Quick questions on Transition Metals (Topic 14) - Edexcel A-Level Chemistry

Transition metals catalyse reactions in two ways: - Homogeneous (catalyst and reactants in the same phase): the metal uses its variable oxidation states to provide an alternative route, for example $\text{Fe}^{2+}/\text{Fe}^{3+}$ catalysing the reaction between $\text{S}_2\text{O}_8^{2-}$ and $\text{I}^-$. - Heterogeneous (different phase): reactants adsorb onto the metal surface, where bonds weaken and react, then products desorb, for example iron in the Haber process and nickel in hydrogenation.

Ligand substitution replaces one ligand with another, sometimes changing the colour and the coordination number. Adding excess ammonia to pale blue $[\text{Cu(H}_2\text{O})_6]^{2+}$ gives the deep blue $[\text{Cu(NH}_3)_4(\text{H}_2\text{O})_2]^{2+}$; adding concentrated $\text{HCl}$ gives the yellow tetrahedral $[\text{CuCl}_4]^{2-}$ (a change of coordination number from 6 to 4).

Define a transition metal. [1 mark]

Explain why aqueous copper(II) ions are blue. [3 marks]