OCR A-Level Chemistry AS Depth in chemistry (02), November 2020: Question 4

9 marks · Medium difficulty · Structured Questions

Predict the bond angle and explain shapes of molecules using VSEPR, write a Kc expression, calculate equilibrium concentrations from stoichiometry and Kc values, and predict the effect of temperature on chemical equilibrium for nitrosyl chloride dissociation.

Practise this question

Question

Chemistry exam question with two main sections. Part (a) shows a dot-and-cross diagram of nitrosyl chloride (ONCl) and asks to predict the O=N-Cl bond angle and explain it. Part (b) provides an equilibrium equation for the thermal dissociation of nitrosyl chloride with enthalpy change, and asks to write the Kc expression, explain a concentration relationship based on stoichiometry, calculate an equilibrium concentration using Kc, and predict the effect of increasing temperature on equilibrium.
Question text

4 Nitrosyl chloride, NOCl, is used in the industrial manufacture of nylon.

(a) The ‘dot-and-cross’ diagram for nitrosyl chloride is shown below.

N

O Cl

Predict the O=N–Cl bond angle and explain why NOCl has this bond angle.

Bond angle …

Explanation …

… [3]

(b) Nitrosyl chloride, NOCl, dissociates into nitrogen monoxide and chlorine as in the equilibrium

below.

2NOCl (g) 2NO(g) + Cl (g) ΔH = +77.0 kJ mol–1

Nitrosyl chloride is added to a container, which is then sealed.

The container is heated to 400 °C, and equilibrium is allowed to be reached.

(i) Write the expression for the equilibrium constant, Kc, for this equilibrium.

[1]

(ii) In the equilibrium mixture at 400 °C, the equilibrium concentration of Cl2(g) is found to be

0.17 mol dm−3.

The student calculates that the equilibrium concentration of NO(g) is 0.34 mol dm−3.

Explain how the student obtained this value for [NO(g)].

… [1]

(iii) At 400 °C, K = 0.015 moldm–3.

c

Calculate the equilibrium concentration of NOCl(g) at 400 °C.

equilibrium concentration of NOCl (g) = … mol dm−3 [2]

(iv) The temperature of the equilibrium mixture is increased above 400 °C while keeping the

pressure constant.

State and explain the effect on the equilibrium concentration of nitrogen monoxide,

NO(g), with these new conditions.

… [2]

Mark scheme

Show the mark scheme Mark scheme for the nitrosyl chloride question. Part (a) awards 3 marks for bond angle (112-120 degrees), mentioning electron pairs/double/single bonds and repulsion. Part (b)(i) awards 1 mark for the correct Kc expression. Part (b)(ii) awards 1 mark for explaining the 2:1 stoichiometric ratio. Part (b)(iii) awards 2 marks for the calculation of [NOCl] resulting in 1.1 mol dm-3. Part (b)(iv) awards 2 marks for stating equilibrium shifts right due to endothermic forward reaction and concentration of NO increases.

AO

Question Answer Marks Guidance

element

4 (a) Bond angle 3 1.1 ×1

112–120º

Explanation

Around N, there is a double bond, a single bond 2.1 ×2 ALLOW 3 bonding pairs and 1 lone pair

and a lone pair OR 2 bonding region and 1 lone pair

Electron pairs repel ALLOW bonding pairs or lone pairs

Seen anywhere

(b) (i) [NO]2[Cl2] 1 1.2 ×1 DO NOT ALLOW curved brackets

(Kc =) 2

[NOCl]

(ii) From equation, n(NO) is 2 × n(Cl2) 1 3.1 ×1 Response MUST refer to stoichiometry of

OR equation and compare molar ratio of both NO

Ratio NO:Cl2 is 2:1 and Cl2

(iii) FIRST CHECK ANSWER ON THE ANSWER LINE 2 2.6 ×2 ALLOW 1.1 up to calculator value of

If answer = √𝟏𝟏. 𝟑𝟑𝟏𝟏 = 1.1 (mol dm–3) award 2 marks 1.144552314

[NO]2[Cl ] 0.342 × 0.17 ALLOW ECF from inverted Kc expression in b(ii)

[NOCl ]2 = OR OR 1.3 -4

Kc 0.015 2.9(478) x 10 1 mark

0.017(1691584) 2 marks

[NOCl ] = √1.3 = 1.1 (mol dm–3)

