Question

How does Le Châtelier's principle describe an equilibrium's response to a stress?

Short answer

Le Châtelier's principle states that when a system at equilibrium experiences a stress, such as changes in concentration, pressure, or temperature, it will adjust to counteract the stress and restore equilibrium. For example, an increase in concentration shifts the equilibrium to consume the extra species, while an increase in temperature consumes the extra heat by shifting the equilibrium toward the endothermic reaction. However, a catalyst does not affect the position of the equilibrium but speeds up the rate at which equilibrium is reached, by lowering the activation energy for both the forward and reverse reactions.

Step-by-step solution

1. Understanding Le Châtelier's principle

Le Châtelier's principle helps us predict how a change in conditions will affect the position of equilibrium for a reaction. If an equilibrium system is disturbed by an external stress, the system will adjust to minimize the effect of the stress and restore the equilibrium state. It's important to note that the principle does not affect the equilibrium constant but instead shifts the position of equilibrium.

2. Effect of change in concentration

If the concentration of a species involved in a reaction is increased, the system will shift the position of equilibrium to consume the extra species and restore the equilibrium. Conversely, if the concentration of a species decreases, the system will shift the equilibrium position to produce more of that species, counteracting the decrease.

3. Effect of change in pressure

For reactions involving gases, changes in pressure can also affect the position of equilibrium. Increasing the pressure of a gas will cause the system to shift its position to reduce the overall pressure, favoring the side with fewer moles of gas. Conversely, decreasing pressure will cause the system to shift towards the side with more moles of gas, increasing the pressure.

4. Effect of change in temperature

When the temperature changes, it affects the equilibrium by favoring either the exothermic or endothermic direction of the reaction. If the temperature increases, the system will consume the extra heat by shifting the equilibrium toward the endothermic reaction (the side that absorbs heat). If the temperature decreases, the system will counteract the loss of heat by shifting the equilibrium toward the exothermic reaction (the side that releases heat).

5. Effects of catalysts on equilibrium

A catalyst does not affect the position of the equilibrium but instead speeds up the rate at which the equilibrium is reached. It does this by lowering the activation energy of both the forward and reverse reactions, allowing the reaction to proceed faster in both directions without favoring one over the other. In conclusion, Le Châtelier's principle helps us understand how an equilibrium system responds to various stresses, shifting its position to counteract these changes and reestablish equilibrium.

Key concepts

Chemical Equilibrium

Chemical equilibrium is a state in a reversible chemical reaction where the forward and backward reactions occur at the same rate. This means the concentration of reactants and products remain constant over time. It's like a balanced seesaw where neither side wins. However, any disturbance can affect this balance, leading to shifts in the reaction's equilibrium position. Understanding equilibrium is crucial for predicting how a system will respond to changes in conditions, as described by Le Châtelier's principle.

Reaction Shift

A reaction shift refers to the movement of a chemical reaction towards the products or reactants in response to a change in the system. This shift aims to counteract the applied change and restore equilibrium. Le Châtelier's principle explains that adding stress to a system, like concentration changes or temperature fluctuations, will cause the system to "shift". For example, if more reactants are added, the system shifts toward products to minimize that change, helping reestablish equilibrium.

Concentration Changes

Concentration changes impact the position of equilibrium by either supplying more of a reactant or removing some. When a concentration change occurs:

• Adding more of a reactant pushes the reaction towards products to reduce the added concentration.
• Removing a reactant causes the system to produce more of it by shifting the balance towards the reactants.

These adjustments help the system maintain its equilibrium while adapting to the concentration change.

Pressure Effects

Changes in pressure predominantly affect reactions involving gases. When pressure is altered:

• Increasing pressure shifts the equilibrium towards the side with fewer moles of gas, reducing total pressure.
• Decreasing pressure favors the side with more moles of gas, increasing volume and pressure.

These shifts demonstrate the system's attempt to counteract the pressure change and maintain equilibrium, according to Le Châtelier's principle.

Temperature Effects

Temperature changes affect chemical equilibrium by favoring either the exothermic or endothermic side of the reaction. When temperature changes:

• Higher temperatures favor endothermic reactions, as these absorb heat, shifting equilibrium in that direction.
• Lower temperatures favor exothermic reactions, which release heat, shifting equilibrium towards this side.

These temperature-induced shifts help balance the reaction's heat energy requirements and maintain equilibrium.

Catalysts

Catalysts speed up chemical reactions but do not affect the position of equilibrium. They lower a reaction's activation energy, enabling both the forward and reverse reactions to occur more rapidly:

• A catalyst doesn't change the equilibrium constant or favor one reaction side over the other.
• It only makes it faster for the system to reach equilibrium.

Thus, while catalysts expedite reaching equilibrium, they don't change where the equilibrium lies.