The cause of a redox reaction is the:

Correct — C. A redox reaction is defined by the transfer of electrons between the reactants: one species loses electrons and is oxidised, the other gains them and is reduced.

The two halves are simultaneous. Nothing is oxidised unless something else is reduced in the same reaction, which is why the transfer must be between reactants.

In Zn + CuSO₄ → ZnSO₄ + Cu, zinc gives up two electrons and copper takes them. The oxidising agent takes electrons, the reducing agent gives them.

Why the others are wrong

• (a)
  transfer of neutrons between two reactants — Neutrons sit in the nucleus and do not move in chemical change. Chemistry rearranges electrons — shifting neutrons is nuclear physics, not a redox reaction.
• (b)
  transfer of electrons between two products — The species are wrong. Electrons move between the reactants while the reaction happens, and the products are what is left afterwards, so they cannot be its cause.
• (d)
  exchange of halogens between two reactants — A halogen displacement such as Cl₂ + 2KBr → 2KCl + Br₂ is one instance of redox, not its definition. Most redox reactions involve no halogen at all.

Oxidation and reduction have been defined three times over, each definition wider than the last.

The oldest is about oxygen and hydrogen: oxidation adds oxygen or removes hydrogen. The modern one is about electrons: oxidation is loss, reduction is gain, remembered as OIL RIG.

The most general test is a change in oxidation number — if no atom's oxidation number changes, the reaction is not redox, whatever else it does.

Key facts

• Oxidation is loss of electrons and reduction is gain of electrons.
• Oxidation and reduction always occur together in the same reaction.
• The oxidising agent is itself reduced and the reducing agent is itself oxidised.
• A change in oxidation number is the general test for a redox reaction.

Common traps

• Picking the products option because it also says transfer of electrons.
• Assuming every decomposition reaction is a redox reaction.

No directly related past PYQ was found.