Question

Assertion: The reaction of $50.0ml$ of $0.1M$ sodium thiosulphate and $5.0ml$ of $2M$ $HCl$ will go to completion more quickly than the reaction of $50.0ml$ of $0.05M$ sodium thiosulphate and $5.0ml$ $2M$ $HCl$ .
Reason: The frequency of effective collisions increases with the increase in the concentration of reactants.
$N{a_2}{S_2}{O_3}\left( {aq} \right) + HCl\left( {aq} \right) \to S\left( g \right) + S{O_2}(g) + 2NaCl$
(A) Both assertion and reason are true and reason is the correct explanation of assertion.
(B) Both assertion and reason are true but reason is not the correct explanation of assertion.
(C) Assertion is true but reason is false.
(D) Assertion is false but reason is true
(E) Both assertion and reason are false.

Answer 1

We know that in a chemical reaction, collisions take place between reactant molecules or atoms in order to form bonds. Collisions that lead to the formation of bonds are known as effective collisions. For the effective collisions to occur, collisions must occur with enough speed energy and force to break the bonds of the reactant.

Complete answer: As we already know that an effective collision occurs when the molecules collide with each other with sufficient amount of energy to break the bonds of the reactants and form the products. This energy is understood as the energy of activation. So, Activation energy is the minimum amount of energy required to break bonds and cause a reaction to occur.
The frequency of effective collisions depends upon the following factors: -
1.Size of particles
2.Concentration of reactants
3.Temperature
4.Addition of catalyst
5.Pressure of a gaseous reactant
In this question we will only see how the concentration of reactant will increase the effective collisions taking place in the above reaction.
So, when we increase the concentration of reactants there will be a probability of increase in collisions between the reactants as there will be several reactants offered for collisions within the same quantity of the solution and they will be closer to each other. Hence new bonds can only be formed when the reactants are close to each other in order to share or transfer the electron.
In the assertion part $0.1M$ of $50.0ml$ sodium thiosulphate is greater than $0.05M$ of $50.0ml$ sodium thiosulphate, so it is clear that the frequency of collisions will increase for $0.1M$concentration of sodium thiosulphate and the reaction will go to completion more quickly.
Thus, both assertion and reason are true and the reason is the correct explanation of assertion.

Hence, the correct option is Option A.

Note: The equation given in the question is not balanced, so the correctly balanced equation will be:
$N{a_2}{S_2}{O_3}\left( {aq} \right) + 2HCl\left( {aq} \right) \to S\left( s \right) + S{O_2}(g) + 2NaCl(aq) + {H_2}O(l)$
By increasing the concentration of reactants, we are increasing the frequency of effective collisions and thus also increasing the rate of reaction.