(iv) As T increases, equilibrium (position) shifts to right 2 2.5 ×2 ALLOW ‘favours the right’, for ‘shifts to right’

AND (forward) reaction is endothermic ALLOW moves to right in endothermic direction

Equilibrium concentration of NO increases

How to answer it

Nitrosyl Chloride Equilibria and Shape

What this question tests

This question assesses core physical and inorganic chemistry concepts: applying Electron Pair Repulsion Theory (VSEPR) to predict bond angles from dot-and-cross diagrams, writing expressions for the equilibrium constant (K_c), interpreting chemical stoichiometry for concentration deductions, carrying out multi-step K_c calculations, and applying Le Chatelier's principle to predict the effects of temperature changes on equilibrium positions.

Question 4 (a)

Bond Angle and Shape of NOCL

✅ Correct Answer

Bond angle: 112 to 120 degrees

Explanation: Around N, there is a double bond, a single bond, and a lone pair (or 3 electron domains / 3 bonding regions and 1 lone pair). Electron pairs repel.

💡 Key Knowledge

  • Count electron domains around the central atom (N). Treat double and single bonds as single electron domains for shape/repulsion purposes.
  • Lone pairs repel more strongly than bonding pairs, compressing the expected bond angle (which would be 120 degrees for trigonal planar).

🧠 Exam Technique

When asked for an explanation involving VSEPR theory, always explicitly state: (1) what surrounds the central atom (bonding pairs & lone pairs), and (2) that electron pairs repel.

❌ Common Errors

Students often state "molecules repel" instead of electron pairs repel, which loses the chemistry marking point. Others forget to account for the lone pair presence on nitrogen.

Marks: 3 marks total (1 mark for angle range, 2 marks for explanation points).
Question 4 (b) (i)

Equilibrium Constant Expression (K_c)

✅ Correct Answer

K_c = [NO]²[Cl₂] / [NOCl]²

❌ Common Errors

Using curved brackets ( ) instead of square brackets [ ] for concentration terms will automatically lose you the mark. Examiners are strict on notation!

Marks: 1 mark.
Question 4 (b) (ii)

Interpreting Stoichiometry

✅ Correct Answer

From the balanced equation, the molar ratio of NO to Cl₂ is 2:1, meaning the concentration of NO must be 2 × [Cl₂] (2 × 0.17 = 0.34 mol dm⁻³).

🧠 Exam Technique

You must explicitly refer to the stoichiometry of the balanced equation or compare the molar ratio of NO and Cl₂ to secure the mark.

Marks: 1 mark.
Question 4 (b) (iii)

Calculating Equilibrium Concentration

📐 Step-by-Step Calculation

  1. Rearrange the K_c expression to make [NOCl]² the subject:
    [NOCl]² = [NO]² [Cl₂] / K_c
  2. Substitute the values:
    [NOCl]² = (0.34)² × 0.17 / 0.015 = 1.311...
  3. Take the square root:
    [NOCl] = sqrt(1.311...) = 1.1 mol dm⁻³

❌ Common Calculation Traps

  • Forgetting to square the [NO] term in the numerator.
  • Carrying out rounding too early before taking the final square root.
  • Error carried forward (ECF) applies if you used an incorrect K_c expression from part (i), but setup must be logical.
Marks: 2 marks (1 for correct rearrangement/substitution, 1 for final correct answer to appropriate significant figures/units).
Question 4 (b) (iv)

Le Chatelier's Principle & Temperature

✅ Correct Answer

Effect: The equilibrium concentration of NO increases.

Explanation: As temperature increases, the equilibrium position shifts to the right (favours the forward reaction) because the forward reaction is endothermic (positive ΔH).

💡 Key Knowledge

Le Chatelier's principle states that if a factor changing the conditions of an equilibrium is applied, the system responds to counteract that change. An increase in temperature favours the endothermic direction to absorb excess heat.

🧠 Exam Technique

Structure your answer in two parts: (1) Direction of shift linked to the endothermic sign of ΔH, and (2) Resulting concentration change for the specific species requested (NO).

Marks: 2 marks (1 mark for explaining the shift due to endothermic nature, 1 mark for stating the concentration of NO increases).

Topics

Module 2: Foundations in chemistry · Module 3: Periodic table and energy · 2.2 Electrons, bonding and structure · 3.2 Physical chemistry

Question and mark scheme from the OCR A-Level Chemistry examination, AS Depth in chemistry (02), November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.