Edexcel A-Level Chemistry Paper 1, June 2019: Question 7

6 marks · Hard difficulty · Extended Writing

Compare and contrast the origin of the colour of a copper(II) complex with the origin of the colour of the copper(II) ion in a flame test, without stating specific colours.

Practise this question

Question

An exam question numbered 7, worth 6 marks. It instructs students to compare and contrast the origin of the colour of a copper(II) complex with the origin of the colour of the copper(II) ion in a flame test, noting that specific colours do not need to be stated. Lined answer space follows.
Question text

7 Colour is often used in chemistry to identify substances.

Compare and contrast the origin of the colour of a copper(II) complex with the origin

of the colour of the copper(II) ion in a flame test.

You do not need to state any specific colours.

(6)

… *P58306A01828*

(Total for Question 7 = 6 marks)

Mark scheme

Show the mark scheme A mark scheme showing a 6-mark extended response broken down into 4 marks for indicative content and 2 marks for structure and lines of reasoning. Indicative points include: (IP1) differences in energy levels determining colour, (IP2) heat energy resulting in electron promotion in flame tests, (IP3) return of excited electron to lower energy state, (IP4) d orbitals splitting in energy due to ligands in complex ions, (IP5) light energy needed for electron promotion in complexes, and (IP6) colour not absorbed is seen.

How to answer it

Edexcel A-Level Chemistry • Transition Metals & Atomic Structure

Origin of Colour: Complexes vs Flame Tests

What this question tests

This 6-mark extended-response question assesses your understanding of electron transitions across two different chemical contexts: d-block aqueous complex ions and emission/absorption in flame tests. It evaluates your ability to compare similarities, contrast mechanisms (heat vs light energy, d-orbital splitting vs general energy levels), and construct a logically structured scientific argument.

Question Breakdown & Mark Scheme Guide

To achieve full marks (4 marks for Indicative Content + 2 marks for Quality of Written Communication/Structure), you must successfully hit the key marking points while maintaining clear comparative linkages throughout your answer.

✅ Indicative Marking Points (IP1 to IP6)

  • IP1 (Similarities): Both phenomena rely on differences in energy levels (delta E) to determine the specific wavelengths of light involved.
  • IP2 (Flame Test - Excitation): Heat energy is supplied, resulting in electron promotion to higher energy levels.
  • IP3 (Flame Test - Emission): The excited electron returns to a lower energy state, emitting light.
  • IP4 (Complex Ion - Splitting): The d-orbitals in a transition metal complex are split into different energy levels by the surrounding ligands.
  • IP5 (Complex Ion - Absorption): Light energy (specifically visible light) is absorbed to promote an electron between these split d-orbitals (d-d transition).
  • IP6 (Complex Ion - Observed Colour): The colour observed is due to the light frequencies that are not absorbed (transmitted/complementary colour).

💡 Core Chemical Principles

  • Energy Changes: ΔE = hf is fundamental to both, but the source of energy differs (thermal energy in flames vs electromagnetic radiation/light in complexes).
  • Terminology: Always refer to copper(II) ions , never copper atoms , when discussing flame tests of ionic compounds.
  • d-Subshell Splitting: Coordinate bonds from ligands cause degeneracy of the 3d orbitals to lift, creating non-degenerate sets with an energy gap ΔE .

🧠 Structuring for Level 3 (2 Structure Marks)

  • Use Comparative Connectives: Use phrases like "Similarly, both..." and "In contrast, the flame test involves..." to secure your reasoning marks.
  • Direct Comparisons: Do not just write two isolated paragraphs. Explicitly contrast how energy is supplied (heat vs light) and where electrons move (general energy levels vs split d-orbitals).

❌ Pitfalls to Avoid

  • Atoms vs Ions: Using the word "atom" instead of "ion" for the flame test will be penalized.
  • Emission vs Fall: Do not attribute complex ion colour to an "emission of light" or an electron "falling to a lower energy d-orbital" via emission. Complexes absorb light; they do not emit it under normal viewing conditions.
  • Incorrect Splitting: Avoid stating that a singular "d-orbital splits" — it is the degenerate set of d-subshell orbitals that splits into different energy sub-levels.
Mark Allocation Breakdown:
• 6 IP points seen: 4 marks (5-4 points = 3 marks, 3-2 points = 2 marks, 1 point = 1 mark)
• Structure & Linkages: Up to 2 additional marks awarded for a coherent, logically structured argument with continuous direct comparisons.

Topics

Inorganic Chemistry · Physical Chemistry · Topic 15: Transition Metals · Topic 1: Atomic Structure and the Periodic Table

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 1, June 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.