AQA A-Level Chemistry Paper 1, 2021: Question 3
12 marks · Medium difficulty · State/Explain/Describe
Answer questions on the reactions, properties, bonding, and structures of Period 3 elements and their oxides.
Practise this questionQuestion
Question text
03 This question is about elements in Period 3 and their compounds.
03.1 When a piece of sodium is added to 200 cm3 of water in a large beaker a vigorous
reaction occurs. The temperature of the water increases by 25 oC
Give an equation, including state symbols, for the reaction of sodium with water.
Suggest why it is dangerous to react a similar piece of sodium with 10 cm3 of water in
a boiling tube.
[2 marks]
Equation
Why it is dangerous
03.2 Give an equation for the reaction of phosphorus(V) oxide with water.
Suggest a pH for the solution formed.
[2 marks]
Equation
pH
03.3 Explain, in terms of crystal structure and bonding, why silicon(IV) oxide has a higher
melting point than phosphorus(V) oxide.
[4 marks]
03.4 An element in Period 3 forms an oxide that is insoluble in water.
This oxide reacts with sulfuric acid and with aqueous potassium hydroxide.
Give the formula for this oxide.
Give an equation for the reaction of this oxide with sulfuric acid.
[2 marks]
Formula
Equation
03.5 Give the formula of a hydroxide of an element in Period 3 used in medicine.
[1 mark]
03.6 Identify the element in Period 3, from sodium to chlorine, that has the
largest atomic radius.
[1 mark]
Mark scheme
Show the mark scheme
Question Answers Additional comments/Guidelines Mark
2 Na(s) + 2 H2O(l) → 2 NaOH(aq) + H2(g) Allow ionic equations 1
Allow multiples
03.1
Temperature will go up more or reactants can shoot out of the tube Allow the mixture could explode or glass could 1
shatter or hydrogen could ignite/is flammable
Ignore reaction is exothermic/vigorous
P4O10 + 6 H2O → 4 H3PO4 Allow ionic equations 1
03.2 Do not allow equations from P2O5
Allow ‒1 to + 1
M1 SiO2 is macromolecular / giant covalent / giant molecule Do not allow giant, giant atomic or giant ionic 1
M2 Strong covalent bonds (between atoms) or covalent 1
bonds need a lot of energy to be broken/overcome
03.3
M3 P4O10 is molecular or simple covalent molecule
M4 Weak van der Waals forces between molecules or van
der Waals forces between molecules break easily 1
– A-LEVEL CHEMISTRY – – JUNE 2021
Al2O3 1
03.4
Al2O3 + 3 H2SO4 → Al2(SO4)3 + 3 H2O 1
or Al O + 6 H+ → 2 Al 3+ + 3 H O
23 2
03.5 Mg(OH)2 1
03.6 Na / sodium 1
How to answer it
Period 3 Elements, Oxides, and Trends
This question assesses comprehensive core knowledge from Inorganic Chemistry: Period 3 (AQA specification 3.2.4):
- Reactivity and hazards of Period 3 elements with water (Sodium).
- Acid-base behaviour, formulas, and aqueous reactions of Period 3 oxides (P₄O₁₀ and Al₂O₃).
- Structure and bonding models explaining differences in physical properties (macromolecular SiO₂ vs molecular P₄O₁₀).
- Practical and medical applications of Period 3 compounds (Mg(OH)₂).
- Periodic trends across Period 3 (atomic radius).
Question 03.1
Reaction of Sodium with Water & Practical Hazard [2 Marks]
✅ Correct Answer
Equation (with state symbols):
2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)
(Multiples or correct ionic equations accepted: Na(s) + H₂O(l) → Na⁺(aq) + OH⁻(aq) + ½H₂(g) )
Why it is dangerous:
Any one of the following:
- Temperature will increase much more / solution may boil.
- Reactants / corrosive liquid can shoot out of the tube.
- Mixture could explode / boiling tube could shatter.
- Hydrogen gas produced could ignite / catch fire.
🧠 Exam Technique
- Check for state symbols: The question specifically demanded state symbols. Omitting them loses the first mark even if the equation is perfectly balanced.
- Proportional reasoning for safety: In 200 cm³, temperature rose by 25 °C. Using only 10 cm³ (a 20-fold reduction in solvent volume) means the heat released will boil the water rapidly, building pressure in a narrow tube.
❌ Common Errors
- Writing Na₂O as a product instead of NaOH .
- Vague statements such as "it is an exothermic reaction" or "the reaction is very vigorous". The question states it is already vigorous; you must explain why having less water in a narrow tube poses an acute hazard (e.g. projection of boiling alkaline solution, tube shattering).
