Question

Which of the following is true at equilibrium?

• A. The reaction stops at equilibrium
• B. The rate of forward reaction is greater than the rate of the reverse reaction.
• C. Amounts of reactants and products are equal.
• D. Amount of reactants and products are constant.

Answer 1

The correct option is D Amount of reactants and products are constant.

At equilibrium, the concentrations (or composition) of all the reactants and products involved remain unchanged i.e., amount of reactants and products do not change with time.
At equilibrium, the rate of forward reaction becomes equal to the rate of backward reaction.


Theory:


Rate of reaction:
The change in concentration of the reactants or the products per unit time is known as the rate of the reaction.
For ex. :-
$A\left(g\right)\leftrightharpoons B\left(g\right)$
For reactant concentration decreases with time $=-\frac{d\left[A\right]}{dt}$
For products concentration increases with time $=+\frac{d\left[A\right]}{dt}$
General reaction:
Consider $A+B\leftrightharpoons C+D$
here $A$ and $B$ are reactants, $C$ and $D$ are products.


In case of reversible reactions $A$ and $B$ combine to form $C$ and $D$,
$C$ and $D$ also combine to form $A$ and $B$
$C+D\leftrightharpoons A+B$
So here $C$ and $D$ are reactants, $A$ and $B$ are products.
These two opposite reactions balance each other. A condition is reached where these two reactions oppose each other they have attained equilibrium.
$A+B\rightleftharpoons C+D$
Forward and backward reaction
Forward reaction:
Forward reaction is a reaction in which products are produced from reactants and it goes from left to right in a reversible reaction.
For ex :-
$A_{\left(g\right)}+B_{\left(g\right)}\leftrightharpoons C_{\left(g\right)}+D_{\left(g\right)}$
$Reactants\leftrightharpoons Products$
Collision Frequency$\propto \left[\frac{n_A}{V}\right]$
Collision Frequency$\propto \left[\frac{n_B}{V}\right]$
Note :- Collisional Frequency is the average rate in which two reactants collide for a given system and is used to express the average number of collisions per unit of time in a defined system.
Backward reaction :
Backward reaction is when reaction goes from products to reactants.
$C_{\left(g\right)}+D\left(g\right)\leftrightharpoons A_{\left(g\right)}+B_{\left(g\right)}$
$Products\leftrightharpoons Reactants$
Collision Frequency $\propto \left[\frac{n_C}{V}\right]$
Collision Frequency$\propto \left[\frac{n_D}{V}\right]$
$R_{f}and{R}_b$:
$A_{\left(g\right)}+B_{\left(g\right)}\rightleftharpoons C_{\left(g\right)}+D\left(g\right)$
Rate of forward reaction $\left(R_f\right)=K_f\left[A\right]\left[B\right]$
Rate of Backward reaction $\left(R_b\right)=K_b\left[C\right]\left[D\right]$
At equilibrium rate of forward equal to rate of backward reaction.