AQA GCSE Chemistry 8462: Key ideas – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

These questions cover section 4.11, Key ideas, of the AQA GCSE Chemistry (8462) specification, for teaching from September 2016 with GCSE exams from June 2018 (version 1.1). The key ideas are assessed across all papers, so these questions link topics that can appear on Paper 1 or Paper 2, at Foundation and Higher tier, every May/June. Parts that use Higher tier only content are labelled “(Higher tier only)”.

Learn the ideas first in the study guide and the revision notes. The course hub is AQA GCSE Chemistry and the printable checklist lists every topic.

How to use this set. Most questions need facts from two or more topics. Before you write, name the key idea involved (for example “outer electrons” or “activation energy”). Write one linked point per mark. Questions 7 and 11 are extended answers: plan the chain first. Use the periodic table for atomic numbers, and show every step of a calculation.

Questions

1.

(a) Give the relative charge of a proton, a neutron and an electron. [1] (b) An atom of fluorine has 9 protons. Explain why the atom has no overall charge. [2]

2. Calculate the numbers of protons, neutrons and electrons in each ion.

(a) ³⁹K⁺ (atomic number 19) [2] (b) ⁷⁹Br⁻ (atomic number 35) [2]

3. A sample of silver is 51.8% silver-107 and 48.2% silver-109. Calculate the relative atomic mass of silver. Give your answer to 1 decimal place. [3]

4. Lithium, sodium and potassium are in Group 1.

(a) Give the electronic structure of potassium (atomic number 19). [1] (b) Explain why lithium, sodium and potassium have similar chemical reactions. [1] (c) Explain why potassium reacts more vigorously with water than sodium does. [3]

5. A student adds chlorine water to colourless potassium iodide solution. The solution turns brown.

(a) Explain why chlorine displaces iodine from potassium iodide solution. [2] (b) (Higher tier only) Complete the ionic equation: Cl₂ + 2I⁻ → ________ + ________ [2] (c) (Higher tier only) Identify the species that is oxidised. Explain your answer in terms of electrons. [2]

6.

(a) Describe what happens to the electrons when magnesium reacts with oxygen to form magnesium oxide. [3] (b) State the type of bonding in hydrogen chloride and describe how the bond forms. [2] (c) Calcium is in Group 2 and bromine is in Group 7. Give the formula of calcium bromide. [1]

7. The table gives data about three substances, A, B and C.

Substance	Melting point / °C	Conducts when solid?	Conducts when molten?
A	1710	No	No
B	801	No	Yes
C	–182	No	No

Use ideas about structure and bonding to explain the properties of A, B and C. [6]

8.

(a) Name the ion that all acids produce in aqueous solution. [1] (b) Write the ionic equation, with state symbols, for the neutralisation of an acid by an alkali. [2] (c) Explain why neutralisation is described as a proton transfer reaction. [1] (d) (Higher tier only) How many times greater is the hydrogen ion concentration in a solution of pH 2 than in a solution of pH 5? [1] (e) (Higher tier only) Solutions of ethanoic acid and hydrochloric acid have the same concentration. The ethanoic acid has the higher pH. Explain why. [2]

9. Marble chips react with dilute hydrochloric acid. In the first 120 s, 66 cm³ of carbon dioxide is collected.

(a) Calculate the mean rate of reaction over the first 120 s. Give the unit. [2] (b) Explain, using collision theory, why warming the acid increases the rate of reaction. [3] (c) The marble chips are replaced by the same mass of marble powder. Explain the effect on the rate. [2]

10. Ammonia is made from nitrogen and hydrogen: N₂ + 3H₂ ⇌ 2NH₃

(a) State what is meant by “energy is conserved in a chemical reaction”. [1] (b) (Higher tier only) Use these bond energies to calculate the energy change for the forward reaction: N≡N 945 kJ/mol, H–H 436 kJ/mol, N–H 391 kJ/mol. [3] (c) (Higher tier only) Use your answer to (b) to state whether the forward reaction is exothermic or endothermic. [1] (d) An iron catalyst is used. Explain how a catalyst increases the rate of a reaction. [2]

11. Sodium burns in chlorine gas to form sodium chloride.

(a) Write a balanced symbol equation, with state symbols, for the reaction. [2] (b) Explain how sodium and chlorine atoms form sodium chloride, and why sodium chloride has a high melting point. Use electronic structures in your answer. [6]

12. A student heats 5.00 g of calcium carbonate until its mass stops changing. The solid left, calcium oxide, has a mass of 2.80 g. CaCO₃ → CaO + CO₂

(a) Calculate the relative formula mass of calcium carbonate (Ca = 40, C = 12, O = 16). [1] (b) Calculate the mass of carbon dioxide given off. [1] (c) No atoms are lost or made in a chemical reaction. Explain why the mass of the solid decreases. [2] (d) The reaction only continues while it is heated. State whether it is exothermic or endothermic. [1]

Answers

1. (a) Proton +1, neutron 0, electron –1 (all three needed) [1] (b) It has 9 electrons, the same number as protons [1]; the positive and negative charges cancel [1]. Examiner insight: “Equal numbers of protons and electrons” alone scores only the first mark; the second needs the idea that the charges cancel.

2. (a) 19 protons and 20 neutrons [1]; 18 electrons [1] (b) 35 protons and 44 neutrons [1]; 36 electrons [1] Examiner insight: The ion’s charge changes only the electrons; changing the proton number as well loses the mark for that ion.

3. 107 × 51.8 = 5542.6 and 109 × 48.2 = 5253.8 [1]; (5542.6 + 5253.8) ÷ 100 = 10796.4 ÷ 100 [1]; = 108.0 [1] Examiner insight: A correct final answer usually earns full marks, but writing “108” when 1 decimal place is asked for loses the accuracy mark.

