What gives the transition metals their colours, complexes and catalytic power?
Definition and electron configurations of transition elements (including the chromium and copper exceptions), complex ions, ligands and shapes, ligand substitution with colour changes, precipitation reactions, the origin of colour, and catalysis.
An OCR H432 module 5 answer on transition elements: the definition and electron configurations (with the chromium and copper exceptions), complex ions and shapes, ligand substitution with colour changes, precipitation reactions, the origin of colour, and catalysis.
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What this topic is asking
OCR specification point 5.3.1 wants you to define a transition element, write electron configurations (including the chromium and copper exceptions), describe complex ions, ligands, coordination number and shapes, write ligand-substitution and precipitation reactions with their colour changes, explain the origin of colour from d-orbital splitting, and explain catalysis. This is the rich descriptive chemistry that rounds off Module 5.
Definition and electron configurations
Complex ions, ligands and shapes
Common shapes: six-coordinate complexes are octahedral (for example ); four-coordinate complexes are usually tetrahedral (for example ) or square planar (for example cisplatin, which shows cis-trans isomerism).
Ligand substitution
Precipitation reactions
The origin of colour
Examples in context
Example 1. Cisplatin in cancer treatment. The square-planar complex cis- binds to DNA and stops cancer cells dividing; only the cis isomer works, a direct application of complex-ion shape and isomerism.
Example 2. Catalytic converters and the Contact process. Transition metals catalyse reactions either heterogeneously (the iron catalyst in the Haber process and in the Contact process work by surface adsorption) or homogeneously (variable oxidation states allow to act as a catalyst), illustrating both catalysis types.
Try this
Q1. Give the electron configuration of a copper atom. [1 mark]
- Cue. .
Q2. State what is meant by a ligand. [2 marks]
- Cue. A species that donates a lone pair of electrons to a metal ion to form a coordinate (dative) bond.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20204 marksAqueous copper(II) sulfate is treated first with a little aqueous ammonia and then with excess aqueous ammonia. (a) Describe what you would see at each stage. (b) Write the equation for the reaction with excess ammonia.Show worked answer β
(a) With a little ammonia, a pale blue precipitate of copper(II) hydroxide forms (1). With excess ammonia, this dissolves to give a deep blue solution (1).
(b) (1)(1).
Markers reward the pale blue precipitate then deep blue solution, and the ligand-substitution equation with four ammonia ligands replacing four water ligands.
OCR 20223 marksExplain why aqueous transition-metal ions are coloured, and why is a different colour from .Show worked answer β
In a complex ion the ligands split the d-orbitals into two energy levels (1). Visible light is absorbed when a d electron is promoted from the lower to the higher level (a d-d transition); the colour seen is the complementary colour of the light absorbed (1).
Changing the ligand from water to ammonia changes the size of the d-orbital splitting, so a different wavelength is absorbed and a different colour is seen (1).
Markers reward d-orbital splitting, the d-d transition absorbing visible light, and the ligand changing the splitting and hence the colour.
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Sources & how we know this
- OCR A-Level Chemistry A (H432) specification β OCR (2015)