Edexcel A-Level Chemistry AS Paper 1, June 2024: Question 7

18 marks · Hard difficulty · Open Response

Compare the chemical and physical properties of Group 1 and Group 2 compounds, including redox reactions with sulfuric acid, flame tests, thermal stability, and electrical conductivity.

Practise this question

Question

Question text

7 This question is about comparing the chemical and physical properties of Group 1

and Group 2 compounds.

*(a) Potassium chloride and potassium bromide are white crystalline solids which

react with concentrated sulfuric acid.

Give the observations in these reactions and an explanation, using oxidation

numbers, of which is the stronger reducing agent.

Include equations for any reactions that occur.

(6)

… *P76893A01424*

(b) A student was asked to confirm the cation present in a sample of white powder

that was known to be a Group 1 compound. The student carried out a flame test 15

using the procedure shown.*P76893A01524*

Procedure

Step 1 A platinum wire was first cleaned by dipping it into concentrated

hydrochloric acid and then heating in a colourless Bunsen flame.

Step 2 After cleaning, the cleaned wire was dipped into a fresh, clean sample of

concentrated hydrochloric acid and then into the white powder to pick

up a sample for testing.

Step 3 The sample was tested by placing the wire in the colourless Bunsen flame.

Result

The flame was coloured lilac.

(i) State a reason why, in Step 2, the acid used was hydrochloric acid.

(1)

(ii) Identify the cation present in this sample of white powder.

(1)

(i) Draw a labelled diagram of apparatus that would enable you to compare the

thermal stability of Group 2 carbonates.

(2)

(ii) State the conditions that must be used with the apparatus shown in your

diagram to ensure that the test is fair.

(2)

(iii) State what data could be obtained in this experiment to compare

thermal stability.

(1)

… *P76893A01624*

(iv) Which pair of responses show the trend in thermal stability of compounds

down Group 2 ?

(1)

Carbonates Nitrates

A increasing decreasing

B decreasing decreasing

C increasing increasing

D decreasing increasing

(d) The table shows the electrical conductivity of some pure substances in the solid

and liquid states.

Electrical conductivity

Substance Solid state Liquid state

potassium chloride poor good

iron good good

water poor poor

Explain the electrical conductivity of potassium chloride, iron and water in the

solid and liquid states. 17

*P76893A01724* (4)

(Total for Question 7 = 18 marks)

Mark scheme

Show the mark scheme

Question

Acceptable Answer Additional Guidance Mark

Number

7(b)(i) An answer that makes reference to the following point: (1)

• chlorides are more volatile (than other compounds). Allow converse argument; compounds formed by

other acids are less volatile

Allow ‘chlorides are more easily vaporised’

Allow ‘it forms a volatile (metal) chloride (from the

salt)’

Do not award hydrochloric acid is more volatile

Question

Acceptable Answer Additional Guidance Mark

Number

7(b)(ii) An answer that makes reference to the following point: (1)

• potassium (ion) / K(+)

Question

Acceptable Answer Additional Guidance Mark

Number

7(c)(i) An answer that makes reference to the following Example of suitable diagram (2)

points:

carbonate

• heating carbonate in suitable container, eg test

tube (1)

• collection of gas produced eg by syringe, gas

burette, inverted test tube / measuring cylinder (1)

etc

Alternative method: (1)

• M2 bubble gas through lime water.

time how long for lime water to go milky

Do not award M1 for conical flask/ beaker

Do not award M1 if additional reagents are in the container

Do not award M2 if gas syringe does not have plunger

Do not award M2 if the apparatus would not work e.g. no

bung or gas cannot move through the apparatus

Question

Acceptable Answer Additional Guidance Mark

Number

7(c)(ii) An answer that makes reference to any two of the following (2)

points

• same Bunsen flame / same Bunsen temperature (1) Ignore just same temperature / heat

Allow same Bunsen setting

• same distance of heat source from test tube (1)

• allow same moles of each carbonate (1) Accept ‘amount’ of each carbonate

Do not award same mass / volume

Do not award heat under reflux for either M1 or M2

Question

Acceptable Answer Additional Guidance Mark

Number

7(c)(iii) An answer that makes reference to the following point: (1)

either

• measure the time taken for lime water to turn milky

• measure the time taken for a particular volume of gas

collected

Question

Answer Mark

Number

7(c)(iv) The only correct answer is C (carbonates: increasing, nitrates: increasing) (1)

A is incorrect because the thermal stability of nitrates increases down the Group 2

B is incorrect because the thermal stability of nitrates increases down the Group 2

D is incorrect because the thermal stability of carbonates increases down the Group 2

Question

Acceptable Answer Additional Guidance Mark

Number

7(d) An explanation that makes reference to the following points: (4)

