Question

Which of the following is an example of redox reaction?
A. $NaCl+KN{{O}_{3}}\to NaN{{O}_{3}}+KCl$
B. $Ca{{C}_{2}}{{O}_{4}}+2HCl\to CaC{{l}_{2}}+{{H}_{2}}{{C}_{2}}{{O}_{4}}$
C. $Mg{{(OH)}_{2}}+2N{{H}_{4}}Cl\to MgC{{l}_{2}}+2N{{H}_{4}}OH$
D. $Zn+2AgCN \to 2Ag +Zn{{(CN)}_{2}}$

Answer 1

Hint: In a redox reaction, one of the reactants gets oxidised and the other reactant gets reduced in the product side, which means both oxidation and reduction occur simultaneously.

Complete answer:
In order to answer the following question, we need to know what oxidation number is and what is a redox reaction in chemistry. Now, each element is made up of atoms and inside these atoms, protons, neutrons and electrons are present. However, the nucleus and the protons are at the centre of the atom and due to the heavy mass, it is immobile. Electrons are the only substances that can move around in the orbit as well as jump from one orbit to the other.
Whenever there is a chemical reaction, an atom either loses or gains electrons. Now the net loss or gain of the electrons in an atom, during a chemical reaction, so as to form a chemical bond, is called oxidation number. For example in the formation of NaCl, sodium has +1 oxidation number and chlorine has -1 oxidation number. That is why when they combine we get the formula as NaCl.
In some chemical reactions, we can observe that the oxidation state of the product is more than that of the reactant whereas, in some cases, the oxidation state of products is more than that of the reactant. The first reaction is called reduction and the latter one is called oxidation reaction.
However, if we combine both of the two, there can be reactions where one reactant is getting oxidised and the other one is getting reduced. Since both reduction and oxidation are taking place, we call these reactions redox reactions.
Now, let us come to the question. In option D, we can observe that zinc has oxidation state of 0 in the reactant side and +2 oxidation state in the product side. Silver has an oxidation state of +1 in reactant side and 0 oxidation state in product side. Hence, zinc is oxidised and silver is reduced, which makes it a redox reaction.

So, we get option D as the correct answer.

Note:
It is to be noted that oxidation refers to adding of oxygen or removal of hydrogen, whereas reduction means removing the oxygen or addition of hydrogen.