Question

Consider the reaction

$A\left(g\right)+B\left(g\right)\rightleftharpoons C\left(g\right)+D\left(g\right)$

A friend asks the following: “I know we have been told that if a mixture of A, B, C, and D is at equilibrium and more of A is added, more C and D will form. But how can more C and D form if we do not add more B?” What do you tell your friend?

Short answer

You need to explain without adding of B how more C and D can form.

You can explain this question with the help of La Chatelier’s principle.

Step-by-step solution

Equilibrium constant for the reaction

According to the question the reaction is,

$A\left(g\right)+B\left(g\right)\rightleftharpoons C\left(g\right)+D\left(g\right)$

You can write the equilibrium constant as

$K=\frac{\left[C\right]\left[D\right]}{\left[A\right]\left[B\right]}$

Where, K = equilibrium constant;

$\left[A\right]$= concentration of A at equilibrium;

$\left[B\right]$= concentration of B at equilibrium;

$\left[C\right]$= concentration of C at equilibrium;

$\left[D\right]$= concentration of D at equilibrium;

La Chatelier’s principle

According to this principle, if a change is done on a system at equilibrium, the system will shift its equilibrium in such a direction to nullify the change.

Explanation of the answer

For this equilibrium,

$A\left(g\right)+B\left(g\right)\rightleftharpoons C\left(g\right)+D\left(g\right)$

If you add more A, then the equilibrium will favored in the forward direction that is in the right hand side.

You can increase the concentration of C and D in two ways…

1. You can add A or B to the reaction mixture;

2. You can remove C or D from the reaction mixture

Conclusion

According to the La Chatelier’s principle, addition of A or B is sufficient to bring the equilibrium to the forward direction. Because any small change in the concentration of reactants is responsible to re-establish a new equilibrium.

Hence, without adding of B, you can increase the concentration of C and D in the reaction mixture.