Question 1

What happens to an equilibrium system when you add more reactant?

Accepted Answer

Adding reactant decreases the reaction quotient (Q), causing the equilibrium to shift toward products. This shift consumes the added reactant and re-establishes equilibrium. The system minimizes the stress by expending the extra reactant molecules to restore balance between forward and reverse reaction rates.

Question 2

How does removing a product affect a system at equilibrium?

Accepted Answer

Removing product decreases the reaction quotient, triggering a forward shift in equilibrium. The system responds by shifting toward products to replace the removed molecules and restore equilibrium. This shift consumes reactants and produces more product until balance is reestablished between reaction rates.

Question 3

What is Le Châtelier's principle and how does it explain concentration changes?

Accepted Answer

Le Châtelier's principle states that when an equilibrium system is stressed by a concentration change, the system shifts to minimize that stress and restore equilibrium. The equilibrium position shifts in the direction that counteracts the stress, whether reactants or products are added or removed, without changing the equilibrium constant value.

Question 4

Why does adding product to an equilibrium mixture cause a leftward shift?

Accepted Answer

Adding product increases the reaction quotient (Q) above the equilibrium constant (K), making Q greater than K. The system responds by shifting left toward reactants to consume the excess product and re-establish equilibrium. This leftward shift reduces product concentration back to its equilibrium value.

Question 5

Does the equilibrium constant change when you alter the concentration of reactants or products?

Accepted Answer

No, the equilibrium constant remains unchanged when concentrations are altered. Although the equilibrium position shifts to a new composition, the value of K stays the same. Concentration changes shift equilibrium without affecting the fundamental relationship defined by the equilibrium constant expression.

Question 6

How can you predict which direction an equilibrium will shift using the reaction quotient?

Accepted Answer

Compare the reaction quotient (Q) to the equilibrium constant (K). If Q is less than K, the equilibrium shifts right toward products. If Q is greater than K, the equilibrium shifts left toward reactants. This comparison allows you to predict the direction of shift before the system reaches equilibrium.

Question 7

What is the difference between a stress on an equilibrium system and the system's response?

Accepted Answer

A stress is the change applied to the system, such as adding or removing a reactant or product. The response is the equilibrium shift that occurs afterward to counteract that stress. The system's shift continues until the forward and reverse reaction rates are equal again, restoring dynamic equilibrium.

Principe de Le Châtelier : modification de la concentration

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