Question

In exoergic reaction, the total mass of product is:
A.More than the total mass of the reactants
B.Less than the total mass of the reactants
C.Equal to the total mass of the reactants
D.None of the above

Answer 1

Exoergic reactions are those reactions in which energy is released in the product side and endoergic reactions are those in which energy has to be supplied in the reactant side for the reaction to occur. Energy is released when mass on the product side is less than on the reactant side.

Complete answer:
In an exoergic reaction, the reactants react with each other on their own without supplying any energy to the reactants. On the product side, heat is released and this happens due to loss of mass. Thus, the mass of product will be less than the mass of the reactants.
Let us see an example
${{M}_{A}}+{{M}_{B}}\to {{M}_{C}}+{{M}_{D}}$
Here, projectile A interacts with molecule B and forms molecule C and outgoing particle D.
Let us apply the law of mass energy conservation,
${{M}_{A}}{{c}^{2}}+{{E}_{A}}+{{M}_{B}}{{c}^{2}}={{M}_{C}}{{c}^{2}}+{{E}_{C}}+{{M}_{D}}{{c}^{2}}+{{E}_{D}}$
Here, energies of projectile A, formed molecule C and outgoing particle D are shown.
Solving the equation further we get,
${{E}_{C}}+{{E}_{D}}-{{E}_{A}}=\left[ \left( {{M}_{A}}+{{M}_{B}} \right)-\left( {{M}_{C}}+{{M}_{D}} \right) \right]{{c}^{2}}$
Now, adding the energies we get the value of Q
$Q=\left[ \left( {{M}_{A}}+{{M}_{B}} \right)-\left( {{M}_{C}}+{{M}_{D}} \right) \right]{{c}^{2}}$
The value of Q depends on the added mass of the projectile and stationary molecule, and outgoing particle and formed molecule.
The Q value of exoergic reaction is positive. For the Q value to be positive, the total mass of the reactants has to be more than the total mass of the products. Thus, it proves that in an exoergic reaction, the total mass of the product is less than the total mass of reactants.
The correct answer to this question is option B, the total mass of product will be less than total mass of the reactants.

Note:
Here, we have proven that the loss of mass is done as heat is rejected in the product side of the reaction, but actually the heat is released in the product side, because the mass is decreased in the product side.