# Reasoning for redox reactions to be classified as such

Could anyone help me out solving this question regarding redox reactions? I am fairly new to this topic.

Which of the following is a redox reaction?
(1) $$\ce{NaCl + KNO3 -> NaNO3 + KCl}$$
(2) $$\ce{CaCO3 -> CaO + CO2}$$
(3) $$\ce{Mg(OH)2 + 2NH4Cl -> MgCl2 + 2NH4OH}$$
(4) $$\ce{Zn + 2AgCN -> 2Ag + Zn(CN)2}$$

I know that the answer here is option (4) but could someone help me understand why this is so? I see a change in oxidation in the other three equations but it is not redox.

• why do you say it is not redox when you "see" a change in oxidation? – Safdar Faisal Apr 12 at 13:58
• I suspect you do not really know what is an oxidation number – Maurice Apr 12 at 16:22
• never mind @Maurice and Safdar. I finally (after much research and frustration) found out that redox reactions don't depend on the change in the presence of oxygen. you know, indian textbooks are very incomplete in terms of content and you could only get full understanding after watching some khanacademy – HyperFluxx Apr 14 at 7:38

$$\ce{Zn + 2AgCN -> 2Ag + Zn(CN)2}$$
Zinc changes its oxidation state from $$0$$ to $$+2$$, and silver changes its oxidation state from $$+1$$ to $$0$$. So, zinc is reducing agent and silver is oxidising agent.
$$\ce{Zn^0 ->[-2e^-] Zn^{+2}}$$ (oxidation);
$$\ce{2Ag^{+1} ->[+2e^-] 2Ag^0}$$ (reduction).