4. (a) 2,8,8,1 [1] (b) They all have one electron in their outer shell [1]. (c) Potassium’s outer electron is in a shell further from the nucleus [1]; it is less strongly attracted to the nucleus [1]; so it is lost more easily [1]. Examiner insight: “Potassium has more electrons” scores nothing; the marks are for distance, weaker attraction and ease of losing the electron.

5. (a) Chlorine is more reactive than iodine (it is higher in Group 7) [1]; a more reactive halogen displaces a less reactive halogen from a solution of its salt [1]. (b) 2Cl⁻ [1]; I₂ [1] (c) Iodide ions are oxidised [1]; because they lose electrons [1]. Examiner insight: Naming “iodine” instead of “iodide ions” as the species oxidised loses the first mark in (c); the reactant is the ion.

6. (a) Each magnesium atom loses two electrons [1]; each oxygen atom gains two electrons [1]; forming Mg²⁺ and O²⁻ ions [1]. (b) Covalent [1]; the hydrogen and chlorine atoms share a pair of electrons [1]. (c) CaBr₂ [1] Examiner insight: In (a), saying electrons are “shared” contradicts the ionic answer and can cancel the marks for transfer.

7. A has a giant covalent structure [1]; many strong covalent bonds need a lot of energy to overcome, so the melting point is very high [1]. B is a giant ionic lattice [1]; when molten the ions are free to move and carry charge, but in the solid they are fixed [1]. C consists of small molecules [1]; only weak intermolecular forces are overcome on melting, and the molecules have no overall charge, so it does not conduct [1]. Examiner insight: Six-mark answers are marked on levels; for the top level each substance needs both a structure and a reason, and “C’s covalent bonds break” is a chemistry error that caps the level.

8. (a) Hydrogen ion, H⁺ [1] (b) H⁺ + OH⁻ → H₂O [1]; with state symbols (aq), (aq) and (l) [1] (c) A hydrogen ion (a proton) is transferred from the acid to a hydroxide ion [1]. (d) 10³ = 1000 times [1] (e) Ethanoic acid is a weak acid, only partially ionised [1]; so it produces a lower concentration of hydrogen ions than fully ionised hydrochloric acid [1]. Examiner insight: In (e), “ethanoic acid is weaker” without the idea of partial ionisation does not score; do not confuse weak with dilute.

9. (a) 66 ÷ 120 [1]; = 0.55 cm³/s [1] (b) Particles move faster, so they collide more frequently [1]; collisions are more energetic [1]; so a greater proportion of collisions have energy equal to or greater than the activation energy [1]. (c) The rate increases [1]; the powder has a larger surface area to volume ratio, so collisions are more frequent [1]. Examiner insight: The unit is needed for the second mark in (a); in (b), “more collisions” without “more frequent” or “per second” is often not credited.

10. (a) Energy is neither created nor destroyed; the total energy before and after the reaction is the same [1]. (b) Bonds broken: 945 + 3 × 436 = 2253 kJ [1]; bonds formed: 6 × 391 = 2346 kJ [1]; energy change = 2253 – 2346 = –93 kJ/mol [1] (c) Exothermic, because more energy is released forming bonds than is needed to break them (negative value) [1]. (d) It provides a different pathway for the reaction [1] with a lower activation energy [1]. Examiner insight: In (b) error carried forward is allowed if one bond total is wrong but the subtraction is right; 2NH₃ contains six N–H bonds, and using three is a common slip.

11. (a) 2Na + Cl₂ → 2NaCl, balanced [1]; state symbols (s), (g), (s) [1] (b) Sodium (2,8,1) loses one electron [1]; chlorine (2,8,7) gains one electron [1]; forming Na⁺ (2,8) and Cl⁻ (2,8,8), each with the structure of a noble gas [1]. The oppositely charged ions are held by strong electrostatic forces of attraction [1] acting in all directions in a giant ionic lattice [1]. A large amount of energy is needed to overcome the many strong forces, so the melting point is high [1]. Examiner insight: Writing “Cl₂ gains an electron” or “molecules of sodium chloride” are chemistry errors that limit the level; describe atoms forming ions and a lattice.

12. (a) 40 + 12 + 3 × 16 = 100 [1] (b) 5.00 – 2.80 = 2.20 g [1] (c) Carbon dioxide is a gas and escapes into the air [1]; the total mass of calcium oxide and carbon dioxide still equals 5.00 g [1]. (d) Endothermic [1] Examiner insight: In (c), “mass is lost” contradicts conservation of mass; say where the mass goes (the gas escapes) to earn both marks.

Where marks are usually lost

• Saying ions form by gaining or losing protons instead of electrons.
• Giving a relative atomic mass without the number of decimal places asked for.
• Explaining group reactivity with “more electrons” instead of distance and attraction.
• Calling the species oxidised “iodine” or “chlorine” when the ion is meant (Higher tier only).
• Saying that covalent bonds break when a simple molecular substance melts or boils.
• Leaving out units on a rate, or writing cm³ instead of cm³/s.
• Saying a catalyst “gives particles more energy” instead of lowering the activation energy.
• Losing the negative sign on an exothermic energy change (Higher tier only).
• Writing that mass or energy is “lost” instead of saying where it is transferred.

Next steps

• Recap with the Key ideas revision notes.
• Relearn anything you missed in the Key ideas study guide.
• See every topic on the AQA GCSE Chemistry hub and tick them off on the printable checklist.
• Try all free 10-minute diagnostics.
• Book a free trial class.

Official syllabus

AQA GCSE Chemistry (8462) specification, for teaching from September 2016, GCSE exams June 2018 onwards, version 1.1 (4 October 2019), published by AQA – section 4.11 Key ideas.