Question

Consider the reaction \(A+B \rightleftharpoons C+D . A\) friend asks the following: "I know we have been told that if a mixture of \(\mathrm{A}, \mathrm{B}, \mathrm{C},\) and \(\mathrm{D}\) is in equilibrium and more A is added, more \(\mathrm{C}\) and \(\mathrm{D}\) 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

When more A is added to the equilibrium mixture without adding more B, the system will adjust to maintain equilibrium according to Le Chatelier's principle. The reaction will shift in the forward direction, consuming the excess A and forming more C and D. Although we do not add more B, the existing B is still able to participate in the reaction, eventually establishing a new equilibrium state with higher concentrations of C and D.

Step-by-step solution

Understand the reaction and equilibrium

In the given reaction, A and B are reactants that combine to form the products C and D: \[A+B \rightleftharpoons C+D\] When this reaction reaches equilibrium, the concentrations of A, B, C, and D become constant, and the forward reaction rate equals the reverse reaction rate.

Introduce Le Chatelier's Principle

Le Chatelier's principle states that if a change is made to a system in equilibrium, the system will adjust in such a way to counteract that change and restore the equilibrium. In this case, by adding more A to the system without changing the amount of B, we are altering the equilibrium concentrations of the reactants and products.

Explain the effect of adding more A

When we increase the concentration of A without adding more B, according to Le Chatelier's principle, the system will try to counteract this change and restore the equilibrium. To do this, the reaction will shift in the direction that consumes the excess A, which is the forward reaction: \[A+B \longrightarrow C+D\] As a result, more C and D will form to maintain the balance of the reaction.

Explain the role of B in the reaction

Even though we are not adding more B, there is still some B present in the equilibrium mixture, which can react with the added A. As more C and D are formed due to the excess A, this will ultimately cause a decrease in the concentration of B. Since the reaction is trying to restore the original equilibrium, it will now shift in the reverse direction to regenerate some of the lost B: \[A+B \longleftarrow C+D\] This back-and-forth shifting will continue until a new equilibrium is established, with higher concentrations of C and D compared to the initial equilibrium state.

Sum up the explanation to your friend

In summary, when more A is added to the equilibrium mixture without adding more B, the reaction shifts in the direction that consumes the excess A, which is the forward direction. This leads to the formation of more C and D. Although we do not add more B, the existing B is still able to participate in the reaction, which eventually results in a new equilibrium state with higher concentrations of C and D. This is in accordance with Le Chatelier's principle, which dictates how the system adjusts to maintain equilibrium in response to external changes.

Key concepts

Le Chatelier's Principle

Le Chatelier's Principle is a fundamental concept in chemistry that explains how equilibrium systems respond to changes. It states that if an external change, such as concentration, pressure, or temperature, is applied to a system in equilibrium, the system will adjust in a way to counteract that change and restore equilibrium.

In our example, when more of reactant A is added to the equilibrium mixture of the reaction \( A + B \rightleftharpoons C + D \), the equilibrium is disturbed. Le Chatelier's principle predicts that the system will try to counter this change by forming more products C and D. This attempt to "use up" the extra A results in a shift of the reaction towards the right, or forward direction.

Thus, Le Chatelier's principle helps us understand how equilibrium reacts to changes, ensuring that systems remain balanced despite external influences.

Reaction Rates

Reaction rates refer to the speed at which reactants are converted into products in a chemical reaction. At equilibrium, the rates of the forward and reverse reactions are equal, meaning that the concentrations of reactants and products remain constant.

In the context of our reaction \( A + B \rightleftharpoons C + D \), the equilibrium is characterized by equal forward and reverse reaction rates. When more A is added, the reaction rate for \( A + B \to C + D \) increases to accommodate the higher concentration of A.

This increase in the forward reaction rate temporarily disrupts the balance, producing more products C and D until a new equilibrium is found. The reaction rates adjust accordingly to maintain the new equilibrium where the forward and reverse reactions are again equal in rate.

Concentration Changes

Concentration changes are crucial in understanding how equilibrium systems react and adjust. In the scenario where more A is added to the mixture, the concentration of A increases, leading to a shift in equilibrium.

Even though B is not added, its existing concentration is still sufficient to react with the additional A. The increase in A's concentration shifts the reaction toward the production of more C and D. Because equilibrium systems tend to balance reactant and product levels, the system "forces" the reaction to move forward to decrease the surplus of A.

As the reaction progresses, both C and D concentrations increase while B concentration decreases, reflecting the consumption of B. Eventually, equilibrium is re-established with new concentration levels.

Equilibrium Systems

Equilibrium systems serve as the basis for understanding how chemical reactions maintain balance. In a reversible reaction like \( A + B \rightleftharpoons C + D \), equilibrium is the state where the formation of reactants and products occurs at the same rate.

Equilibrium systems are dynamic. Even though concentrations of reactants and products remain steady, the individual molecules continuously convert between states. When external changes are imposed, such as adding more A, the equilibrium adjusts dynamically.

This dynamic adjustment reflects the system's tendency to counteract changes, whether they're in concentration, pressure, or temperature. The "goal" is always to return to a state of balance, even if the levels of specific substances change to achieve it.