Question

Which of the following statements is(are) true? Correct the false statement(s). a. When a reactant is added to a system at equilibrium at a given temperature, the reaction will shift right to reestablish equilibrium. b. When a product is added to a system at equilibrium at a given temperature, the value of \(K\) for the reaction will increase when equilibrium is reestablished. c. When temperature is increased for a reaction at equilibrium, the value of \(K\) for the reaction will increase. d. When the volume of a reaction container is increased for a system at equilibrium at a given temperature, the reaction will shift left to reestablish equilibrium. e. Addition of a catalyst (a substance that increases the speed of the reaction) has no effect on the equilibrium position.

Short answer

True statements: (a) and (e) False statements and corrections: (b) Corrected statement: When a product is added to a system at equilibrium at a given temperature, the value of \(K\) for the reaction remains the same when equilibrium is reestablished. (c) Corrected statement: When temperature is increased for a reaction at equilibrium, the value of \(K\) for the reaction will either increase or decrease depending on whether the reaction is endothermic or exothermic, respectively. (d) Corrected statement: When the volume of a reaction container is increased for a system at equilibrium at a given temperature, the reaction will shift either left or right depending on the number of moles of gases on each side of the balanced chemical equation to reestablish equilibrium.

Step-by-step solution

Statement (a)

Statement: When a reactant is added to a system at equilibrium at a given temperature, the reaction will shift right to reestablish equilibrium. This statement is true. According to Le Châtelier's principle, when a system at equilibrium is disturbed by a change in concentration, the system will adjust its equilibrium position to partially counteract the disturbance. In this case, adding more reactant will cause the reaction to shift to the right (towards the products) to reestablish equilibrium.

Statement (b)

Statement: When a product is added to a system at equilibrium at a given temperature, the value of \(K\) for the reaction will increase when equilibrium is reestablished. This statement is false. The equilibrium constant (\(K\)) is dependent only on the temperature, and it remains the same regardless of the changes in concentrations of reactants or products. Therefore, when a product is added to the system at a constant temperature, the system will adjust its equilibrium position to restore the original value of \(K\). Corrected statement: When a product is added to a system at equilibrium at a given temperature, the value of \(K\) for the reaction remains the same when equilibrium is reestablished.

Statement (c)

Statement: When temperature is increased for a reaction at equilibrium, the value of \(K\) for the reaction will increase. This statement is not necessarily true for all reactions. The effect of temperature on \(K\) depends on the nature of the reaction (exothermic or endothermic). For an exothermic reaction, increasing the temperature will decrease the value of \(K\), while for an endothermic reaction, increasing the temperature will increase the value of \(K\). Corrected statement: When temperature is increased for a reaction at equilibrium, the value of \(K\) for the reaction will either increase or decrease depending on whether the reaction is endothermic or exothermic, respectively.

Statement (d)

Statement: When the volume of a reaction container is increased for a system at equilibrium at a given temperature, the reaction will shift left to reestablish equilibrium. This statement is not always true. The shift in equilibrium upon changing the volume of the system depends on the number of moles of reactants and products in the balanced chemical equation. When increasing the volume, the reaction will shift in the direction that has more moles of gas to reduce the pressure. If there are more moles of gas on the left side, the reaction will shift left, but if there are more moles of gas on the right side, the reaction will shift right. Corrected statement: When the volume of a reaction container is increased for a system at equilibrium at a given temperature, the reaction will shift either left or right depending on the number of moles of gases on each side of the balanced chemical equation to reestablish equilibrium.

Statement (e)

Statement: Addition of a catalyst (a substance that increases the speed of the reaction) has no effect on the equilibrium position. This statement is true. A catalyst speeds up both the forward and reverse reactions by lowering the activation energy of the reaction. It does not change the equilibrium position or the value of the equilibrium constant (\(K\)), but it helps the system reach equilibrium faster.

Key concepts

Le Châtelier's Principle

Understanding Le Châtelier's Principle is essential when studying chemical reactions. It describes how a system at equilibrium responds to changes in concentration, temperature, pressure, or volume. Simply put, if an external condition is altered, the system will adjust in a way that offsets the change. For example, if more reactants are added to a reaction, the system shifts towards the product side, to use up the added reactants and re-establish equilibrium. This concept aids in predicting the behavior of a reaction when subjected to different stresses, ensuring students can anticipate the direction in which a reaction will shift.

Le Châtelier's principle can be demonstrated in a classroom experiment where changing the concentration of reactants or products visibly shifts the color of an indicator, reflecting the reaction shift. Visualizing this principle helps students to better grasp how dynamic and responsive chemical systems are.

Equilibrium Constant (K)

The equilibrium constant, denoted as (\(K\)), is paramount in equilibrium calculations. It quantifies the ratio of the concentrations of products to reactants at equilibrium, raised to the power of their stoichiometric coefficients. Importantly, (\(K\)) does not change with varying concentrations of reactants or products, but it is temperature-dependent. This means that as long as the temperature remains constant, no matter how much you increase or decrease the concentration of reactants or products, (\(K\) will remain the same.

To clarify with an example, imagine a simple chemical reaction where increasing the product's concentration doesn't affect (\(K\)); instead, the system will adjust by shifting to decrease the product concentration and restore equilibrium. This illustrates that while the constants can feel abstract, they show a very consistent and measurable property of reactions.

Reaction Shift

A reaction shift occurs when the equilibrium of a reaction moves towards either the reactants or the products. The direction of the shift is determined by changes in reaction conditions, following Le Châtelier's principle. Adjusting concentrations, temperature, and pressure can induce shifts, with the reaction seeking to counter these changes and maintain equilibrium.

Understanding the conditions that cause a reaction to shift can be valuable for students conducting experiments. For instance, by increasing the reactant concentration, you'd typically expect the reaction to shift towards forming more products. You can't see molecules interacting, but by calculating reaction quotients and comparing them with the equilibrium constant, students can predict and describe the invisible shifts happening within a reaction mixture.

Effects of Temperature on Equilibrium

The effects of temperature on equilibrium are significant and variable. Temperature changes can alter the equilibrium constant, (\(K\)), thereby shifting the position of equilibrium. For endothermic reactions, where heat is absorbed, an increase in temperature leads to a higher value of (\(K\)), which favors product formation. Conversely, for exothermic reactions, which release heat, a higher temperature results in a lower (\(K\)), favoring reactant formation.

These effects are important when considering industrial processes, like the Haber process for synthesizing ammonia, where temperature must be carefully managed to maintain an optimal output of products. Classroom demonstrations that involve temperature changes can make this concept more tangible for students, illustrating real-world applications.

Catalysts and Equilibrium

The role of catalysts in equilibrium is often misunderstood. While catalysts speed up the rate at which equilibrium is reached by lowering the activation energy, they do not change the equilibrium position or the value of the equilibrium constant, (\(K\)). Their ability to expedite both the forward and reverse reactions equally prevents any shift in the balance of reactants and products at equilibrium.

By using a catalyst in a classroom setting, students can observe the quick establishment of equilibrium. This can be particularly useful in slow reactions where the equilibrium would take an impractical amount of time to reach. In these cases, a catalyst can demonstrate the dynamics of equilibrium in a more classroom-friendly timeframe.