Heating iron(III) oxide with carbon reduces it to iron, but no amount of heating sodium oxide with carbon will produce sodium metal. Which statement correctly explains this difference, in terms of the tendency of each species to form positive ions?

IGCSE Chemistry 0620: Why Carbon Can Reduce Iron Oxide but Not Sodium Oxide

Step 1: Recall what the reactivity series tells us about ions

A metal’s position in the reactivity series reflects how strongly its atoms tend to lose electrons and form positive ions. Carbon is not a metal, but it can be placed within the series based on this same tendency: carbon sits below sodium (and calcium, magnesium, aluminium) but above iron (and zinc, hydrogen, copper).

Step 2: Apply this to iron(III) oxide

Since carbon has a greater tendency to form positive ions than iron does, carbon atoms can transfer electrons to $\text{Fe}^{3+}$ ions in the oxide, reducing them to iron atoms, while the carbon itself is oxidised (forming carbon monoxide or carbon dioxide). This is exactly why heating iron(III) oxide with carbon (as in the blast furnace) successfully extracts iron.

Step 3: Apply this to sodium oxide

Sodium, however, has a far greater tendency to form positive ions than carbon does. Sodium is more reactive than carbon. This means sodium ions “hold on” to their ionic state much more strongly than carbon can compete with; carbon simply cannot pull electrons away from $\text{Na}^+$ ions to reduce them back to sodium atoms. This is why sodium oxide cannot be reduced by heating with carbon, and why sodium must instead be extracted by electrolysis.

Why the other options are wrong

B: If carbon had a greater tendency to form ions than sodium, sodium oxide reduction would work (just perhaps slowly), but in reality it does not happen at all. The relationship, not the speed, is the issue.
C: This reverses the comparison between sodium and carbon; a metal with a greater tendency to form ions than carbon cannot have its oxide reduced by carbon, not an easier reduction.
D: Carbon genuinely does chemically reduce iron(III) oxide (this is real, well-established chemistry, not merely a temperature effect), and sodium oxide’s resistance to reduction is about reactivity/ion-forming tendency, not melting point.

Final answer

Carbon reduces iron(III) oxide because carbon has a greater tendency than iron to form positive ions, but cannot reduce sodium oxide because sodium has a greater tendency than carbon to form positive ions, option A.