Q4. Assertion: iron does not rust in dry air. Reason: rusting needs water as well as oxygen. Are both true, and does the reason explain the assertion?
Both are true, and the reason explains the assertion: corrosion of iron occurs in the presence of water and air (NCERT page 57).
Q5. In the rusting of iron, which spot is the anode, and what is oxidised there? Which is the cathode, and what is reduced?
At the anodic spot iron is oxidised, Fe → Fe2+ + 2e−; at the cathodic spot oxygen is reduced, O2 + 4H+ + 4e− → 2H2O. Both spots are on the same piece of iron (NCERT page 57).
Q6. Show that 2Fe2+ + 2H2O + ½O2 → Fe2O3 + 4H+ is balanced, and give the change in iron’s oxidation state.
Fe: 2 = 2; O: 2 + 1 = 3; H: 4 = 4; charge: +4 = +4. Iron goes from +2 to +3, so the ½O2 takes 2 electrons (equation from NCERT Fig. 2.13; check reasoned).
Q7. Why are magnesium blocks, and not copper ones, attached to iron objects that must not corrode?
Magnesium (−2.36 V) is oxidised more easily than iron (−0.44 V), so it corrodes in place of the iron. Copper (+0.34 V) is less easily oxidised than iron and would make the iron the anode, speeding up its corrosion (reasoned from NCERT Table 2.1, page 37).
Q8. How does a coat of paint prevent rusting?
It stops the iron surface from coming into contact with the atmosphere, so water and oxygen cannot reach it (NCERT page 57).
Q9. Why does a carbon dioxide-rich, or otherwise acidic, film of water speed up rusting?
At fixed other activities, more H+ raises the equilibrium oxygen-reduction potential, and dissolved ions increase conductivity (reasoned from NCERT page 57, NCERT Eq. (2.13), page 38, and NCERT Section 2.4). These factors can accelerate corrosion, but emf alone does not determine its rate; oxygen supply and surface films also matter (reasoned).
Q10. Why do iron objects near the sea rust faster?
The water film on them holds dissolved salts. The extra ions make it a better electrolyte, so the cell between the anodic and cathodic spots works more easily (reasoned from NCERT page 57 and NCERT Section 2.4).