁺ + Fe[1]

Galvanising = zinc barrier + sacrificial protection; zinc displaces iron: Zn + Fe²⁺ → Zn²⁺ + Fe.

About 78% nitrogen and about 21% oxygen (the remaining ~1% is mostly argon with a little carbon dioxide).

Add anhydrous copper(II) sulfate — it turns from white to blue in the presence of water (or blue cobalt chloride paper turns pink).

Carbon dioxide is a greenhouse gas; it absorbs and re-emits heat (infrared radiation) that would otherwise escape, so the extra CO₂ traps more heat and raises average global temperatures.

1. Water (moisture)[1]
2. Oxygen (air)[1]
3. Galvanising coats the iron with a layer of zinc[1]
4. The zinc is more reactive than iron, so it corrodes/loses electrons in preference (sacrificial protection)[1]
5. so the iron is protected even where the coating is scratched[1]
6. Painting / oiling / greasing / coating with plastic (any one)[1]

Water + oxygen needed; galvanising = zinc coating that sacrificially protects (zinc more reactive) even if scratched; also painting/oiling.

Sulfur impurities in the coal burn to sulfur dioxide, which dissolves in rainwater to form an acidic solution (sulfurous/sulfuric acid). This acid rain damages plants and forests, acidifies lakes killing fish, or corrodes limestone buildings and metals.

1. Carbon monoxide forms by incomplete combustion[1]
2. when there is not enough oxygen to make CO₂[1]
3. it is toxic because it prevents the blood from carrying oxygen[1]
4. The converter provides a catalyst surface where pollutants react[1]
5. carbon monoxide is oxidised to carbon dioxide[1]
6. 2CO + O₂ → 2CO₂ (and NOₓ is reduced to N₂)[1]

CO from incomplete combustion, toxic (stops blood carrying O₂); catalytic converter oxidises CO to CO₂ (2CO + O₂ → 2CO₂) and reduces NOₓ to N₂.

A family of organic compounds with the same general formula and similar chemical properties, where each member differs from the next by CH₂.

Add bromine water: an alkene decolourises it (orange to colourless); an alkane does not (stays orange).

Many ethene monomers (with C=C double bonds) join together, with no other product, to form a long-chain polymer. Repeat unit: –(CH₂–CH₂)–.

1. C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂[1]
2. correct formulae / balanced[1]
3. Steam / water, ~300 °C, ~60 atm, phosphoric acid catalyst[1]
4. C₂H₄ + H₂O → C₂H₅OH[1]
5. balanced equation[1]
6. It is a continuous process / faster / gives purer ethanol (any one)[1]

Fermentation: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂; hydration: C₂H₄ + H₂O → C₂H₅OH (steam, ~300 °C, ~60 atm, H₃PO₄); hydration is continuous/faster/purer.

Add bromine water: ethene (an alkene) decolourises it from orange to colourless; ethane (an alkane) leaves it orange (no reaction).

1. Cracking breaks long-chain hydrocarbons into shorter ones[1]
2. using heat and a catalyst[1]
3. to make more useful small molecules (fuels) and alkenes for plastics[1]
4. Many ethene monomers (with C=C double bonds) join together[1]
5. the double bonds open and link into a long chain, with no other product[1]
6. The product is poly(ethene)[1]

Cracking breaks long hydrocarbons (heat + catalyst) into useful short molecules and alkenes; addition polymerisation joins many ethene monomers into poly(ethene).

Free-radical substitution. Initiation: Cl₂ → 2Cl• (UV). Propagation: Cl• + CH₄ → •CH₃ + HCl, then •CH₃ + Cl₂ → CH₃Cl + Cl•.

1. The mechanism is electrophilic addition[1]
2. the electron-rich C=C double bond attracts the electrophile (the δ+ H of HBr)[1]
3. The H can add to either carbon, giving either a secondary or a primary carbocation[1]
4. the secondary carbocation is more stable than the primary[1]
5. so the H adds to the terminal (CH₂) carbon (Markovnikov's rule)[1]
6. giving mainly 2-bromobutane[1]

Electrophilic addition; the C=C attracts the electrophile; via the more stable secondary carbocation the major product is 2-bromobutane.

C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂, using yeast at about 30 °C in the absence of air. An advantage is that it uses a renewable resource (sugar/plant crops) rather than crude oil.

1. Oxidise the ethanol with acidified potassium dichromate (warm/reflux)[1]
2. (or ferment ethanol in air) to give ethanoic acid[1]
3. The magnesium fizzes / dissolves, giving off hydrogen gas[1]
4. Mg + 2CH₃COOH → (CH₃COO)₂Mg + H₂[1]
5. Ethyl ethanoate is formed, plus water[1]
6. using a concentrated sulfuric acid catalyst[1]

Oxidise ethanol → ethanoic acid; with magnesium it fizzes giving H₂ (Mg + 2CH₃COOH → (CH₃COO)₂Mg + H₂); with ethanol it makes ethyl ethanoate + water (conc. H₂SO₄ catalyst).

By simple distillation: heat the solution so water boils off, then condense the vapour back to liquid; the salt remains behind.

A pure substance produces a single spot on the chromatogram; a mixture separates into two or more spots.

The Rf value is the ratio of the distance moved by the spot to the distance moved by the solvent front. Rf = distance moved by spot ÷ distance moved by solvent.

1. Add water and stir to dissolve the copper(II) sulfate[1]
2. Filter — sand is the residue in the filter paper[1]
3. Wash and dry the sand[1]
4. Heat the filtrate to evaporate water to the point of crystallisation[1]
5. Leave to cool so crystals form[1]
6. Filter off and dry the crystals[1]

Dissolve, filter (sand = residue), wash and dry it; evaporate the filtrate to crystallisation, cool, filter and dry the crystals.

Dip a clean wire in the salt and hold it in a blue flame: sodium gives a yellow flame, potassium gives a lilac flame.

1. Dissolve the solid and add sodium hydroxide solution[1]
2. a green precipitate forms[1]
3. confirming the iron(II) ion, Fe²⁺[1]
4. To a fresh sample add dilute (hydrochloric or nitric) acid, then barium chloride solution[1]
5. a white precipitate forms[1]
6. confirming the sulfate ion, SO₄²⁻[1]

Fe²⁺: NaOH gives a green precipitate. SO₄²⁻: acid + barium chloride gives a white precipitate.