AQA A-Level Chemistry Paper 3, June 2018: Question 14

1 mark Ā· Easy difficulty Ā· Multiple Choice

Identify the equation representing the standard enthalpy of atomisation of iodine.

Practise this question

Question

Multiple choice question asking which equation represents the process occurring when the standard enthalpy of atomisation of iodine is measured. Four options are given: A: 1/2 I2(s) gives I(g); B: I2(s) gives 2I(g); C: 1/2 I2(g) gives I(g); D: I2(g) gives 2I(g).
Question text

14 Which equation represents the process that occurs when the standard enthalpy of

atomisation of iodine is measured?

[1 mark]

A 2 I2(s) I(g)

B I2(s) 2I(g)

C I2(g) I(g)

D I2(g) 2I(g)

Mark scheme

Show the mark scheme Mark scheme table indicating question 14 has the correct answer option A.

14 A

How to answer it

Standard Enthalpy of Atomisation of Iodine

šŸ“Œ What this question tests

This question assesses your understanding of fundamental energetic definitions in A-Level Energetics / Thermodynamics, specifically:

  • The exact definition and stoichiometry of standard enthalpy of atomisation (Ī”atHā¦µ).
  • Correct identification of standard physical states of Group 7 elements at 298 K and 100 kPa.
  • Distinguishing between atomisation and bond dissociation enthalpy.

Question 14 Analysis

Multiple Choice (1 mark)

āœ… Correct Answer

A: Ā½ Iā‚‚(s) ā†’ I(g)

Mark Scheme: Award 1 mark for option A.

The definition requires forming exactly 1 mole of gaseous atoms from the element in its standard state under standard conditions.

šŸ’” Key Knowledge

  • Standard Enthalpy of Atomisation (Ī”atHā¦µ): The enthalpy change when one mole of gaseous atoms is formed from an element in its standard state under standard conditions (298 K, 100 kPa).
  • Standard State of Iodine: At 298 K, iodine exists as a solid diatomic molecular crystal, Iā‚‚(s) .
  • Stoichiometric Constraint: Exactly 1 mol of I(g) must appear on the product side. Therefore, the reactant must be Ā½ Iā‚‚(s) .

šŸ“ Step-by-Step Breakdown of All Options

  • A: Ā½ Iā‚‚(s) ā†’ I(g) [CORRECT]
    Starts with the standard state of iodine, Iā‚‚(s) , and forms exactly 1 mol of gaseous iodine atoms, I(g) .
  • B: Iā‚‚(s) ā†’ 2I(g) [INCORRECT]
    Produces 2 moles of gaseous atoms. The enthalpy change for this equation is equal to 2 Ɨ Ī”atHā¦µ(I) .
  • C: Ā½ Iā‚‚(g) ā†’ I(g) [INCORRECT]
    Starts from gaseous iodine, Iā‚‚(g) , not its standard state ( s ). This represents Ā½ Ɨ E(Iā€“I) (half the bond dissociation enthalpy of gaseous iodine).
  • D: Iā‚‚(g) ā†’ 2I(g) [INCORRECT]
    Starts from gas and forms 2 moles of gaseous atoms. This represents the bond dissociation enthalpy, E(Iā€“I) , or Ī”dissHā¦µ .

šŸ§  Exam Technique

  • Look at the product first: Atomisation always yields exactly 1 mole of gaseous atoms. Rule out any option where the product is not 1 I(g) (eliminates B and D immediately).
  • Check the standard state: Recall the states of the halogens at room temperature:
    ā€¢ Fā‚‚: gas  |  Clā‚‚: gas
    ā€¢ Brā‚‚: liquid  |  Iā‚‚: solid
  • Since iodine is a solid, eliminate C. Only A remains!

āŒ Common Errors & Pitfalls

  • Confusing standard states: Many students forget that iodine is solid at 298 K and incorrectly choose C.
  • Confusing atomisation with bond enthalpy: Bond dissociation enthalpy applies specifically to gaseous molecules breaking into gaseous atoms ( Iā‚‚(g) ā†’ 2I(g) ), whereas atomisation must start from the standard state ( s ).
  • Forgetting "1 mole of product": Choosing B because they balance the equation using whole numbers rather than keeping the product as 1 I(g) .

Topics

Physical Chemistry Ā· 3.1.8 Thermodynamics

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.