Question

Consider an equilibrium mixture of four chemicals (A, B, C, and D, all gases) reacting in a closed flask according to the equation: $$\mathrm{A}(g)+\mathrm{B}(g) \rightleftharpoons \mathrm{C}(g)+\mathrm{D}(g)$$ a. You add more A to the flask. How does the concentration of each chemical compare to its original concentration after equilibrium is reestablished? Justify your answer. b. You have the original setup at equilibrium, and you add more D to the flask. How does the concentration of each chemical compare to its original concentration after equilibrium is reestablished? Justify your answer.

Short answer

a. After adding more A, the new equilibrium will have lower concentrations of A and B and higher concentrations of C and D than the original equilibrium. b. After adding more D, the new equilibrium will have higher concentrations of A and B and lower concentrations of C and D than the original equilibrium.

Step-by-step solution

a. Adding more A to the flask

According to Le Chatelier's principle, when more of the reactant A is added, the equilibrium will shift to minimize the disturbance. In this case, the system will move toward consuming more A by producing more C and D, and in the process, reactant B will also be consumed. This shift will occur until a new equilibrium is established. As a result, the concentration of A will decrease from its initial value, while the concentrations of C and D will increase. The concentration of B will decrease as well since it reacts with A. After the new equilibrium is established, the concentrations of A and B will be lower than their original values, while the concentrations of C and D will be higher than their original values.

b. Adding more D to the flask

In this scenario, more of product D is added to the system. Similar to the previous case, the system will adjust according to Le Chatelier's principle to minimize the disturbance. Adding more D will cause the equilibrium to shift in the opposite direction, moving toward the reactants to consume more D. Thus, more A and B will be produced, and in the process, C will also be consumed. As a result, the concentration of D will decrease from its initial value, while the concentrations of A and B will increase. The concentration of C will decrease as well since it reacts with D. After the new equilibrium is established, the concentrations of A and B will be higher than their original values, while the concentrations of C and D will be lower than their original values.

Key concepts

Understanding Chemical Equilibrium

Chemical equilibrium refers to a state in a chemical reaction where the concentrations of reactants and products remain constant over time. This occurs in a closed system where the forward and reverse reactions are happening at the same rate. This means that while the reactions continue to occur, there are no net changes in the concentration of the chemical substances involved. Some key points to remember about chemical equilibrium include:

• It can only be achieved in a closed system where no substances can leave or enter.
• The rate of the forward reaction equals the rate of the reverse reaction.
• Even though concentrations are constant, the system is not static—molecules are continuously reacting and being formed.

Le Chatelier's principle plays a crucial role when disturbances occur in an equilibrium system. It predicts how the system will respond to regain equilibrium.

Exploring Reaction Shifts

Reaction shifts occur when a change in conditions disturbs a system at equilibrium. According to Le Chatelier's Principle, the system will react in a way that counteracts the change. This can mean shifting to the right, favoring the formation of products, or shifting to the left, favoring the formation of reactants. When the concentration of a reactant or product changes:

• Adding a reactant typically causes the system to shift towards the products to decrease the added substance.
• Conversely, adding a product often causes the system to shift towards the reactants.

These shifts help the system reestablish equilibrium under the new conditions. In gaseous reactions, this might include changes in pressure or volume, further affecting equilibrium.

Impact of Concentration Changes

Concentration changes can significantly affect the position of equilibrium in a chemical reaction. By understanding these impacts, one can predict the shifts in the reaction. In Le Chatelier's framework:

• Increasing the concentration of reactants causes the equilibrium to shift towards the products. More reactants will be consumed, producing more products until equilibrium is restored.
• Increasing the concentration of products drives the system to shift back towards the reactants, producing more reactants as the products are consumed.

The system's ability to self-adjust highlights the dynamic nature of chemical reactions at equilibrium.

Characteristics of Gaseous Reactions

Gaseous reactions at equilibrium have unique characteristics due to the gases involved. They are sensitive to changes in pressure and volume, as well as temperature. Key characteristics include:

• Any change in the volume of the container in which the reaction takes place will affect the equilibrium according to the ideal gas law. For instance, decreasing the volume increases the pressure, causing the equilibrium to shift towards the side with fewer moles of gas.
• Since gaseous molecules must collide with each other to react, their concentrations and partial pressures play a significant role in determining the position of equilibrium.
• Temperature changes also affect equilibrium, potentially favoring either the exothermic or endothermic direction of the reaction.

Understanding these concepts helps predict how gaseous reactions will adjust to maintain equilibrium under different conditions.