Q4. Give two reasons why transition metals and their compounds make good catalysts, with an example of each.
They can adopt several oxidation states, as iron(III) does when it catalyses the reaction of iodide with persulphate, passing through iron(II) and back; and they form bonds to reactant molecules at their surface, using 3d and 4s electrons, which concentrates and weakens them, as nickel does in hydrogenation (NCERT pages 104 and 114).
Q5. What does NCERT say the UK ‘copper’ and ‘silver’ coins are made of, and what are 1p/2p and newly issued 5p/10p coins made of today?
NCERT: the UK ‘copper’ coins are copper-coated steel and the ‘silver’ coins “a Cu/Ni alloy” (page 114); give that in an exam. Beyond the NCERT text: 1p and 2p coins are copper-plated steel, and 5p and 10p coins have been issued in nickel-plated steel since 2012 (Wikipedia, “Coins of the pound sterling”), so NCERT’s ‘silver’ coin statement is now dated.
Q6. Find the oxidation state of manganese in MnO2 and in the MnO(OH) it becomes in a dry cell, and say whether MnO2 is oxidised or reduced there.
MnO2: x − 4 = 0, +4. MnO(OH): x − 2 − 1 = 0, +3. Manganese falls from +4 to +3, so MnO2 is reduced, as the cathode reaction MnO2 + NH4+ + e− → MnO(OH) + NH3 shows (NCERT Class 12 Chemistry, Unit 2, page 54).