Question Video: Identifying the Factor that Does Not Affect the Equilibrium Position Chemistry • 10th Grade

Which of the following will never affect the position of an equilibrium? [A] The pressure [B] A catalyst [C] The temperature [D] The concentration of reactants [E] The concentration of products

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Video Transcript

Which of the following will never affect the position of an equilibrium? (A) The pressure, (B) a catalyst, (C) the temperature, (D) the concentration of reactants, or (E) the concentration of products.

Equilibrium position is the ratio of the product and reactant concentrations at equilibrium. To understand which of the conditions will never affect the position of an equilibrium, let’s consider the equilibrium reaction between dinitrogen tetroxide and nitrogen dioxide.

When this reaction is at dynamic equilibrium, the rates of the forward and reverse reactions will be the same and the concentrations of the reactants and products will remain constant. At dynamic equilibrium, this reaction will have a certain equilibrium position depending on the reaction conditions. The dynamic equilibrium will continue and the equilibrium position will remain the same, unless the conditions are changed. If the pressure is increased, the equilibrium will shift towards the side of the reaction that has the fewest moles of gas. So, some of the nitrogen dioxide will be converted into dinitrogen tetroxide. This will continue until a new dynamic equilibrium is established.

In this example, when the pressure was increased, the concentration of the product decreased and the concentration of the reactant increased. So, changing the pressure can change the equilibrium position. As we want to know which of the conditions will never affect to the position of an equilibrium, answer choice (A) cannot be the answer to this question.

Now, let’s consider what would happen if we change the temperature. We can see from the provided positive enthalpy change value that the forward reaction is endothermic, meaning that heat is absorbed from the surroundings during the reaction. The reverse reaction therefore is exothermic, meaning that heat is released to the surroundings during the reaction. Heating the reaction vessel provides energy to the system, which favors the endothermic reaction. So, some of the dinitrogen tetroxide will be converted into nitrogen dioxide until dynamic equilibrium is reestablished.

In this example, when the temperature was increased, the concentration of the product increased and the concentration of the reactant decreased. So, changing the temperature can change the equilibrium position. Therefore, answer choice (C) cannot be the answer to this question.

Now, let’s consider what would happen if the concentration of reactants is increased after dynamic equilibrium has been established. Some of these newly added reactants would turn into products until dynamic equilibrium is reestablished. Once again, the concentrations of the reactant and product changed, changing the equilibrium position. So, answer choice (D) is not the answer to this question.

Now, let’s consider what would happen if we decreased the concentration of the products by removing them from the reaction vessel entirely. In order to reestablish equilibrium, some of the reactants must be converted into products. Thus, changing the concentration of the products will change the equilibrium position. So, answer choice (E) cannot be the answer to this question.

This leaves us with answer choice (B), a catalyst. A catalyst is a substance that increases the rate of reaction. A catalyst speeds up the rate of the forward and reverse reactions to the same degree. So, when a catalyst is added, dynamic equilibrium will be established more quickly, but it will have no effect on the concentrations of the reactants and products at equilibrium. Therefore, adding a catalyst will have no effect on the equilibrium position.

In conclusion, the condition that will never affect the position of an equilibrium is answer choice (B), a catalyst.

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