S2O82+2r2SO42+I2 It can be speeded up considerably by the presence of either iron(II) ions or iron(III) ions in the solution. a) What type of catalysis is this? b) Why is the reaction so slow in the absence of a catalyst? c) Use equations to help you to explain what happens in the presence of iron(II) ions. Nickel catalyses the hydrogenation of a carbon-carbon double bond. For example, with ethene: CH2=CH2+H2NiCH3CH3 Draw a series of labelled diagrams to show the mechanism for this reaction, showing clearly the role of adsorption