Question

A reversible reaction is in equilibrium. If a factor is changed which affects it, then
A. The speed of forward and backward reactions increases
B. The speed of forward and backward reactions decreases
C. Only the speed of that reaction increases which nullifies the factor causing an increase in speed
D. No difference

Answer 1

Hint: The behavior of any system at equilibrium follows Le-Chatelier’s principle. According to Le-Chatelier’s principle when in any system, pressure, temperature, and concentration are changed at equilibrium. Then the system will shift in such a direction it will adjust to compensate for the effect of change.

Complete answer:A reversible reaction proceeds in both forward and backward directions. Every reaction tends to reach a state of equilibrium at which the rate of forward and backward reactions are equal. At equilibrium, the overall concentration of reactants and products remains constant.

Le-Chatelier’s principle tells that if any system is disturbed by pressure, concentration, or temperature, then the position of equilibrium shifts to neutralize the change to reestablish a new equilibrium.
The factors that can change the equilibrium position of any system are:
a.Pressure
b.Concentration
c.Temperature

Any system tries to reach an equilibrium state where its energy change will be minimum. There is no tendency for the system to change spontaneously.
Therefore when a reversible reaction is in equilibrium and if a factor is changed which affects that system then only the speed of that reaction increases which nullifies the factor causing the increase in speed according to Le-Chatelier’s principle.

Thus, option (C) is correct.

Note: Le-chatelier’s principle is only useful to identify what happens when the condition of a system is changed in a reaction at equilibrium i.e at equilibrium any change to a system will adjust to compensate for that particular change But it never tells the reason of their changes at the molecular level.