Edexcel A-Level Chemistry Paper 1, November 2021: Question 10

6 marks · Hard difficulty · Extended Writing

Discuss whether thermodynamic feasibility calculated using the Gibbs free energy equation determines if a reaction occurs in practice, referencing enthalpy, entropy terms, and kinetic factors.

Practise this question

Question

An extended response question worth 6 marks. It presents a statement: 'Whether or not a reaction occurs depends only on the thermodynamic feasibility calculated using ΔG = ΔH - TΔS_system'. Students are instructed to discuss this statement, including reference to the changes in values and signs of the terms in the equation for both endothermic and exothermic reactions, and circumstances where a reaction predicted to be thermodynamically feasible may not occur in practice.
Question text

*10 A student wrote:

Whether or not a reaction occurs depends only on the thermodynamic feasibility

calculated using

∆G = ∆H — T∆Ssystem

Discuss this statement.

Include reference to:

changes in the values and signs of the terms in the equation for both endothermic

and exothermic reactions

circumstances where a reaction that is predicted to be

thermodynamically feasible may not occur in practice.

(6)

… 26

… *P65463A02628*

(Total for Question 10 = 6 marks)

Mark scheme

Show the mark scheme The mark scheme for question 10, showing a 6-mark levels-based marking grid combining indicative content and structure/lines of reasoning. Indicative content points cover the requirement for ΔG to be negative, the four combinations of ΔH and ΔS_system signs affecting feasibility, and kinetic factors/high activation energy preventing reactions despite thermodynamic feasibility.

Question

Answer Additional Guidance Mark

Number

This question assesses the student’s ability to show a coherent and Guidance on how the mark scheme should be (6)

logically structured answer with linkages and fully sustained applied:

reasoning. The mark for indicative content should be

added to the mark for lines of reasoning. For

Marks are awarded for indicative content and for how the answer is example, a response with four indicative

structured and shows lines of reasoning. marking points that is partially structured with

some linkages and lines of reasoning scores 4

The following table shows how the marks should be awarded for marks (3 marks for indicative content and 1

indicative content. mark for partial structure and some linkages

Number of indicative marking Number of marks awarded for and lines of reasoning).

points seen in answer indicative marking points If there were no linkages between the points,

64 then the same indicative marking points would

5-4 3 yield and overall score of 3 marks (3 marks for

3-2 2 indicative content and zero marks for

11 linkages).

The following table shows how the marks should be awarded for

structure and lines of reasoning

Number of marks awarded

for structure of answer and

sustained lines of reasoning

Answer shows a coherent logical

structure with linkages and fully

sustained lines of reasoning

demonstrated throughout

Answer is partially structured

with some linkages and lines of 1

reasoning

Answer has no linkages between

0 Accept any six indicative content points

points and is unstructured

More than one indicative marking point may

be made within the same comment or

explanation

Indicative content

• ∆G needs to be negative for a reaction to be feasible

• if ∆Ssystem and ∆H are both negative then a reaction is

feasible if the magnitude of ∆H > (magnitude of) T∆Ssystem

• if ∆Ssystem and ∆H are both positive then a reaction is

feasible if the magnitude of T∆Ssystem> (magnitude of) ∆H

• if ∆Ssystem is positive and ∆H is negative then the reaction is

(always) feasible

• if ∆Ssystem is negative and ∆H is positive then the reaction is

never feasible

• even if it is feasible the reaction may not occur because of High activation energy/energy barrier/physical

kinetic factors barrier e.g. oxide layer

Ignore just reference to slow rate of reaction

Ignore reference to non-standard conditions

(Total Question 10 = 6 marks)

How to answer it

Thermodynamic Feasibility & Kinetics Discussion

Edexcel A-Level Chemistry — 6-Mark Extended Response

What this question tests

This question assesses your understanding of thermodynamic feasibility using Gibbs Free Energy (ΔG = ΔH - TΔS_system), how different combinations of enthalpy change (ΔH) and entropy change (ΔS_system) affect feasibility across varying temperatures, and the crucial distinction between thermodynamic predictions and actual kinetic outcomes (activation energy barriers).

Question Breakdown & Mark Scheme Analysis

Question 10 (6 Marks total)

The question requires a 6-mark extended written response (often marked using a levels-of-response grid combining indicative chemistry points and communication structure).

💡 Key Knowledge

  • For a reaction to be thermodynamically feasible, ΔG must be negative (< 0).
  • The role of temperature ( T ) in scaling the entropy term ( TΔS_system ).
  • Kinetics vs. Thermodynamics: A feasible reaction may not happen due to a high activation energy barrier.

🧠 Exam Technique

  • Structure your answer logically by systematically working through the possible combinations of ΔH and ΔS_system signs.
  • Explicitly mention the magnitude comparisons (e.g., magnitude of ΔH vs magnitude of TΔS_system).
  • Conclude with kinetic factors to address the second bullet point in the prompt.

❌ Common Errors

  • Stating that a reaction with positive ΔG will occur slowly (confusing thermodynamics with rate).
  • Saying a reaction won't happen because of "non-standard conditions" or just a "slow rate" (examiners penalize vague phrasing; you must reference high activation energy or energy barriers).
  • Forgetting to state that ΔG must be negative for feasibility.

Indicative Content & Marking Points

To achieve full marks for indicative content (up to 4 marks), candidates must include at least 4 to 6 of the following points, supported by 2 marks for structure and coherent reasoning:

✅ Point 1: The Feasibility Condition

A reaction is only thermodynamically feasible if ΔG is negative .

✅ Point 2: Both Negative (ΔH < 0, ΔS < 0)

If both ΔS_system and ΔH are negative, the reaction is feasible only if the magnitude of ΔH is greater than the magnitude of TΔS_system (typically at lower temperatures).

✅ Point 3: Both Positive (ΔH > 0, ΔS > 0)

If both ΔS_system and ΔH are positive, the reaction is feasible only if the magnitude of TΔS_system is greater than the magnitude of ΔH (typically at higher temperatures).

✅ Point 4: Exothermic & Positive Entropy

If ΔS_system is positive and ΔH is negative, ΔG is always negative regardless of temperature, meaning the reaction is always feasible.

✅ Point 5: Endothermic & Negative Entropy

If ΔS_system is negative and ΔH is positive, ΔG is always positive regardless of temperature, meaning the reaction is never feasible.

✅ Point 6: Kinetic Factors

Even if a reaction is predicted to be thermodynamically feasible ( ΔG < 0 ), it may not occur in practice due to kinetic factors, such as a high activation energy or a physical barrier (e.g., an unreactive oxide layer).

Examiner Note on Structure: The final 2 marks are awarded for clear linkages and sustained lines of reasoning. Ensure your paragraphs flow logically from general conditions into specific sign combinations, and finally into kinetic limitations.

Topics

Physical Chemistry · Topic 13: Energetics II

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 1, November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.