AQA GCSE Chemistry Chemistry Paper 1 (Higher), 2024: Question 9
12 marks · Standard Demand difficulty · Short Answer
This question covers properties of weak acids, pH changes during dilution, titration procedures, a titration calculation involving moles and concentration, and an explanation of the reactivity of group 2 metals.
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Question text
09 This question is about acids and their reactions.
Acids can be either weak or strong.
09.1 What is meant by ‘a weak acid’?
[2 marks]
09.2 Explain what happens to the pH of an acid as the acid is diluted with water.
[2 marks]
09.3 A student does a titration to find the volume of acid needed to neutralise an alkali.
The student fills a burette with the acid.
Give three more steps the student must do before adding the acid to the alkali from
the burette.
You should name any equipment used.
[3 marks]
09.4 The student titrated a solution containing 0.0045 moles of sodium hydroxide with
*27* 0.15 mol/dm3 hydrochloric acid.
The equation for the reaction is:
NaOH + HCl → NaCl + H2O
Calculate the volume of hydrochloric acid in cm3 needed in the titration.
[2 marks]
29 Volume of acid = cm3
09.5 A calcium atom is larger than a magnesium atom.
Explain why calcium reacts more vigorously than magnesium with hydrochloric acid of
the same concentration.
[3 marks]
Mark scheme
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Question 9
AO /
Question Answers Extra information Mark
Spec. Ref.
09.1 (an acid which) is partially allow (an acid which) is partially 1 AO1
ionised dissociated 4.4.2.4
4.4.2.6
in aqueous solution allow (when dissolved) in water 1
MP2 is dependent on the award
of MP1
AO /
Spec. Ref.
09.2 pH increases 1 AO2
4.4.2.6
(because) the concentration of 1
hydrogen ions decreases
AO /
Spec. Ref.
09.3 use a (volumetric) pipette to add 1 AO1
the alkali 4.4.2.5
RPA2
ignore beaker
any two from:
• into a conical flask
allow named indicator
• add an indicator (to the do not accept add universal
alkali) indicator
• take the initial burette
reading
• use a white tile (under a
conical flask) – – –
AO /
Spec. Ref.
09.4 0.0045 AO2
(volume of HCl = ) 4.3.4
0.15
= 0.030 (dm3) 1 4.4.2.5
RPA2
(conversion 0.030 dm3 =)
30 (cm3) allow correct conversion of an 1
incorrectly determined volume in
dm3
alternative approach
0.15
(concentration = =)
1000
0.00015 (mol/cm3) (1)
0.0045
(volume = =)
0.00015
30 (cm3) (1) allow correct use of an
incorrectly determined
concentration in mol/cm3
AO /
Spec. Ref.
09.5 allow converse arguments in AO2
terms of magnesium 4.1.2.3
4.2.1.2
allow energy level for shell 4.4.1.2
(calcium’s) outer shell / allow calcium has more shells 1
electrons are further from the
nucleus ignore calcium atoms are larger
(so) the outer electrons are less allow (so) the outer electrons 1
strongly attracted to the nucleus are more shielded from the
nucleus
(so) positive ions are formed allow (so) electrons are more 1
more easily easily lost
Total Question 9 12
How to answer it
Acids, Titrations, and Reactivity
What this question tests
- Definitions: Understanding the difference between strong and weak acids.
- pH Scale: Explaining how dilution affects the concentration of H⁺ ions.
- Required Practical 2: Recalling the precise steps for a titration.
- Quantitative Chemistry: Calculating volumes using moles and concentration.
- Periodic Table Trends: Explaining reactivity in Group 2 based on atomic structure.
Defining a Weak Acid
💡 Key Knowledge
Acids release hydrogen ions (H⁺) in solution. The "strength" of an acid refers to how many of its molecules split up into ions.
✅ Correct Answer
- An acid that is partially ionised (or dissociated).
- In aqueous solution (or when dissolved in water).
❌ Common Errors
Do not confuse "weak" with "dilute". A weak acid is about the percentage of ionisation, not how much water is mixed with it. You must mention "aqueous solution" to get the second mark.
pH and Dilution
🧠 Exam Technique
When asked to "explain what happens," state the change first, then give the reason using scientific terminology.
✅ Correct Answer
- The pH increases.
- Because the concentration of hydrogen ions (H⁺) decreases.
Titration Procedure
Steps before adding acid from the burette
💡 Key Knowledge
Titrations require high precision. You must use a volumetric pipette for the fixed volume (alkali) and a burette for the variable volume (acid).
✅ Correct Answer (Pick Three)
- Use a pipette to add a fixed volume of alkali to a conical flask.
- Add an indicator (e.g., phenolphthalein or methyl orange) to the alkali.
- Place the flask on a white tile to see the colour change clearly.
- Take the initial reading on the burette.
❌ Common Errors
Never suggest using "Universal Indicator" for a titration. It changes colour gradually. You must use a single-colour change indicator. Also, "beaker" is usually ignored; always use "conical flask" for titrations to allow for swirling.
Titration Calculation
📐 Step-by-Step Calculation
The Goal: Find the volume of 0.15 mol/dm³ HCl needed to react with 0.0045 moles of NaOH.
1 Check the Ratio: From the equation NaOH + HCl → NaCl + H₂O , the ratio is 1:1. So, 0.0045 moles of NaOH reacts with 0.0045 moles of HCl.
2 Calculate Volume in dm³:
Volume (dm³) = Moles / Concentration
Volume = 0.0045 / 0.15 = 0.03 dm³
3 Convert to cm³:
0.03 dm³ × 1000 = 30 cm³
❌ Calculation Traps
Students often forget to convert dm³ to cm³. Always check the units requested in the answer line! 1 dm³ = 1000 cm³.
Reactivity Trends (Group 2)
Why is Calcium more reactive than Magnesium?
🧠 Exam Technique
For "reactivity" questions in Groups 1 or 2, always follow this 3-step logic: Distance → Attraction → Ease of loss.
✅ Correct Answer
- Calcium's outer shell/electrons are further from the nucleus (it has more shells).
- There is less attraction between the nucleus and the outer electrons.
- Outer electrons are lost more easily to form a positive ion.
💡 Examiner Commentary
Simply saying "Calcium is a larger atom" is not enough for a mark. You must explain why that size matters—specifically mentioning the distance from the nucleus or shielding by inner shells.
Topics
Chemistry · Required Practicals
Question and mark scheme from the AQA GCSE Chemistry examination, Chemistry Paper 1 (Higher), 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.