AQA GCSE Chemistry Chemistry Paper 1 (Higher), 2022: Question 7
15 marks · High Demand difficulty · Short Answer
Analyze questions about silicon and its compounds, including reactivity, extraction costs, stoichiometry calculations, covalent bonding diagrams, and gas volume calculations.
Practise this questionQuestion
Question text
07 This question is about silicon and compounds of silicon.
07.1 The reactivity series sometimes includes non-metals such as carbon, hydrogen
and silicon.
Silicon can be extracted by reducing silicon dioxide with different substances.
The equation for one possible reaction is:
2C(s) + SiO2(s) → Si(s) + 2CO(g)
Explain what this reaction shows about the position of silicon in the reactivity series.
[2 marks]
07.2 Aluminium also reduces silicon dioxide.
Carbon is used rather than aluminium to reduce silicon dioxide because carbon is
cheaper than aluminium.
Carbon can be obtained by heating coal.
Aluminium is obtained from aluminium oxide.
Explain why aluminium is more expensive than carbon.
[2 marks]
Magnesium also reduces silicon dioxide.
The equation for the reaction is:
2Mg(s) + SiO2(s) → Si(s) + 2MgO(s)
07.3 Give one reason why the products are difficult to separate if magnesium is used to
*21* reduce silicon dioxide.
[1 mark]
07.4 Calculate the minimum mass in grams of magnesium needed to completely reduce
1.2 kg of silicon dioxide.
Relative atomic masses (Ar): O = 16 Mg = 24 Si = 28
[5 marks]
Minimum mass of magnesium = g
Si2H6 is a covalent compound of silicon and hydrogen.
07.5 Complete Figure 9 to show the outer shell electrons in a molecule of Si2H6
[1 mark]
Figure 9
07.6 Si2H6 reacts with oxygen.
The equation for the reaction is:
2Si2H6(g) + 7O2(g) → 4SiO2(s) + 6H2O(g)
30 cm3 of Si H is reacted with 150 cm3 (an excess) of oxygen.
Calculate the total volume of gases present after the reaction.
All volumes of gases are measured at the same temperature and pressure.
[4 marks]
Volume of gases = cm3
Mark scheme
Show the mark scheme
Question 7
AO /
Question Answers Extra information Mark
Spec. Ref.
ignore references to hydrogen
07.1 AO3
silicon is less reactive than allow converse 1 4.4.1.3
carbon
allow silicon is below carbon (in
the reactivity series)
(because) carbon displaces 1
silicon (from silicon dioxide)
ignore (because) carbon
reduces silicon dioxide
AO /
Spec. Ref.
07.2 more energy is needed (to ignore references to electricity 1 AO3
obtain aluminium) 4.4.1.3
4.4.3.3
(because) aluminium is obtained 1
(from aluminium oxide) by
electrolysis
AO /
Spec. Ref.
07.3 both products are solid 1 AO3
4.4.1.3
AO /
Spec. Ref.
07.4 (Mr of SiO2 = 28 + (2 × 16)) = 60 1 AO2
4.3.1.2
4.3.2.1
(conversion 1.2 kg =) 1200 (g) 1 4.3.2.2
(number of moles of SiO2 = allow correct use of an 1
1200 incorrectly converted or
) = 20 unconverted mass of SiO2
allow correct use of an
incorrectly calculated Mr of SiO2
(number of moles of Mg allow correct use of an 1
= 20 × 2) = 40 incorrectly calculated number of
moles of SiO2
allow correct use of an 1
(mass of Mg = 40 × 24)
= 960 (g) incorrectly calculated number of
moles of Mg
alternative approach:
(Mr of SiO2 = 28 + (2 × 16)) = 60
(1)
48 g Mg reacts with 60 g SiO2 allow correct use of an
(1) incorrectly calculated Mr of SiO2
(conversion 1.2 kg =) 1200 (g)
(1)
1200
48 × (g Mg reacts allow correct use of an
60 incorrectly calculated mass of
with 1200 g SiO2) (1)
Mg and / or incorrectly
converted or unconverted mass
of SiO2
= 960 (g) (1)
AO /
Spec. Ref.
