Question

A mixture of magnesium chloride and calcium carbide on heating gives magnesium metal. In this process calcium carbide is:
A.Oxidising agent
B.Reducing agent
C.Neither oxidising or reducing agent
D.Dehydrating agent

Answer 1

Hint: This reaction is a type of redox reaction. This involves the transfer of electrons between compounds. Oxidation and reduction are taking place in these reactions based on the transfer of electrons. Through analysing the changes in the oxidation states of the interacting material, this transition of electrons can be established. So, here we have to find out the role of calcium carbide by analysing the redox reaction happening.

Complete step by step answer:
Let us first understand clearly about redox reactions;
Redox reactions are chemical reactions of oxidation-reduction in which the reactants undergo a transition in their oxidation states. A short form of reduction-oxidation is the word 'redox'. It is possible to break down all redox reactions into two separate processes, a reduction process and an oxidation process.
Now, what is oxidation and reduction?
Oxidation is called the loss of electrons and the resulting rise in the oxidation state of a given reactant. The electron gain and the resulting decrease in a reactant 's oxidation state is called reduction.
However, there are another species called oxidizing agents and reducing agents which is an opposite kind of things, let us learn them too;
Oxidizing agents are called electron-accepting species that appear to undergo a reduction in redox reactions. An electron-donating species that appears to hand over electrons may be referred to as a reducing agent. Such species appear to undergo oxidation.
Now, coming into the question;
The chemical reaction that takes place here is;
$\,MgC{l_2} + Ca{C_2} \to CaC{l_2} + Mg + 2C\,$
$\,MgC{l_2}\,$ is magnesium chloride and $\,Ca{C_2}\,$ is calcium carbide.
The oxidation number of $\,Cl\,$is $\, - 1\,$ as per the rules of calculating oxidation number, here we have two chlorine atoms so the oxidation number of chlorine would be $\, - 2\,$. Since magnesium chloride is a neutral molecule, the oxidation state of magnesium is $\, + 2\,$ as the sum of the oxidation states of neutral molecules is always zero as per the rules. In this reaction, we can see that magnesium has an oxidation state of $\, + 2\,$ in $\,MgCl\,$ which is getting reduced by gaining of electrons to form $\,Mg\,$. This reduction is carried out by calcium carbide. Hence, it is a reducing agent here.
Also, calcium carbide is undergoing oxidation by losing electrons as follows;
Let us analyse calcium carbide here;
The oxidation state of calcium is \[\, + 2\,\] because calcium is present in group two of the periodic table, so we know that it forms $\,2 + \,$ cations in ionic compounds. The oxidation number for ions is equal to the total ion charge. Then we have two carbon atoms here so the oxidation number would be $\, - 2\,$ as the sum of the oxidation number of neutral atoms is $\,0\,$ . So, when this gets converted to carbon the oxidation number changes to $\, - 4\,$ as per the number of valence electrons present in it. As a result, carbon is undergoing oxidation and so it is the reducing agent here.

So, Option B is correct.

Note: It is to be noted that any redox reaction can be broken down into two half-reactions namely the oxidation half-reaction and the reduction half-reaction. Each of them must be balanced in a way that all the electrons are accounted for when writing these half-reactions separately. With the aid of redox reactions, certain metals are isolated from their ores. The smelting of metal sulphides in the presence of reducing agents is one such example.