conc. sulphur 一問

Question

An iron is heated with concentrated sulphuric acid , the gas given off is sulphur dioxide , and the colour of solution formed is yellow

why it is yellow but not green ?
i want the answer with equation and explain, thanks !

Answer 1

Iron can be oxidized to iron(II) or iron(III) under different conditions. If it reacts with the solution of a non-oxidizing acid (e.g. HCl(aq), CH3COOH(aq) and dilute H2SO4(aq)), iron will react to give iron(II) ion. But if it reacts with an oxidizing agent (e.g. heated concentrated sulphuric acid, concentrated or moderately concentrated nitric acid, and chlorine gas), iron will be oxidized to iron(III) ion.

Since the heated concentrated sulphuric acid is an oxidizing agent, the iron (O.N. of Fe = 0) is oxidized to iron(III) ion (O.N. of Fe = +3), while the concentrated sulphuric acid (O.N. of S = +6) is reduced to sulphur dioxide (O.N. of S = +4).
2Fe + 6H2SO4 → Fe2(SO4)3 + 6H2O + 3SO2