Problem 73 - Introductory Chemistry Atoms First

Question

When a reaction at equilibrium is disturbed by the addition of products, (a) Which way will the reaction shift? (b) After the reaction is done shifting, will the product concentration be the same as before the disturbance, greater than before the disturbance, or less than before the disturbance? Explain your answer. (c) Repeat part (b) but for the reactant concentration.

Step-by-Step Solution

Answer

(a) The reaction will shift to the left (toward the reactants) when products are added due to Le Chatelier's principle. (b) The product concentration will be greater than before the disturbance but less than immediately after the disturbance, as the reaction shifts to consume some of the added products. (c) The reactant concentration will increase compared to before the disturbance, as the reaction shifts to consume products and form reactants. However, it won't return to its original value due to the remaining increase in product concentration.

Step 1: (a) Identifying the shift direction

Le Chatelier's principle states that if a system at equilibrium is subjected to a change in concentration, temperature, or pressure, the equilibrium will shift to counteract the imposed change and restore the equilibrium. In this case, we are increasing the concentration of products. According to Le Chatelier's principle, the reaction will shift, trying to minimize the change by consuming the added products. So, the reaction will shift to the left (in the direction of the reactants).

Step 2: (b) Product concentration after disturbance

After the reaction has shifted to the left (toward the reactants) to counteract the change, some of the added products will be consumed and transformed back into reactants. Therefore, the concentration of the products will be less than it was immediately after the disturbance but greater than before the disturbance. The increase in concentration will not be completely compensated for by the shift, but it will no longer be as high as it was right after the addition of products.

Step 3: (c) Reactant concentration after disturbance

As the reaction shifts to the left, products are consumed, and reactants are formed. Thus, the concentration of reactants increases compared to the equilibrium state before the disturbance. However, the reactant concentration won't typically return to the value it had before the disturbance, as the system will still need to maintain a new equilibrium with the remaining increased product concentration.

Key Concepts

Chemical Equilibrium

Chemical equilibrium occurs when the rate of the forward reaction is equal to the rate of the reverse reaction in a chemical process, resulting in no net change in the concentrations of the reactants and products. This dynamic state is crucial because it's the point at which the system's energy is at a minimum, and the concentrations become constant, though not necessarily equal. In a closed system, this can be represented by a balanced chemical equation. An important characteristic of equilibrium is that it can be disrupted by external changes like temperature, pressure, or concentration, but the system will attempt to restore equilibrium, as per Le Chatelier's principle.

Think of a seesaw perfectly balanced with two children of equal weight. If one child were to suddenly have extra weight added, the seesaw would tilt — this is similar to adding a product or reactant in a chemical equation. The system will then try to compensate, much like how the children might adjust their positions on the seesaw to balance it once again.

Reaction Shift

When the chemical equilibrium in a reaction is disturbed, the system experiences what is called a 'reaction shift.' This is the process by which the reaction adjusts itself to a new equilibrium state. According to Le Chatelier's principle, the direction of this shift will be towards the side (reactants or products) that will counteract the change.

If the concentration of products is increased, the reaction will shift towards the reactants to reduce the product concentration, much like adding more riders on one end of a balanced see-saw causes it to tip, and riders must shift their positions to restore balance. This shift can be either to the left, indicating a favor towards the reactants (as in this example), or to the right, indicating a preference towards producing more products when reactants are added. These shifts are fundamental in understanding how various conditions affect the yield of products in industrial and biochemical processes.

Concentration Changes

Changes in the concentration of reactants or products can dramatically affect the position of chemical equilibrium. When the concentration of a substance on one side of a balanced equation is increased, the system will shift to the opposite side to try and reduce the effect of this change. This shift leads to a change in the concentration of all substances involved in the reaction.

For instance, if more product is added to a system at equilibrium, the reaction shifts to the left, meaning that some of the product is converted back to reactants, thereby increasing the reactant concentration and decreasing the product concentration. However, the change is not entirely reversed to the original starting conditions. The new equilibrium will be established at different concentrations but the ratio of the concentrations, known as the equilibrium constant, remains unchanged unless affected by temperature changes. This concept is crucial for predicting how chemical reactions will behave when subjected to different conditions, and it has wide-reaching applications in chemical engineering, pharmacy, and environmental science.