A-Level · Chemistry · AQA · Mark scheme decoded
AQA A-Level Chemistry: Transition Metal Characteristics and Complex Formation — mark scheme explained
The short answer
In A-Level Chemistry, understanding the characteristics of transition metals is crucial for grasping their unique properties and applications. Transition metals are elements that have an incomplete d sub-level in either their atoms or ions. This section focuses on four key characteristics: complex formation, formation of coloured ions, variable oxidation states, and catalytic activity.
The question
Identify the ligands and co-ordination number in the complex [Cu(NH 3 ) 4 ]SO 4 . [Paraphrased for study — not reproduced from any exam paper.]
Mark scheme, decoded
What each mark is really for — in plain English — and the wording trap that loses it.
- S1
Step 1: Identify the central metal ion.
- S2
The central metal ion is Cu 2+ (copper(II)).
- S3
Step 2: Identify the ligands.
- S4
The ligands are ammonia (NH 3 ) molecules.
- S5
Step 3: Determine the co-ordination number.
- S6
There are four NH 3 molecules bonded to the Cu 2+ ion, so the co-ordination number is 4.
Model answer
Worked through, with each step tagged to the mark it earns.
- S1
Step 1: Identify the central metal ion.
- S2
The central metal ion is Cu 2+ (copper(II)).
- S3
Step 2: Identify the ligands.
- S4
The ligands are ammonia (NH 3 ) molecules.
- S5
Step 3: Determine the co-ordination number.
- S6
There are four NH 3 molecules bonded to the Cu 2+ ion, so the co-ordination number is 4.
Final answer: Ligands: NH 3 , Co-ordination number: 4
Common mistakes
- Misidentifying ligands in a complex. — Carefully identify the central metal ion and the molecules or ions directly bonded to it. Ligands are those that form co-ordinate bonds with the metal.
- Incorrectly determining the co-ordination number. — Count only the number of co-ordinate bonds between the central metal and the ligands to determine the co-ordination number.
- Failing to correctly calculate oxidation states. — Set up an equation that accounts for the charge on the complex and the charges on the ligands. Solve the equation step-by-step to find the oxidation state of the central metal.
- Not explaining why transition metals form coloured ions clearly. — Practice explaining that the presence of partially filled d orbitals in transition metals leads to the absorption of specific wavelengths of light, resulting in the characteristic colours observed.
- Confusing catalytic activity with other properties of transition metals. — Remember that transition metals act as catalysts by providing a surface for reactants to adsorb, lowering the activation energy. Practice explaining this role clearly with examples from industry.
- Incorrectly identifying the central metal ion in a complex. — Carefully identify the central metal ion by looking at the formula of the complex and recognizing the transition metal element. The central metal is typically the atom or ion surrounded by ligands.
Where the marks go
- Full worked solution (all marking points)3 marks