07.5 allow any combination of 1 AO2
(–) 4.2.1.423
x, , o, e for electrons
AO /
Spec. Ref.
(volume of oxygen for 30 cm3 AO2
07.6
Si2H6 = 3.5 × 30) 4.3.2.4
= 105 (cm3) 1 4.3.5
(volume of excess oxygen allow correct use of an
= 150 – 105) incorrectly calculated volume of
= 45 (cm3) oxygen for 30 cm3 Si H 1
(volume of water (vapour)
= 3 × 30)
= 90 (cm3) 1
(volume of gases = 45 + 90) allow correct use of incorrectly
= 135 (cm3) calculated volumes of excess 1
oxygen and / or water vapour
allowed alternative approach:
0.03
(moles Si2H6 = =)
0.00125 (1)
(moles water vapour formed = allow correct use of an
3 × 0.00125 =) 0.00375 incorrectly calculated number of
and moles of Si2H6
(moles oxygen used =
3.5 × 0.00125 =) 0.004375 (1)
(moles excess oxygen = allow correct use of an
0.15 incorrectly calculated number of
- 0.004375 =)
24 moles of oxygen used
0.001875 (1)
(volume of gases = allow correct use of an
24 × (0.00375 + 0.001875) = incorrectly calculated number of
24 3
0.135 dm =) moles of excess oxygen and / or
135 (cm3) (1) moles of water vapour formed
Total Question 7 15
How to answer it
Silicon, Reactivity, and Quantitative Calculations
What this question tests
This question assesses your understanding of the reactivity series, displacement reactions, and the economics of metal extraction. It also tests your quantitative skills through a 5-mark reacting mass calculation, drawing covalent dot-and-cross diagrams, and solving a challenging 4-mark gas volume stoichiometry problem involving excess reactants and state symbols.
Reactivity & Displacement
Analyzing a reduction reaction to determine reactivity position
Correct Answer
- Silicon is less reactive than carbon [1 mark]
- Because carbon displaces silicon from silicon dioxide [1 mark]
Key Knowledge
A more reactive element will always displace a less reactive element from its compound. In the reaction:
2C(s) + SiO₂(s) → Si(s) + 2CO(g)
Carbon takes the oxygen away from silicon, meaning carbon is the stronger reducing agent and sits higher in the reactivity series.
Exam Technique
To get both marks, you must make a comparative statement about reactivity (e.g., "silicon is below carbon") AND use the chemical equation to explain why (e.g., "carbon removes oxygen from silicon dioxide").
Common Errors
❌ Writing "carbon is more reactive than silicon dioxide" (always compare element to element, not element to compound).
❌ Simply stating "carbon reduces silicon" without mentioning displacement.
Extraction Economics
Why is aluminium more expensive to extract than carbon?
Correct Answer
- More energy is needed to obtain aluminium [1 mark]
- Because aluminium is obtained by electrolysis [1 mark]
Key Knowledge
Aluminium is highly reactive and must be extracted using electrolysis of molten aluminium oxide. This requires massive amounts of electrical energy to melt the ore and run the current, which is extremely expensive. Carbon, however, is simply mined as coal and requires much less energy to process.
Exam Technique
Always link the method of extraction (electrolysis) directly to the energy cost. Mentioning "electricity" alone is often ignored; focus on the high energy demand of the process.
Separation of Products
Why are the products of the magnesium reduction difficult to separate?
Correct Answer
Both products are solids [1 mark]
Key Knowledge
Look at the state symbols in the equation:
2Mg(s) + SiO₂(s) → Si(s) + 2MgO(s)
Both Si(s) and MgO(s) are solids. Unlike the carbon reduction where carbon monoxide escapes as a gas, these two solid products remain mixed together.
