Question

Consider this reaction at equilibrium: $$2 \mathrm{BrNO}(g) \rightleftharpoons 2 \mathrm{NO}(g)+\mathrm{Br}_{2}(g)$$ Predict whether the reaction will shift left, shift right, or remain un- changed after each disturbance. \begin{equation}\begin{array}{l}{\text { a. } \mathrm{NO} \text { is added to the reaction mixture. }} \\ {\text { b. BrNO is added to the reaction mixture. }} \\ {\text { c. } \mathrm{Br}_{2} \text { is removed from the reaction mixture. }}\end{array}\end{equation}

Short answer

a. The reaction will shift left. b. The reaction will shift right. c. The reaction will shift right.

Step-by-step solution

Analyze the Effect of Adding NO

When NO (a product) is added to the reaction mixture, the concentration of NO increases. This disturbs the equilibrium by increasing the amount of product. According to Le Chatelier's principle, the system will react to this change by shifting the reaction to the left to reduce the concentration of NO by converting some back into BrNO.

Analyze the Effect of Adding BrNO

When BrNO (a reactant) is added to the reaction mixture, the concentration of BrNO increases. This disturbs the equilibrium by increasing the amount of reactant. According to Le Chatelier's principle, the system will react by shifting the reaction to the right to produce more products (NO and Br2) from the added BrNO.

Analyze the Effect of Removing Br2

When Br2 (a product) is removed from the reaction mixture, the concentration of Br2 decreases. This disturbs the equilibrium by decreasing the amount of product. According to Le Chatelier's principle, the system will react to this change by shifting the reaction to the right to replace the removed Br2 by converting more BrNO into NO and Br2.

Key concepts

Le Chatelier's Principle

Le Chatelier's Principle is a foundational concept in chemical equilibrium that allows us to predict how a system at equilibrium responds to various disturbances. It states that if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counteract the change. This principle is essential for understanding how reactions adjust in response to changes in concentration, pressure, or temperature.

For example, imagine a seesaw perfectly balanced at its fulcrum. If a weight is added to one side, the balance is disturbed, and the seesaw will tilt to one side. The system will naturally shift to try and regain balance. Similarly, a chemical reaction at equilibrium will 'shift' to reduce the disturbance and re-establish equilibrium, just as the seesaw will tilt to counterbalance the added weight.

Reaction Shift

A 'reaction shift' refers to the change in direction a chemical reaction takes in response to an external disturbance. When a system in equilibrium is disturbed, the reaction can either shift left, towards the reactants, or right, towards the products. A shift to the left means the reaction is generating more reactants and less products, and vice versa for a shift to the right.

In the context of the given exercise, adding a product like NO causes the reaction to shift left, favoring the formation of reactants. Conversely, adding a reactant like BrNO, or removing a product like Br₂, causes the reaction to shift right, producing more products.

Equilibrium Disturbance

Equilibrium disturbance occurs when the conditions of a reaction in equilibrium are altered. This can be due to changes in the concentration of reactants or products, pressure, volume, or temperature. The disturbance disrupts the balance between the forward and reverse reactions. If the system at equilibrium is disturbed, it will undergo a shift to re-establish equilibrium according to Le Chatelier's principle.

It is important to recognize that equilibrium does not mean the reactions stop; rather, it signifies that the rates of the forward and backward reactions are equal. When disturbed, the rates are no longer equal, and the system will adjust by changing the rate of either the forward or the reverse reaction to reach a new state of equilibrium.

Concentration Changes

Changes in concentration of reactants or products play a significant role in disturbing chemical equilibrium. An increase in the concentration of reactants pushes the reaction to produce more products, thereby shifting the equilibrium to the right. On the other hand, increasing the concentration of products pushes the reaction to form more reactants, shifting the equilibrium to the left.

Thinking of a chemical reaction as a marketplace can be helpful. If the supply of a product (like NO) suddenly increases, the market (or reaction) may respond by decreasing production (shifting left) to bring supply and demand back into balance. Similarly, if more of a reactant (like BrNO) is provided, the market will likely respond by increasing production (shifting right), utilizing the additional supply to create more product.