Which one of the following is not an example of a redox reaction?
- (a)AlCl₃ + 3H₂O → Al(OH)₃ + 3HCl
- (b)2NaH → 2Na + H₂
- (c)4Fe + 3O₂ → 2Fe₂O₃
- (d)CuSO₄ + Zn → Cu + ZnSO₄
Correct — A, AlCl₃ + 3H₂O → Al(OH)₃ + 3HCl. In a redox reaction the oxidation numbers of some elements change. Here aluminium stays +3, chlorine stays −1, hydrogen +1 and oxygen −2 throughout — it is a hydrolysis (double-displacement) reaction with no change in oxidation state, so it is not redox. In (b), (c) and (d) elements do change oxidation number, so those are all redox reactions.
- (b)2NaH → 2Na + H₂ — This is redox — sodium goes from +1 to 0 (reduced) and hydrogen from −1 to 0 (oxidized), so oxidation numbers change.
- (c)4Fe + 3O₂ → 2Fe₂O₃ — This is redox — iron is oxidized from 0 to +3 and oxygen reduced from 0 to −2 (this is rusting).
- (d)CuSO₄ + Zn → Cu + ZnSO₄ — This is a redox displacement — zinc is oxidized from 0 to +2 and copper reduced from +2 to 0.
A redox (reduction-oxidation) reaction involves simultaneous oxidation and reduction, shown by a change in the oxidation numbers of the elements involved. Reactions in which no oxidation state changes — such as hydrolysis, precipitation, or acid-base neutralisation — are not redox.
The test is mechanical: assign oxidation numbers on both sides. Three of these show clear changes (metal displacement, rusting, hydride decomposition); only the hydrolysis of aluminium chloride leaves every element's oxidation number unchanged.
- Redox = oxidation (loss of electrons / rise in oxidation number) plus reduction (gain / fall).
- Hydrolysis of a salt (AlCl₃ + H₂O) keeps all oxidation states fixed, so it is not redox.
- Rusting (Fe + O₂) and metal displacement (Zn + CuSO₄) are common redox reactions.
- Decomposition of a hydride (NaH → Na + H₂) is also redox.
- Assuming every reaction that forms new products is redox — hydrolysis and neutralisation are not.
- Missing that hydrolysis of a salt keeps every oxidation state unchanged.
Identify the non-redox reaction by checking which equation has no change in any element's oxidation number.
No directly related past PYQ was found.
- practice — not a real PYQ
Which one of the following is a redox reaction?
- (a)NaOH + HCl → NaCl + H₂O
- (b)Zn + CuSO₄ → ZnSO₄ + Cu
- (c)AgNO₃ + NaCl → AgCl + NaNO₃
- (d)CO₂ + H₂O → H₂CO₃
Answer(b) Zn + CuSO₄ → ZnSO₄ + Cu — zinc is oxidized and copper reduced; the others have no change in oxidation number.
- practice — not a real PYQ
In the reaction Zn + CuSO₄ → ZnSO₄ + Cu, the reducing agent is
- (a)Cu
- (b)Zn
- (c)SO₄²⁻
- (d)CuSO₄
Answer(b) Zn — it loses electrons (is oxidized) and so reduces copper.