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 \(\mathrm{C}\) and \(\mathrm{D}\) will form. But how can more \(\mathrm{C}\) and \(\mathrm{D}\) form if we do not add more \(\mathrm{B}\) ?' What do you tell your friend?

Le Chatelier's principle states that if a system at equilibrium is disturbed by changing the conditions (e.g., concentration, temperature, or pressure), the equilibrium will shift to counteract the change and reach a new equilibrium state. In this reaction, we have: \(A(g) + B(g) \rightleftharpoons C(g) + D(g)\)

At equilibrium, the concentrations of A, B, C, and D are constant. According to Le Chatelier's principle, if we add more A to the system, the equilibrium will shift to consume the added A, reestablishing the equilibrium. In this case, the equilibrium will shift to the right, forming more C and D.

The reaction quotient, Q, is a measure of the relative amounts of products and reactants present in a reaction at a specific time. For the given reaction, Q is defined as: \(Q = \frac{[C][D]}{[A][B]}\) Where [A], [B], [C], and [D] represent the concentrations of the respective substances at a given time.

At equilibrium, Q equals the equilibrium constant, K. When more A is added to the system, the concentration of A increases, making Q smaller than K. In order to reestablish equilibrium, the reaction needs to progress in the forward direction until Q = K, meaning more C and D will be formed.

When more A is added to the system, the reaction will shift to the right, consuming both A and B to form more C and D. Even though the concentration of B remains unchanged initially, the consumption of B will cause the reaction to progress in the forward direction, ultimately leading to the formation of more C and D. B still plays a crucial role in the formation of C and D, but the increase in A drives the system to consume more B to counteract the disturbance and reestablish equilibrium. In conclusion, when more A is added to the system at equilibrium, the system reacts according to Le Chatelier's principle by shifting to the right, consuming both A and B to form more C and D until the equilibrium is reestablished. Even though more B is not added, it will still contribute to the formation of C and D, as the increase in A drives the reaction to consume more B to restore the balance in the system.