AQA GCSE Chemistry Chemistry Paper 2 (Higher), November 2021: Question 2
8 marks · Low Demand difficulty · Short Answer
Plot and analyze alkane boiling points, deduce the formula of nonane, and explain its fractional distillation behavior.
Practise this questionQuestion
Question text
02 This question is about alkanes.
Table 1 shows information about some alkanes.
Table 1
Number of carbon atoms Boiling point of alkane
in alkane molecule in °C
5 36
6 69
7 X
8 126
9 151
02.1 Plot the data from Table 1 on Figure 2.
[2 marks]
Figure 2
02.2 Predict the boiling point X of the alkane with seven carbon atoms in a molecule.
Use Table 1 and Figure 2.
*06* [1 mark]
X = °C
02.3 Figure 2 is not suitable to show the boiling point of the alkane with three carbon
atoms in a molecule.
Suggest one reason why.
[1 mark]
02.4 What is the state at 20 °C of the alkane with four carbon atoms in a molecule?
Use Table 1.
[1 mark]
Table 1 is repeated below.
Table 1
Number of carbon atoms Boiling point of alkane
in alkane molecule in °C
5 36
6 69
7 X
8 126
9 151
*07* The alkane with nine carbon atoms in a molecule is called nonane.
02.5 Complete the formula of nonane.
[1 mark]
C9H___
02.6 Nonane will condense lower in a fractionating column during fractional distillation
than the other alkanes in Table 1.
Explain why.
You should refer to the temperature gradient in the fractionating column.
[2 marks]
Mark scheme
Show the mark scheme
Question 2
AO /
Question Answers Extra information Mark
Spec. Ref.
02.1 all five points plotted correctly allow a tolerance of ± ½ a small 2 AO2
square 4.7.1.3
allow 1 mark for three or four
points plotted correctly
AO /
Spec. Ref.
02.2 98 (°C) allow a value in the range 92 to 1 AO3
104 (°C) 4.7.1.3
AO /
Spec. Ref.
02.3 the boiling point is lower allow the graph cannot show 1 AO3
than 0 (°C) negative temperatures 4.7.1.3
AO /
Spec. Ref.
02.4 gas allow (g) 1 AO2
4.2.2.1
AO /
Spec. Ref.
02.5 C9H20 1 AO2
4.7.1.1
Question 2 continued
AO /
Spec. Ref.
02.6 allow converse for the other AO2
alkanes 4.7.1.2
(nonane) has a higher boiling 1
point
(so nonane) condenses where allow (so nonane) collects 1
the column has a higher where the column has a higher
temperature temperature
Total 8
How to answer it
Alkanes: Boiling Point Trends, Formulas & Fractional Distillation
This question assesses fundamental organic chemistry skills and physical properties of homologous series:
- Graph Plotting & Interpolation: Reading grid scales precisely, plotting coordinates accurately within a tolerance, and using trends to estimate missing data.
- States of Matter: Deducing the physical state of a substance at room temperature (20 °C) by comparing it to its boiling point.
- Alkane General Formula: Applying CₙH₂ₙ₊₂ to determine molecular formulas.
- Fractional Distillation Principles: Explaining condensation points using molecular size, boiling points, and the temperature gradient in a fractionating column.
Plotting Data on Figure 2
Data presentation and graph skills
✅ Mark Scheme Requirements
- All 5 points plotted correctly: [2 marks]
- 3 or 4 points plotted correctly: [1 mark]
- Tolerance: Within ± ½ a small square.
- Points to plot: (4, 0), (5, 36), (6, 69), (8, 126), and (9, 151).
🧠 Exam Technique: Checking the Grid Scale
- y-axis scale: 10 small squares = 20 °C, meaning 1 small square = 2 °C.
- Plotting 36 °C: Exactly 18 small squares up from 0.
- Plotting 69 °C: 34.5 small squares up (halfway between 68 and 70).
- Plotting 126 °C: 3 small squares above 120.
- Plotting 151 °C: Half a small square above 150.
❌ Common Errors
- Plotting carbon atom 7: Carbon 7 is unknown ( X ). Do not guess or plot a point at 7 in this part!
- Blunt pencils: Markers penalise large blobs. Always use a sharp HB pencil to draw neat, precise crosses (×).
Predicting Boiling Point X (7 Carbons)
Data interpolation from a trend
✅ Correct Answer
X = 98 °C
🧠 How to Get the Mark
Draw a smooth best-fit curve through your plotted points. Find 7 on the horizontal axis (Number of carbon atoms), move vertically up to meet your line, and read directly across to the vertical axis.
Suitability of the Graph for Propane (3 Carbons)
Evaluating graph axes and limits
✅ Correct Answers (Any 1)
- The boiling point is lower than 0 °C.
- The graph/axis does not show negative temperatures.
- The y-axis starts at 0 °C (cannot show values below zero).
💡 Key Knowledge
As carbon chain length decreases, boiling point decreases. Propane has 3 carbons and has a boiling point of -42 °C. Because the vertical axis stops at 0 °C, negative temperatures cannot be plotted.
❌ Common Errors
Vague answers such as "the scale isn't big enough" or "it doesn't fit" score 0. You must specify that the temperature is below 0 °C or negative.
Physical State of Butane at 20 °C
States of matter and boiling points
✅ Correct Answer
gas (or (g) )
🧠 Decision Rule for States of Matter
- From Table 1: Boiling point of 4-carbon alkane = 0 °C.
- Compare room temperature (20 °C) to boiling point (0 °C):
- Since 20 °C > 0 °C, the substance has already boiled and turned into a gas.
Molecular Formula of Nonane
Applying the general formula of alkanes
✅ Correct Answer
C₉H₂₀ (Number on line: 20)
📐 Step-by-Step Calculation
- Recall the general formula for alkanes: CₙH₂ₙ₊₂
- Nonane has 9 carbon atoms, so n = 9
- Calculate hydrogen atoms:
Number of H = (2 × 9) + 2 = 18 + 2 = 20
❌ Common Errors
Confusing the alkane formula ( CₙH₂ₙ₊₂ ) with the alkene formula ( CₙH₂ₙ ), leading to the incorrect answer C₉H₁₈ .
Fractional Distillation & Temperature Gradient
Linking intermolecular forces to industrial separation
✅ Mark Scheme Breakdown
- Mark 1: Nonane has a higher boiling point (than the other alkanes).
- Mark 2: So nonane condenses where the column has a higher temperature (near the bottom / lower down).
💡 How the Fractionating Column Works
- Temperature Gradient: It is hot at the bottom and gets progressively cooler towards the top.
- Vapours rise up the column until the temperature drops below their specific boiling point.
- Nonane has 9 carbons (longest chain in Table 1), so it has the strongest intermolecular forces and highest boiling point (151 °C). It condenses first, at high temperatures near the bottom.
❌ Common Errors
- Forgetting the gradient: Simply stating "nonane is heavier" or "it is dense" misses both chemistry marks.
- Confusing boiling and condensing: Substances condense when they cool down to their boiling point. Nonane condenses at a high temperature, so it condenses low down where it is still hot.
Topics
Chemistry · C7: Organic Chemistry · C2: Bonding, Structure and the Properties of Matter
Question and mark scheme from the AQA GCSE Chemistry examination, Chemistry Paper 2 (Higher), November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.