• solid potassium chloride is a poor conductor because the Allow just ‘the ions are not mobile’

ions are in fixed positions (1) Allow lattice for fixed position

• liquid potassium chloride conducts because the ions are

free to move (1) Allow delocalised ions for ions are free to move

• iron is a good conductor when solid or liquid because it Allow electrons that are free to move for

has delocalised electrons (which move and carry charge) (1) delocalised electrons

• water is a poor conductor because there are no charge

carriers / electrons that are free to move/ ions that are free (1) Allow very few ions in pure water

to move

Do not award marking points if incorrect

bonding referred to

(Total for Question 7 = 18 marks)

How to answer it

Properties of Group 1 and Group 2 Compounds

What this question tests

This comprehensive AS Chemistry exam question assesses your understanding of Group 1 and Group 2 trends, analytical techniques (flame tests), redox reactions involving halide ions and concentrated sulfuric acid, thermal stability trends across Group 2, and the charge carriers responsible for electrical conductivity in different bonding types (ionic, metallic, covalent).

Question 7 (a)

Reactions of Halides with Concentrated Sulfuric Acid & Reducing Ability

✅ Expected Observations & Equations

KCl + H₂SO₄: Steamy white fumes of hydrogen chloride gas ( HCl ).
KCl + H₂SO₄ → KHSO₄ + HCl
KBr + H₂SO₄: Steamy white fumes ( HBr ), red-brown vapour/fumes of bromine ( Br₂ ), and choking acidic gas ( SO₂ ).
2KBr + 2H₂SO₄ → K₂SO₄ + Br₂ + SO₂ + 2H₂O
Stronger Reducing Agent: Bromide ( Br⁻ ) is a stronger reducing agent than chloride ( Cl⁻ ).

💡 Key Knowledge

Reducing power of halide ions increases down Group 7 ( Cl⁻ < Br⁻ < I⁻ ) because ions become larger, outer shell electrons are further from the nucleus, experience more shielding, and are more easily lost.
Chloride ions are not strong enough reducers to reduce sulfur in sulfuric acid, so only an acid-base reaction occurs. Bromide ions reduce S(+6) in H₂SO₄ down to S(+4) in SO₂ .

🧠 Exam Technique & Oxidation Numbers

Explicitly state the oxidation numbers to secure full explanation marks: Sulfur goes from +6 in H₂SO₄ to +4 in SO₂ (reduction), while Bromine goes from -1 in Br⁻ to 0 in Br₂ (oxidation).

❌ Common Errors

Just stating "bromide is a bigger ion" without linking it to ease of losing electrons/oxidation.
Confusing observations (e.g., calling Br₂ gas purple instead of red-brown/orange).

Question 7 (b)

Flame Test Procedure & Cation Identification

✅ Correct Answers

(i) Reason for concentrated HCl: Chlorides are volatile (more easily vaporised), allowing the metal ions to enter the Bunsen flame effectively so the characteristic colour can be observed.

(ii) Cation identified: Potassium ion / K⁺ (linked to the lilac flame).

💡 Key Knowledge

Concentrated hydrochloric acid reacts with metal compounds on the wire to form metal chlorides, which vaporise readily at lower temperatures compared to other salts.

Question 7 (c)

Thermal Stability of Group 2 Compounds

✅ Answers & Setup

(i) Diagram Setup: A side-arm test tube containing the solid carbonate connected via delivery tube to a collection apparatus (gas syringe, inverted measuring cylinder filled with water, or bubbling through lime water).
(ii) Fair Test Conditions: Same Bunsen burner temperature/setting, same distance of heat source to test tube, and same moles (or amount) of each carbonate.
(iii) Data Obtained: Time taken for lime water to turn milky, OR volume of gas collected in a set time.
(iv) Multiple Choice: C (Carbonates: increasing, Nitrates: increasing down Group 2).

❌ Common Errors in Diagrams

Using a conical flask instead of a heat-resistant test tube, or sealing the system completely so gas cannot escape (leading to an explosion hazard in real labs!).

Question 7 (d)

Electrical Conductivity Explanation

✅ Marking Points Breakdown

Solid KCl : Poor conductor because ions are in fixed positions within the giant ionic lattice and cannot move.
Liquid KCl : Good conductor because ions are molten/free to move and carry charge.
Iron ( Fe ): Good conductor in both solid and liquid states because it possesses delocalized electrons that are free to move and carry charge.
Water ( H₂O ): Poor conductor because there are no free charge carriers (no free electrons or mobile ions).

🧠 Exam Technique

Always structure conductivity answers around two requirements: (1) presence of charged particles, and (2) mobility of those particles. Mentioning "delocalized electrons" for metals and "mobile ions" for molten ionic compounds gains automatic credit.

Topics

Physical Chemistry · Inorganic Chemistry · Topic 3: Redox I · Topic 2: Bonding and Structure · Topic 4: Inorganic Chemistry and the Periodic Table

Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 1, June 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.