Question 03.2
Reaction of Phosphorus(V) Oxide with Water & Solution pH [2 Marks]
✅ Correct Answer
Equation:
P₄O₁₀ + 6H₂O → 4H₃PO₄
Suggested pH:
Any value between -1 and +1 (e.g., 0, 1, or 0.5).
💡 Key Knowledge
- Phosphorus(V) oxide exists as the tetramer P₄O₁₀ . AQA requires the molecular formula P₄O₁₀ , NOT the empirical formula P₂O₅ .
- The reaction with water is violently exothermic and forms phosphoric(V) acid ( H₃PO₄ ), which is a moderately strong acid that dissociates sufficiently to yield a very low pH around 0 to 1.
❌ Common Errors
- Using P₂O₅ : The mark scheme explicitly states: "Do not allow equations from P₂O₅".
- Estimating the pH too mildly (e.g. pH 4–6). Phosphoric(V) acid solution formed from concentrated addition has a very low pH (-1 to 1).
Question 03.3
Structure & Bonding Comparison: SiO₂ vs P₄O₁₀ [4 Marks]
✅ Model 4-Mark Response
To achieve full marks, address structure, bonding type, and energy required to break bonds for both substances:
- M1 (SiO₂ structure): Silicon(IV) oxide is a macromolecular (or giant covalent / giant molecule) structure.
- M2 (SiO₂ forces): It has many strong covalent bonds (between atoms) that require a large amount of energy to break.
- M3 (P₄O₁₀ structure): Phosphorus(V) oxide is a molecular (or simple covalent molecular) substance.
- M4 (P₄O₁₀ forces): It has weak van der Waals forces between molecules that require little energy to overcome / break easily.
🧠 Exam Technique: "Structure & Bonding" Structure
Always format comparison questions using a structured 4-step checklist:
- State crystal structure type for Substance A.
- State the exact bond/force being broken and its strength for Substance A.
- State crystal structure type for Substance B.
- State the exact bond/force being broken and its strength for Substance B.
❌ Common Errors & Examiner Traps
- Vague descriptors: Describing SiO₂ as simply "giant" or "giant atomic" loses M1. You must state giant covalent or macromolecular.
- Confusing intermolecular vs intramolecular: Stating that covalent bonds break in P₄O₁₀ when it melts. Melting P₄O₁₀ only breaks weak van der Waals forces between molecules; covalent bonds within the molecule remain intact.
- Missing the phrase "between molecules" when identifying van der Waals forces.
Question 03.4
Amphoteric Oxide Identification & Reaction [2 Marks]
✅ Correct Answer
Formula of oxide: Al₂O₃
Equation with sulfuric acid:
Al₂O₃ + 3H₂SO₄ → Al₂(SO₄)₃ + 3H₂O
Ionic equation also accepted: Al₂O₃ + 6H⁺ → 2Al³⁺ + 3H₂O
💡 Key Knowledge: Amphoteric Nature of Al₂O₃
- An oxide that reacts with both acids ( H₂SO₄ ) and bases ( KOH ) is amphoteric.
- Across Period 3 oxides:
• Basic: Na₂O , MgO
• Amphoteric: Al₂O₃ (insoluble in water)
• Acidic: SiO₂ , P₄O₁₀ , SO₂ , SO₃
❌ Common Errors
- Incorrect formula for aluminium sulfate: writing AlSO₄ instead of Al₂(SO₄)₃ (Al forms 3+ ions, SO₄ forms 2- ions).
- Forgetting to balance the 3 sulfate groups and 3 water molecules.
Questions 03.5 & 03.6
Period 3 Applications & Periodic Trends [2 Marks Total]
03.5 Formula of Hydroxide in Medicine
Answer: Mg(OH)₂
Context: Magnesium hydroxide ("Milk of Magnesia") is sparingly soluble, making it safe to ingest as an antacid to neutralise excess stomach acid without causing tissue damage.
03.6 Largest Atomic Radius (Na to Cl)
Answer: Na (or sodium)
Context: Atomic radius decreases across Period 3 from Na to Cl because nuclear charge increases (more protons) while electron shielding remains approximately constant, pulling electrons closer.
❌ Common Errors
- 03.5: Writing name instead of formula (the question asks explicitly for formula), or forgetting brackets: MgOH₂ is incorrect.
- 03.6: Choosing Argon (excluded by the question: "from sodium to chlorine") or confusing atomic radius with ionic radius / first ionisation energy trends.
Topics
Inorganic Chemistry · Physical Chemistry · 3.2.4 Properties of Period 3 Elements and Their Oxides · 3.2.1 Periodicity · 3.1.3 Bonding · 3.2.2 Group 2, The Alkaline Earth Metals · 3.1.2 Amount of Substance
Question and mark scheme from the AQA A-Level Chemistry examination, Paper 1, 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.