Problem 1 - Chemistry An Atoms First Approach

Question

Consider an equilibrium mixture of four chemicals \((\mathrm{A}, \mathrm{B}, \mathrm{C}\) and \(\mathrm{D},\) 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 \(\mathrm{D}\) to the flask. How does the concentration of each chemical compare to its original concentration after equilibrium is reestablished? Justify your answer.

Step-by-Step Solution

Answer

a. After adding more A and reestablishing equilibrium: - The concentration of A will be lower than its original concentration. - The concentration of B will be lower than its original concentration. - The concentration of C will be higher than its original concentration. - The concentration of D will be higher than its original concentration. b. After adding more D and reestablishing equilibrium: - The concentration of A will be higher than its original concentration. - The concentration of B will be higher than its original concentration. - The concentration of C will be lower than its original concentration. - The concentration of D will be lower than its original concentration.

Step 1: Understanding Le Châtelier's Principle

Le Châtelier's Principle states that when a change is applied to a system at equilibrium, the system will shift its equilibrium position to counteract the change. In this case, we have a closed flask where the chemicals A, B, C, and D are in equilibrium. When we disturb this equilibrium by adding more of one chemical, the reaction will respond by shifting its position either towards the reactants or the products.

Step 2: Analyze the effect of adding more A to the flask

a. When more A is added to the flask, the equilibrium is disturbed, and the system will try to counteract this change. In this case, the reaction will shift towards the products (C and D) to consume the excess A. As the reaction shifts towards the products, the concentration of A will decrease, and the concentration of B will also decrease since it reacts with A. Concurrently, the concentration of the products C and D will increase. After equilibrium is reestablished: - The concentration of A will be lower than its original concentration. - The concentration of B will be lower than its original concentration. - The concentration of C will be higher than its original concentration. - The concentration of D will be higher than its original concentration.

Step 3: Analyze the effect of adding more D to the flask

b. Now, let's consider the initial setup at equilibrium, and we add more D to the flask. The equilibrium is again disturbed, and the system will try to counteract this change. In this case, the reaction will shift towards the reactants (A and B) to consume the excess D. As the reaction shifts towards the reactants, the concentration of D will decrease, and the concentration of C will also decrease since it reacts with D. Concurrently, the concentration of the reactants A and B will increase. After equilibrium is reestablished: - The concentration of A will be higher than its original concentration. - The concentration of B will be higher than its original concentration. - The concentration of C will be lower than its original concentration. - The concentration of D will be lower than its original concentration.

Key Concepts

Le Chatelier's Principle

Le Chatelier's Principle is fundamental in understanding how a chemical reaction at equilibrium responds to various changes. Imagine a seesaw in balance; this is akin to a reaction in equilibrium. If you add more weight to one side, the system will adjust to regain its balanced state. In chemical terms, when you introduce changes such as adding reactants or products, increasing pressure, or changing the temperature, the reaction will shift in a direction that 'opposes' the change. This allows the system to reach a new equilibrium position where the forward and reverse reactions occur at the same rate once again.

Equilibrium Shift

An 'equilibrium shift' is simply the reaction's response to re-establish a state of balance after a change. For instance, if you add more of a reactant to a closed system, the seesaw tips, and the equilibrium will shift forward, toward producing more products. Conversely, if a product is added, the equilibrium will shift toward the reactants. This shift is how the system mitigates the imposed change and restores equilibrium. A helpful way to visualize this is by thinking of a dynamic dance, where the dancers (reactants and products) adjust their steps to maintain the rhythm of the dance (equilibrium), regardless of changes in the music (external conditions).

Reaction Concentration Changes

When the concentrations of reactants or products in a reaction change, it's like altering the number of dancers on the dance floor. Using Le Chatelier's Principle, we can predict the outcome: adding more reactants 'invites' more products to form. This increase in reactants causes the reaction to shift towards the products, and the concentrations adjust accordingly. On the flip side, adding more product leads to a 'retreat' of reactants, shifting equilibrium back to compensate for the excess. The reaction continually works towards a balanced dance, adjusting the concentration of substances to maintain harmony.

Closed System Reactions

Closed system reactions are ones that occur in a sealed environment, with no exchange of substances with the outside. Because nothing 'enters' or 'leaves' the dance floor, the total number of particles remains constant. However, within this closed space, reactions can proceed forward and reverse as needed to achieve equilibrium in response to added or removed reactants or products.

Imagine a chemical dance happening in a ballroom with locked doors; as conditions change inside (such as adding a reactant), the dancers must adapt their movements (concentrations) to ensure the dance (reaction) reaches a new rhythmical balance (equilibrium).