Which of the following oxides can be reduced by hydrogen?

Analyze the question: We need to find which oxide hydrogen can reduce, meaning hydrogen takes the oxygen from the metal oxide and leaves the free metal.

Concept: Hydrogen can only reduce the oxides of metals that are less reactive than hydrogen (those below hydrogen in the reactivity series). Metals above hydrogen hold on to oxygen too strongly, so hydrogen cannot remove it.

Reactivity series (most to least reactive): K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Ag > Au

Check each option:

• A. Aluminium oxide (Al₂O₃): Aluminium is above hydrogen, so it is not reduced by hydrogen.
• B. Magnesium oxide (MgO): Magnesium is above hydrogen, so it is not reduced.
• C. Sodium oxide (Na₂O): Sodium is far above hydrogen, so it is not reduced.
• D. Silver oxide (Ag₂O): Silver is below hydrogen, so it is reduced.

Equation: Ag₂O(s) + H₂(g) → 2Ag(s) + H₂O(l)

Hydrogen is oxidised to water, and silver(I) is reduced to silver metal.

Tip: Remember the rule "Only metals below hydrogen lose their oxygen to hydrogen." Typical examples that WASSCE likes to test are CuO, PbO, Fe₂O₃, and Ag₂O. Oxides of K, Na, Ca, Mg, and Al are never reduced by hydrogen. These need electrolysis (or, for some, a stronger reducing agent like carbon at high temperature).