Consider the hypothetical reaction: A + B + heat ⇌ C + D
(All species are in the gaseous phase)
What will happen to the concentration of A, B, C, and D under each of the following conditions?
a) A catalyst is added to the system, which is at equilibrium
b) Either C or D is added to the system, which is initially at equilibrium
c) Either C or D is removed from the system, which is initially at equilibrium
d) Either A or B is added to the system, which is initially at equilibrium
e) The system, which is initially at equilibrium, is cooled
f) The system, which is initially at equilibrium, is heated
a) When a catalyst is added to the system, the catalyst increases the rate of both the forward and reverse reactions, but it does not affect the equilibrium concentrations of A, B, C, and D. Therefore, there will be no change in the concentrations of A, B, C, and D.
b) If either C or D is added to the system initially at equilibrium, according to Le Chatelier's principle, the equilibrium will shift in the direction that minimizes the added substance. So, the equilibrium will shift to the left, resulting in an increase in the concentration of A and B, and a decrease in the concentration of C and D.
c) If either C or D is removed from the system initially at equilibrium, the equilibrium will shift in the direction that replaces the removed substance. So, the equilibrium will shift to the right, resulting in a decrease in the concentration of A and B, and an increase in the concentration of C and D.
d) If either A or B is added to the system initially at equilibrium, according to Le Chatelier's principle, the equilibrium will shift in the direction that minimizes the added substance. So, the equilibrium will shift to the left, resulting in an increase in the concentration of C and D, and a decrease in the concentration of A and B.
e) If the system initially at equilibrium is cooled, the equilibrium will shift in the exothermic direction to compensate for the removal of heat. In this case, the equilibrium will shift to the left, resulting in an increase in the concentration of A and B, and a decrease in the concentration of C and D.
f) If the system initially at equilibrium is heated, the equilibrium will shift in the endothermic direction to compensate for the addition of heat. In this case, the equilibrium will shift to the right, resulting in a decrease in the concentration of A and B, and an increase in the concentration of C and D.
To understand what happens to the concentration of each species under different conditions, we need to apply Le Chatelier's principle. This principle states that when a system at equilibrium is subjected to a change, it will adjust to counteract the change and establish a new equilibrium.
a) When a catalyst is added to the system at equilibrium, it does not affect the concentration of any species. A catalyst increases the rate of both the forward and reverse reactions equally but does not shift the position of the equilibrium.
b) If either C or D is added to the system at equilibrium, the equilibrium will shift to the left. According to Le Chatelier's principle, an increase in the concentration of the products (C or D) will cause the reaction to produce more reactants (A or B) to counteract the change.
c) If either C or D is removed from the system at equilibrium, the equilibrium will shift to the right. Removing a product will cause the reaction to produce more of that product to compensate for the loss.
d) If either A or B is added to the system at equilibrium, the equilibrium will shift to the right. Adding more reactants will cause the reaction to produce more products to restore equilibrium.
e) If the system at equilibrium is cooled, the equilibrium will shift in the exothermic direction, which is the forward reaction (A + B + heat ⇌ C + D). Heat is considered a product in this reaction. According to Le Chatelier's principle, decreasing the temperature will favor the endothermic direction to compensate for the loss of the heat. Therefore, the concentration of A and B will increase, while the concentration of C and D will decrease.
f) If the system at equilibrium is heated, the equilibrium will shift in the endothermic direction, which is the reverse reaction (C + D ⇌ A + B + heat). Heat is considered a reactant in this reaction. Increasing the temperature will favor the exothermic direction to compensate for the increase in heat. As a result, the concentration of A and B will decrease, while the concentration of C and D will increase.
Remember that these effects on the concentration of each species will continue until a new equilibrium is established to offset the changes introduced.