Given the balanced equation representing a reaction:

2HCl(aq) + Na2S2O3(aq)--> S(s) + H2SO3(aq) + 2NaCl(aq)

Decreasing the concentration of Na2S2O3(aq) decreases the rate of reaction

because the

(1) activation energy decreases

(2) activation energy increases

(3) frequency of effective collisions decreases

(4) frequency of effective collisions increases

According to the collision theory , the number of collisions that take place per unit volume of the reaction mixture is called collision frequency. The effective collisions are ones which result into the formation of products.

For the given reaction :

[tex]2HCl(aq)+Na_2S_2O_3(aq)\rightarrow S(s)+H-2SO_3(aq)+2NaCl(aq[/tex]

According to collision theory , the rate at which a reaction takes place is proportional to rate of collision of reactants i.e [tex]HCl[/tex] and [tex]Na_2S_2O_3[/tex] in the given case.

Thus according to collision theory, with decrease in concentration of reactants , the rate of reaction also decreases. This is because if the concentration of reactants decreases , the chances of effective collision between molecules also decreases. This decreases the reaction rate.