AQA GCSE Chemistry Chemistry Paper 1 (Higher), November 2020: Question 8
16 marks · High Demand difficulty · Extended Answer
Explain the trends and properties of halogens and use experimental mass data to determine the mole ratio and balanced equation for the reaction of iron with chlorine.
Practise this questionQuestion
Question text
08 This question is about the halogens.
Table 5 shows the melting points and boiling points of some halogens.
Table 5
Element Melting point in °C Boiling point in °C
Fluorine –220 –188
Chlorine –101 –35
Bromine –7 59
08.1 What is the state of bromine at 0 °C and at 100 °C?
[1 mark]
Tick ( ) one box.
State at 0 °C State at 100 °C
Gas Gas
Gas Liquid
Liquid Gas
Liquid Liquid
Solid Gas
Solid 23 Liquid
08.2 Explain the trend in boiling points of the halogens shown in Table 5.
[4 marks]
08.3 Why is it not correct to say that the boiling point of a single bromine molecule
is 59 °C?
[1 mark]
Iron reacts with each of the halogens in their gaseous form.
Figure 5 shows the apparatus used.
Figure 5
08.4 Give one reason why this experiment should be done in a fume cupboard.
[1 mark]
08.5 Explain why the reactivity of the halogens decreases going down the group.
[3 marks]
08.6 A teacher investigated the reaction of iron with chlorine using the apparatus
in Figure 5.
The word equation for the reaction is:
iron + chlorine → iron chloride
The teacher weighed:
*24* • the glass tube
• the glass tube and iron before the reaction
• the glass tube and iron chloride after the reaction.
Table 6 shows the teacher’s results.
Table 6
Mass in g
Glass tube 51.56
Glass tube and iron 56.04
Glass tube and iron chloride 64.56
Calculate the simplest whole number ratio of:
moles of iron atoms : moles of chlorine atoms
Determine the balanced equation for the reaction.
Relative atomic masses (Ar): Cl = 35.5 Fe = 56
[6 marks]
Moles of iron atoms : moles of chlorine atoms = :
Equation for the reaction
Mark scheme
Show the mark scheme
Question 8
AO /
Question Answers Extra information Mark
Spec. Ref.
08.1 liquid gas 1 AO2
4.2.2.1
08.2 (boiling point) increases (down 1 AO1
the table / group) 4.1.2.6
4.2.2.1
(because) the relative formula / 1 4.2.2.4
molecular mass increases
or
(because) the size of the
molecule increases
(so) the intermolecular forces allow (so) the forces between 1
increase (in strength) molecules increase (in strength)
(so) more energy is needed to allow (so) more energy is 1
overcome the intermolecular needed to separate the
forces molecules
do not accept a reference to
breaking bonds unless
specifically between molecules
08.3 boiling point is a bulk property allow boiling point is related to 1 AO1
intermolecular forces (so more 4.2.2.1
than one molecule is involved)
08.4 the gas / halogen is toxic allow the gas / halogen is 1 AO3
poisonous / harmful 4.1.2.6
allow to prevent inhalation of the
gas / halogen
ignore deadly / lethal
Question 8 continued
AO /
Spec. Ref.
24 08.5 allow energy level for shell AO1
throughout 4.1.2.6
(going down the group) allow the atoms become larger 1
the outer electrons / shell allow the number of shells
become further from the nucleus increases
ignore the number of outer
shells increases
(so) the nucleus has less allow (so) the nucleus has less 1
attraction for the outer electrons attraction for the incoming
/ shell electron
allow (so) increased shielding
between the nucleus and the
outer electrons / shell
allow (so) increased shielding
between the nucleus and the
incoming electron
(so) an electron is gained less 1
easily
Question 8 continued
08.6 4.48 (g iron) and 8.52 (g 1 AO2
chlorine) 4.1.1.1
4.1.2.6
4.48 allow correct calculation using 1 4.3.2.3
(moles Fe = =) 0.08
incorrectly calculated mass of
iron
8.52 allow correct calculation using 1
(moles Cl = =) 0.24
35.5 incorrectly calculated mass of
chlorine
8.52
allow (moles Cl2 = =) 0.12
(Fe : Cl = 0.08 : 0.24 =) 1 : 3 allow correct calculation using 1
incorrectly calculated moles of
iron and / or chlorine
2 Fe + 3 Cl2 → 2 FeCl3 allow multiples / fractions 2
allow a correctly balanced
equation including Fe and Cl2
from an incorrect ratio of Fe : Cl
allow 1 mark for Fe and Cl2
(reactants) and FeCl3 (product)
or
allow 1 mark for Fe and Cl2
(reactants) and a formula for
iron chloride correctly derived
from an incorrect ratio of Fe : Cl
(product)
Total 16
How to answer it
Group 7 Halogens: Trends, Bonding & Mole Stoichiometry
Core GCSE Chemistry specifications: C1 (Atomic Structure & Periodic Table), C2 (Structure & Bonding), and C4 (Quantitative Chemistry).
- Physical States: Predicting state from melting and boiling point thresholds.
- Intermolecular Forces: Explaining boiling point trends in simple covalent molecular substances.
- Bulk vs. Particulate Properties: Understanding why state changes apply only to collections of molecules.
- Laboratory Safety: Identifying hazard risks associated with Group 7 halogens.