Reacting Mass Calculation
Calculate the minimum mass of magnesium needed to reduce 1.2 kg of SiO₂
Step-by-Step Calculation
- Calculate the relative formula mass (Mᵣ) of SiO₂:
Mᵣ = 28 + (2 × 16) = 60 [1 mark] - Convert the mass of SiO₂ from kg to g:
1.2 kg = 1.2 × 1000 = 1200 g [1 mark] - Find the moles of SiO₂:
Moles = mass / Mᵣ = 1200 / 60 = 20 moles [1 mark] - Use the balanced equation ratio to find moles of Mg:
The equation is: 2Mg + SiO₂ → Si + 2MgO
The molar ratio of Mg : SiO₂ is 2 : 1 .
Moles of Mg = 20 × 2 = 40 moles [1 mark] - Calculate the mass of Mg needed:
Mass = moles × Aᵣ = 40 × 24 = 960 g [1 mark]
Calculation Traps
⚠️ Unit conversion: Forgetting to convert 1.2 kg into 1200 g . Chemistry calculations must use grams!
⚠️ Molar ratio: Forgetting to multiply the moles of SiO₂ by 2 to find the moles of Mg .
Tutor Tip
Even if you make a mistake early on, show all your working! The mark scheme allows "error carried forward" (ecf), meaning you can still score 4 out of 5 marks if your method is correct but you used a wrong number.
Covalent Bonding Diagram
Completing the dot-and-cross diagram for Si₂H₆
How to Draw the Correct Answer
To complete the diagram, you must show shared pairs of electrons in each overlap:
- Place one pair of electrons (e.g., x • ) in the central overlap between the two Si atoms.
- Place one pair of electrons (e.g., x • ) in each of the six overlaps between Si and H atoms.
- Do not add any extra lone pairs (Silicon is in Group 4 and uses all 4 of its outer electrons for bonding).
Key Knowledge
Silicon is in Group 4 (like carbon) and forms 4 covalent bonds. Hydrogen is in Group 1 and forms 1 covalent bond. In Si₂H₆ , there are 7 single covalent bonds in total, meaning 14 shared electrons are displayed.
Gas Volume Stoichiometry
Calculate the total volume of gases present after the reaction
Step-by-Step Calculation
Equation: 2Si₂H₆(g) + 7O₂(g) → 4SiO₂(s) + 6H₂O(g)
Reactants: 30 cm³ of Si₂H₆ and 150 cm³ of O₂ (excess).
- Find the volume of O₂ that actually reacts:
The ratio of Si₂H₆ : O₂ is 2 : 7 (or 1 : 3.5 ).
Volume of O₂ reacted = 30 cm³ × 3.5 = 105 cm³ [1 mark] - Calculate the volume of excess O₂ left over:
Excess O₂ remaining = 150 cm³ - 105 cm³ = 45 cm³ [1 mark] - Calculate the volume of gaseous product (H₂O) formed:
The ratio of Si₂H₆ : H₂O is 2 : 6 (or 1 : 3 ).
Volume of H₂O(g) produced = 30 cm³ × 3 = 90 cm³ [1 mark]
Note: SiO₂ is a solid (s) , so it does not contribute to gas volume! - Calculate total gas volume at the end:
Total Gas Volume = Excess O₂ + H₂O(g) produced = 45 cm³ + 90 cm³ = 135 cm³ [1 mark]
The State Symbol Trap! ⚠️
The most common way students lose marks here is by trying to calculate a gas volume for SiO₂ . Look closely at the state symbols: SiO₂(s) is a solid. Only gases ( g ) have volume in this calculation!
Avogadro's Law Tip
You do not need to convert volumes to moles using 24 dm³ ! Avogadro's Law states that equal volumes of gases contain equal moles. You can use the ratio of the coefficients in the balanced equation directly as a ratio of volumes.
Topics
Chemistry · C1: Atomic Structure and the Periodic Table · C2: Bonding, Structure and the Properties of Matter · C3: Quantitative Chemistry · C4: Chemical Changes
Question and mark scheme from the AQA GCSE Chemistry examination, Chemistry Paper 1 (Higher), 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.