Consider the reaction A + B ∆ AB. The vessel on the right contains an equilibrium mixture of A molecules (red spheres), B molecules (blue spheres), and AB molecules. If the stopcock is opened and the contents of the two vessels are allowed to mix, will the reaction go in the forward or reverse direction? Explain.

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Identify the initial conditions in both vessels. The left vessel contains a mixture of A, B, and AB molecules, while the right vessel contains only AB molecules.

Determine the direction of the reaction based on the initial concentrations. The left vessel has a higher concentration of A and B molecules compared to the right vessel.

Apply Le Chatelier's Principle, which states that the system will shift to counteract the change. Since the right vessel has only AB molecules, opening the stopcock will increase the concentration of AB in the left vessel.

Predict the direction of the reaction. The system will shift to reduce the concentration of AB by converting some AB molecules back into A and B molecules.

Conclude that the reaction will proceed in the reverse direction (AB -> A + B) when the stopcock is opened, as the system seeks to re-establish equilibrium.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Chemical Equilibrium

Chemical equilibrium occurs when the rates of the forward and reverse reactions are equal, resulting in constant concentrations of reactants and products. In the given reaction A + B ⇌ AB, the system reaches equilibrium when the formation of AB from A and B occurs at the same rate as the decomposition of AB back into A and B. Understanding this concept is crucial for predicting how changes in conditions affect the reaction.

Le Chatelier's Principle

Le Chatelier's Principle states that if a dynamic equilibrium is disturbed by changing the conditions, the system will adjust to counteract the change and restore a new equilibrium. In this scenario, opening the stopcock allows the mixing of reactants and products, potentially shifting the equilibrium position depending on the concentrations of A, B, and AB in the combined vessel.

Concentration Changes

Changes in concentration of reactants or products can influence the direction of a chemical reaction. When the stopcock is opened, the concentrations of A and B may decrease while the concentration of AB may increase, depending on the initial amounts in each vessel. This shift in concentration can drive the reaction either forward or backward, depending on the relative amounts of each species present.

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Related Practice

Textbook Question

Consider the interconversion of A molecules (red spheres) and B molecules (blue spheres) according to the reaction A ∆ B. Each of the series of pictures at the right represents a separate experiment in which time increases from left to right:(b) What is the value of the equilibrium constant Kc for the reaction A ∆ B?

Textbook Question

The reaction A2 + B ∆ A + AB has an equilibrium con-stant Kc = 2. The following pictures represent reaction mix- tures that contain A atoms (red), B atoms (blue), and A2 and AB molecules.

(a) Which reaction mixture is at equilibrium?

Textbook Question

The following pictures represent the initial state and the equilibrium state for the reaction of A₂ molecules (red) with B atoms (blue) to give AB molecules. (a) Write a balanced chemical equation for the reaction.

Textbook Question

The following pictures represent the composition of the equi- librium mixture for the reaction A + B ∆ AB at 300 K and at 400 K.

Is the reaction exothermic or endothermic? Explain using Le Châtelier's principle.

Textbook Question

The following picture represents the composition of the equi- librium mixture for the endothermic reaction A2 ∆ 2 A at 500 K. Draw a picture that represents the equilibrium mixture after each of the following changes.(b) Increasing the volume

Textbook Question

The following picture represents the equilibrium state for the reaction 2 AB ∆ A2 + B2. Which rate constant is larger, kf or kr? Explain.