- Periodic Trends in Reactivity: Linking atomic radius, electron shielding, and nuclear attraction to halogen reactivity.
- Quantitative Chemistry: Deriving empirical formulas and writing full balanced chemical equations from experimental gravimetric data.
Deducing Physical States of Bromine
Identifying state of matter at 0 °C and 100 °C
✅ Correct Answer
Tick box 3: Liquid at 0 °C and Gas at 100 °C.
🧠 Exam Technique
- Below melting point (-7 °C): Solid
- Between melting and boiling point (-7 °C to 59 °C): Liquid
- Above boiling point (59 °C): Gas
- 0 °C lies between -7 °C and 59 °C → Liquid
- 100 °C lies well above 59 °C → Gas
Explaining the Trend in Halogen Boiling Points
Simple molecular structures and intermolecular forces
✅ 4-Mark Model Response
- Boiling point increases down the group (from fluorine to bromine).
- This is because the molecules get larger / the relative molecular mass increases.
- Therefore, the intermolecular forces increase in strength.
- More energy is needed to overcome these intermolecular forces (separate the molecules).
❌ Common Errors (Zero Marks Alert)
- Fatal Mistake: Stating that covalent bonds break when halogens boil. Covalent bonds within molecules are NOT broken during a change of state!
- Omitting the trend itself (stating that boiling point increases scores mark 1).
- Vaguely stating "forces" without specifying intermolecular forces or "forces between molecules".
Bulk Properties vs. Single Molecules
Understanding the nature of boiling points
✅ Correct Answer
Boiling point is a bulk property.
Also acceptable: Boiling point involves overcoming intermolecular forces, which requires more than one molecule to exist.
💡 Key Knowledge
Individual atoms and molecules do not possess states of matter (solid, liquid, gas) or melting/boiling points. These properties arise exclusively from the interactions between large collections (aggregates) of particles.
Laboratory Safety: Halogen Reaction
Justifying the use of a fume cupboard
✅ Correct Answer
The halogen gas is toxic / poisonous (or to prevent inhalation of harmful gas).
❌ Examiner Pitfall
Do NOT write that the gas is "deadly" or "lethal" or "flammable". Use precise chemistry vocabulary: toxic or poisonous.
Explaining the Reactivity Trend of Group 7
Atomic radius, shielding, and electron gain
✅ 3-Mark Model Response
- Going down Group 7, the atoms have more electron shells, so the outer shell is further from the nucleus (larger atomic radius).
- There is greater shielding, so the nucleus has less attraction for an incoming electron.
- Therefore, an electron is gained less easily.
💡 Key Knowledge
- Halogens react by gaining 1 electron into their outer shell to form a 1- halide ion.
- Electrostatic attraction depends on distance and shielding: smaller halogen atoms (e.g., fluorine) pull incoming electrons much more strongly than larger ones (e.g., iodine).
Reacting Masses and Balanced Equation
Determining empirical formula and reaction stoichiometry
📐 Step-by-Step Calculation
Step 1: Calculate the mass of iron and mass of chlorine reacted
- Mass of Fe = (tube + Fe) - (tube) = 56.04 g - 51.56 g = 4.48 g
- Mass of iron chloride = 64.56 g - 51.56 g = 13.00 g
- Mass of Cl = (tube + product) - (tube + Fe) = 64.56 g - 56.04 g = 8.52 g [Mark 1]
Step 2: Calculate moles of iron atoms (Ar = 56)
Moles of Fe = 4.48 / 56 = 0.08 mol [Mark 2]
Step 3: Calculate moles of chlorine atoms (Ar = 35.5)
Moles of Cl = 8.52 / 35.5 = 0.24 mol [Mark 3]
Alternative: Moles of Cl₂ molecules = 8.52 / 71 = 0.12 mol.
Step 4: Find simplest whole number ratio (Fe : Cl)
Fe : Cl = (0.08 / 0.08) : (0.24 / 0.08) = 1 : 3 [Mark 4]
This gives the empirical formula: FeCl₃ (Iron(III) chloride).
Step 5: Write and balance the chemical equation
- Chlorine gas exists as diatomic molecules ( Cl₂ ).
- Unbalanced equation: Fe + Cl₂ → FeCl₃ [Mark 5]
- Balanced equation: 2 Fe + 3 Cl₂ → 2 FeCl₃ [Mark 6]
✅ Final Answers Required on Lines
Moles of iron atoms : moles of chlorine atoms = 1 : 3
Equation for the reaction: 2 Fe + 3 Cl₂ → 2 FeCl₃
❌ Common Calculation Traps
- Diatomic Confusion: The question asks for the ratio of moles of chlorine atoms, so divide mass by 35.5 (not 71). If using 71, you find moles of Cl₂ (0.12), which gives Fe : Cl₂ = 2 : 3, meaning Fe : Cl atoms = 1 : 3.
- Incorrect Product: Writing FeCl or FeCl₂ instead of using your calculated 1 : 3 ratio to give FeCl₃.
- Forgetting Diatomic Reactant: Writing Fe + 3 Cl → FeCl₃ loses the final equation mark because chlorine gas reactant is Cl₂.
Topics
Chemistry · C1: Atomic Structure and the Periodic Table · C2: Bonding, Structure and the Properties of Matter · C3: Quantitative Chemistry
Question and mark scheme from the AQA GCSE Chemistry examination, Chemistry Paper 1 (Higher), November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.