OCR A-Level Chemistry AS Depth in chemistry (02), June 2022: Question 2
9 marks · Medium difficulty · Structured Questions
Write equations, explain redox behaviour, compare reactivity down Group 2 with water, predict pH of barium oxide solution, and describe the reaction of barium hydroxide with dilute sulfuric acid.
Practise this questionQuestion
Question text
2 This question is about some Group 2 elements and their compounds.
(a) Strontium and calcium both react with water.
(i) Write an equation for the reaction of strontium with water.
… [1]
(ii) Using oxidation numbers, explain why the reaction of strontium with water is a redox
reaction.
… [2]
(iii) Explain why calcium reacts more slowly with water than strontium does.
… [3]
(b) A student adds barium oxide, BaO, to water.
A reaction takes place forming a colourless solution.
(i) Predict the approximate pH of the colourless solution.
pH = … [1]
(ii) A student adds a few drops of dilute sulfuric acid to the colourless solution.
Describe what the student would observe, and give the formula of the barium
compound produced.
Observation …
Formula of barium compound …
[2]
Mark scheme
Show the mark scheme
AO
Question Answer Marks Guidance
element
2 (a) (i) Sr + 2H2O → Sr(OH)2 + H2 1 AO2.6 IGNORE STATE SYMBOLS
All formulae and balancing correct ALLOW multiples
IGNORE state symbols (even if wrong)
(ii) Oxidation 2 AO2.1 ALLOW 2+ for +2 and 1+ for +1
Sr from 0 to +2 ×2 ‘+’ is required in +2 and +1 oxidation numbers
Reduction
H from +1 to 0 ALLOW H2 for hydrogen
ALLOW 1 mark for elements AND all oxidation
numbers correct but oxidation and reduction wrong
way round OR not given.
IGNORE numbers around equation in (i)
(treat as rough working)
12 AO
element
(iii) 3 FULL ANNOTATIONS MUST BE USED
Atomic radius AO1.2 -----------------------------------------------------------
Ca has smaller atomic radius ORA in terms of Sr
OR fewer shells Comparison needed for each mark.
ALLOW ‘fewer energy levels’
ALLOW ‘electrons closer to nucleus’
IGNORE fewer orbitals OR fewer sub-shells
OR different shell
AO1.2
Effect of nuclear charge/shielding
Ca has less/decreased shielding ALLOW more electron repulsion from inner shells
Nuclear attraction AO1.2
Ca has greater nuclear attraction (for electrons) IGNORE nuclear charge/effective nuclear charge
OR Ca has a higher ionisation energy ALLOW ‘less nuclear pull’
OR more energy is required to lose the outer OR ‘electrons held less tightly’
electrons
(b) (i) Any value in range: 8–14 1 AO1.1
(ii) White precipitate/white solid 2 AO3.1
BaSO4 AO3.2
Total 9
How to answer it
Group 2 Elements and Their Compounds
What this question tests
This question assesses your knowledge of Group 2 chemistry, specifically focusing on the reactions of Group 2 metals with water, assigning oxidation numbers to define redox processes, explaining periodic trends down Group 2 using atomic structure and shielding, and predicting the acid-base properties and precipitation reactions of Group 2 oxides and sulfates.
Reaction of Strontium with Water
✅ Correct Answer
Sr + 2H₂O → Sr(OH)₂ + H₂
🧠 Exam Technique
State symbols are not requested here, so focus entirely on correct chemical formulae and balancing. Remember that Group 2 hydroxides have the formula M(OH)₂ because Group 2 metals form 2+ ions.
❌ Common Errors
Writing the formula of strontium hydroxide as SrOH due to forgetting the 2+ charge of strontium ions, which leads to an unbalanced equation.
Explaining Redox Using Oxidation Numbers
✅ Correct Answer
Oxidation: Strontium (Sr) changes oxidation number from 0 to +2.
Reduction: Hydrogen (H) changes oxidation number from +1 to 0.
💡 Key Knowledge
Elements in their uncombined state always have an oxidation number of 0. In compounds, hydrogen is almost always +1. You must explicitly state both the element and the change in number (including the '+' sign for positive numbers).
❌ Common Errors
Dropping the '+' sign in front of the positive oxidation numbers, or mixing up which species is oxidized and which is reduced.
Comparing Reactivity of Calcium and Strontium with Water
✅ Correct Answer
- Atomic radius: Calcium has a smaller atomic radius / fewer shells than strontium.
- Shielding: Calcium has less/decreased shielding.
- Nuclear attraction: Calcium has greater nuclear attraction for its outer electrons / higher ionization energy / electrons require more energy to be lost. (Note: Answers can be framed in terms of ORA for strontium).
🧠 Exam Technique
This is a classic 3-mark periodic trend explanation. Use the "magic three" criteria: atomic radius, shielding, and nuclear attraction/ionization energy. Always make a comparative statement (e.g., using "smaller", "less", "greater").
❌ Common Errors
Describing the trend down the group instead of answering the specific comparison between calcium and strontium. Forgetting to link nuclear attraction back to how easily electrons are lost.
Alkalinity of Barium Oxide Solution
✅ Correct Answer
Any numerical value in the range: 8 to 14 (inclusive)
💡 Key Knowledge
Group 2 oxides react with water to form alkaline hydroxide solutions (e.g., BaO + H₂O → Ba(OH)₂). Because hydroxides are alkaline, the pH must be in the alkaline range (above 7).
Reaction with Sulfuric Acid
✅ Correct Answer
Observation: White precipitate / white solid
Formula of barium compound: BaSO₄
🧠 Exam Technique
Make sure to distinguish between visual observations (what you see, e.g., "white precipitate") and chemical identities (formulas like BaSO₄ ). Barium sulfate is famously insoluble and forms a dense white precipitate.
❌ Common Errors
Writing "cloudy solution" instead of specifying a "white precipitate", or writing the formula incorrectly as BaSO3 (sulfite) instead of BaSO₄ (sulfate).
Topics
Module 3: Periodic table and energy · 3.1 The periodic table
Question and mark scheme from the OCR A-Level Chemistry examination, AS Depth in chemistry (02), June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.