OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2017: Question 9

1 mark · Medium difficulty · Multiple Choice

Calculate the bond enthalpy of the H-I bond given the reaction enthalpy and other bond enthalpies.

Practise this question

Question

Multiple choice question 9 showing the equation H2(g) + I2(g) -> 2HI(g) with a reaction enthalpy of -9 kJ mol^-1. A table provides bond enthalpies for H-H (+436 kJ mol^-1) and I-I (+151 kJ mol^-1). Four options A (-596), B (-298), C (+298), and D (+596) are listed for the bond enthalpy of the H-I bond.
Question text

9 Enthalpy values are provided below.

H (g) + I (g) 2HI(g) Δ H = –9 kJ mol–1

22 r

Bond enthalpy

Bond –1

/ kJ mol

H–H +436

I–I +151

What is the bond enthalpy, in kJ mol–1, of the H–I bond?

A –596

B –298

C +298

D +596

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme indicating the correct answer is C.

9 C 1

How to answer it

Calculating Bond Enthalpies from Enthalpy Change of Reaction

What this question tests

This question assesses your ability to apply the relationship between enthalpy change of reaction (ΔᵣH) and mean bond enthalpies. You must correctly account for stoichiometry (reacting ratios), rearrange algebraic formulas involving bonds broken minus bonds formed, and handle positive and negative signs correctly.

Question 9 — Multiple Choice

Full Worked Solution & Examiner Breakdown

✅ Correct Answer

C (+298 kJ mol⁻¹)

The correct bond enthalpy for the H–I bond is +298 kJ mol⁻¹. Bond enthalpies are always quoted as positive values because energy is required to break bonds.

💡 Key Knowledge

  • Definition: Enthalpy change of reaction can be calculated using:
    ΔᵣH = Σ(Bonds broken) - Σ(Bonds formed)
  • Bonds broken: Reactants require energy input (endothermic, positive values).
  • Bonds formed: Products release energy upon formation (exothermic, negative values).
  • Sign convention: Individual bond enthalpies are strictly positive.

🧠 Exam Technique

When given a multiple-choice question with mixed signs, eliminate obviously incorrect options first. Since bond enthalpies must be positive, options A (-596) and B (-298) can be instantly discarded, cutting your guessing odds in half!

❌ Common Errors

  • Forgetting stoichiometry: Forgetting to multiply the H–I bond enthalpy by 2, leading to half the correct value (+596 vs +298 confusion).
  • Sign confusion: Treating bond enthalpies as negative values or mixing up the subtraction order in the bond enthalpy equation.

📐 Step-by-Step Calculation

  1. Write out the equation using bond enthalpy terms:
    ΔᵣH = [BE(H–H) + BE(I–I)] - [2 × BE(H–I)]
  2. Substitute the known values from the question:
    -9 = [+436 + +151] - [2 × x] (where x is the bond enthalpy of H–I)
  3. Simplify the bonds broken term:
    -9 = 587 - 2x
  4. Rearrange to solve for 2x :
    2x = 587 + 9
    2x = 596
  5. Divide by 2 to find the single bond enthalpy ( x ):
    x = 596 / 2 = +298 kJ mol⁻¹
Examiner Note: Top-level responses set out the algebraic formula clearly with brackets before substituting numbers, avoiding sign errors caused by the subtraction of the bonds formed term.

Topics

Module 3: Periodic table and energy · 3.2 Physical chemistry

Question and mark scheme from the OCR A-Level Chemistry examination, Periodic table, elements and physical chemistry (01), June 2017. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.