Consider the reaction $\mathrm{A}(g)+\mathrm{B}(g) \rightleftharpoons \mathrm{C}(g)+\mathrm{D}(g) . \mathrm{A}$ 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?

Your friend has a valid question about the reaction at equilibrium. To understand how the reaction responds when more A is added, it is crucial to recall Le Chatelier's Principle. Le Chatelier's Principle states that if a chemical system at equilibrium is perturbed, the system will adjust to minimize the effect of the change.

Initially, the system is at equilibrium, which means that the forward and reverse reaction rates are equal. As a result, the concentrations of A, B, C, and D remain constant over time.

When more A is added to the system, the concentration of A increases. According to Le Chatelier's Principle, the system will respond by shifting in the direction that minimizes the effect of this change. In this case, the system will shift towards the right to form more C and D to decrease the concentration of A.

The reaction can proceed without adding more B because the overall reaction is still at equilibrium, and the initial B concentration was high enough to react with the added A. As the reaction proceeds, more C and D are formed, while the concentrations of A and B decrease. This change in concentrations continues until a new equilibrium is established.

After the reaction has proceeded, a new equilibrium state is reached where the forward and reverse reaction rates are again equal, and the concentrations of A, B, C, and D are constant. This new equilibrium will have a higher concentration of C and D than the original equilibrium because A was added. The concentrations of A and B will end up lower as some of their moles have been converted to form C and D. In conclusion, by applying Le Chatelier's Principle, we can understand that the system can still shift toward forming more products (C and D) when more reactant A is added, even without adding more of reactant B, because the reaction will proceed in a way that counteracts the disturbance in the equilibrium and establish a new equilibrium state.