Question

Consider this reaction at equilibrium. $$ 2 \operatorname{BrNO}(g) \rightleftharpoons 2 \mathrm{NO}(g)+\mathrm{Br}_{2}(g) $$ Predict the effect (shift right, shift left, or no effect) of these changes. (a) adding BrNO to the reaction mixture (b) adding NO to the reaction mixture (c) adding \(\mathrm{Br}_{2}\) to the reaction mixture

Short answer

Adding BrNO shifts the equilibrium to the right; adding NO shifts the equilibrium to the left; adding \(\mathrm{Br}_{2}\) also shifts the equilibrium to the left.

Step-by-step solution

Understand Le Chatelier's Principle

Le Chatelier's Principle states that if an external change is applied to a system at equilibrium, the system adjusts in such a way as to partially counteract the change. This can involve shifting the equilibrium position either to the right, towards products, or to the left, towards reactants.

Analyzing the Addition of BrNO

Adding more BrNO (reactant) will increase the concentration of reactants, which will shift the equilibrium to the right, towards the formation of more products to re-establish equilibrium.

Analyzing the Addition of NO

Adding more NO (product) will increase the concentration of products, which will shift the equilibrium to the left, towards the formation of more reactants to counter the increase in product concentration and re-establish equilibrium.

Analyzing the Addition of \(\mathrm{Br}_{2}\)

Adding more \(\mathrm{Br}_{2}\) (product) will increase the concentration of products, which will shift the equilibrium to the left, towards the formation of more reactants to counter the increase in product concentration and re-establish equilibrium.

Key concepts

Chemical Equilibrium

Chemical equilibrium is a state in a reversible chemical reaction where the rates of the forward and reverse reactions are equal, resulting in no overall change in the concentrations of the reactants and products over time. It's a dynamic balance, not a static one, meaning that molecules continue to react, but at a rate that sustains a constant macroscopic composition.

For the reaction \(2 \operatorname{BrNO}(g) \rightleftharpoons 2 \mathrm{NO}(g)+\mathrm{Br}_{2}(g)\), equilibrium is reached when the rate at which the reactants \(\operatorname{BrNO}\) are converting into the products \(\mathrm{NO}\) and \(\mathrm{Br}_{2}\) is equal to the rate at which the products revert back to \(\operatorname{BrNO}\). Understanding this concept is key when predicting the effect of adding or removing components from the reaction mixture.

Reaction Quotient

The reaction quotient, \(Q\), is a measure used to determine the direction in which a reaction at non-equilibrium will proceed to reach equilibrium. It is calculated similarly to the equilibrium constant, \(K\), using the formula that involves concentrations of the products raised to the power of their stoichiometric coefficients, divided by the concentrations of the reactants raised to their powers. However, unlike \(K\), which is a fixed value at a given temperature, \(Q\) can change as the concentrations of the reactants and products change.

When \(Q\) is compared to \(K\), if \(Q > K\), the system will shift left to reach equilibrium, reducing the concentration of products. Conversely, if \(Q < K\), the system will shift right to increase the concentration of products, moving toward equilibrium. For the provided reaction, adding any of the reactants or products will change the value of \(Q\) and thus the system must adjust to return to equilibrium.

Equilibrium Shift

An equilibrium shift refers to the change in the position of equilibrium as a response to external changes, such as variations in concentration, temperature, or pressure. According to Le Chatelier's Principle, if any of these changes are applied to a mixture at equilibrium, the system will adjust to minimize the effect of the change.

In the exercise, the addition of \(\operatorname{BrNO}\) will shift the equilibrium right (towards products) to decrease the reactant concentration and restore equilibrium. Similarly, adding \(\mathrm{NO}\) or \(\mathrm{Br}_{2}\) increases the product concentration, causing a shift left (towards reactants) to achieve equilibrium. This visual and impactful point of shifting reactions can help students visualize and understand the tangible effects that changes in concentration have on a chemical system in equilibrium.