OCR A-Level Chemistry AS Depth in chemistry (02), November 2021: Question 5

✅ Correct Answer

Kc = [CH₃OH] / ([CO] × [H₂]²)

💡 Key Knowledge

• Products go on the numerator (top); reactants go on the denominator (bottom).
• Powers correspond directly to the stoichiometric balancing numbers from the chemical equation. H₂ has a balancing number of 2, so it must be raised to the power of 2.

❌ Common Errors

• Using curved brackets ( ) instead of square brackets [ ] . Examiners strictly penalise this as square brackets denote concentration in mol dm⁻³.

📐 Calculations (Part i)

Step 1: Rearrange the Kc expression to make the unknown concentration the subject:

[CH₃OH] = Kc × [CO] × [H₂]²

Step 2: Substitute the known values given in the table and question:

[CH₃OH] = 15.4 × 0.57 × (0.40)²

Step 3: Calculate the final numerical value and check units:

[CH₃OH] = 1.40448 mol dm⁻³ (Accept 1.4)

✅ Correct Answer (Part ii)

To the right (or "towards the products / CH₃OH")

🧠 Exam Technique

For magnitude of Kc questions, remember: A large Kc value (>1) indicates the equilibrium lies far to the right, favouring the products. A small Kc value (<1) indicates it lies to the left, favouring reactants.

✅ Correct Answers (Any two)

• Less fossil fuel used / less energy used.
• Reduction in CO₂ emissions.

🧠 Exam Technique

Keep your environmental answers direct and focused on energy consumption and greenhouse gas emissions. Vague statements like "it's better for the ozone layer" do not gain credit.

✅ Correct Answers

(i) Block: d-block

(ii) Electron Configuration of Gallium (Ga):

1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p¹

❌ Common Errors

• Using argon shorthand [Ar] when the question explicitly prints 1s² at the start. You must write out the full configuration starting from 1s² .
• Writing 4s² 3d¹⁰ instead of filling 3d before 4s in energy order notation, though standard spectroscopic order is 3d¹⁰ 4s² 4p¹ . (Note: examiners accept 4s before 3d if written, but follow standard filling order conventions).

✅ Correct Answer

Element A: Silicon / Si

💡 Explanation & Reasoning

• There is a large increase in ionisation energy between the 4th and 5th ionisation energies.
• This sharp jump proves that the 5th electron is removed from a shell closer to the nucleus (a new, lower principal quantum shell).
• Therefore, element A has 4 electrons in its outer shell, placing it in Group 4 and Period 3, which is Silicon.

🧠 Exam Technique

When analyzing successive ionisation energy tables, always look down the row for massive numerical jumps. Count the number of electrons removed before the huge jump to instantly deduce the group number!