14.3
Le Chatelier's Principle: Changing Concentration
A system at equilibrium is in a state of dynamic balance, with forward and reverse reactions taking place at equal rates. If an equilibrium system is…
A system is in dynamic equilibrium when the rate of the forward reaction equals the rate of the reverse reaction. Though the reaction is still proceeding in both directions, the net concentrations of reactants and products remain constant.
At this point, if a reactant or product’s concentration is changed, for example by adding additional molecule A or by removing some molecule C, the equilibrium is disturbed. Such changes are stresses on the system.
According to Le Châtelier’s principle, when a system at equilibrium is stressed due to a change in concentration, a change in volume or pressure, or a change in temperature, the equilibrium position shifts in a direction to minimize the stress and restore equilibrium.
To understand a system’s response to changes in concentration, consider two connected water tanks, A and B. Initially, the water level in the two tanks is equal, and the system is in equilibrium. Then, water is added to tank B.
This stress raises the water level in tank B and disturbs the equilibrium. To offset this stress, some of the water flows into tank A to restore the equilibrium.
Conversely, if water is removed from tank B, the lowering of the water level also triggers stress.
Consequently, water from tank A flows into tank B to restore the equilibrium.
For a system at equilibrium, the reaction quotient equals the equilibrium constant. Adding more reactants to the equilibrium mixture decreases Q. According to Le Châtelier’s principle, the system minimizes this stress by shifting the equilibrium position towards the products to expend the added reactant and re-establish equilibrium.
Likewise, removing a product from the equilibrium mixture will lead to a similar response in order to restore equilibrium.
In contrast, adding more products increases Q. Thus, the equilibrium position shifts towards the reactants to expend the added product and restore equilibrium.
A change in concentration shifts the equilibrium position without changing the value of K.
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Q1: What happens to an equilibrium system when you add more reactant?
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.
Q2: How does removing a product affect a system at equilibrium?
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.
Q3: What is Le Châtelier's principle and how does it explain concentration changes?
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.
Q4: Why does adding product to an equilibrium mixture cause a leftward shift?
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.
Q5: Does the equilibrium constant change when you alter the concentration of reactants or products?
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.
Q6: How can you predict which direction an equilibrium will shift using the reaction quotient?
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.
Q7: What is the difference between a stress on an equilibrium system and the system's response?
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.