Le Chatelier's Principle: Videos & Practice Problems

Le Chatelier's principle states that a system at equilibrium will shift to counteract disturbances, such as changes in concentration, pressure, or temperature. Increasing reactants or decreasing products shifts the equilibrium to the right, producing more products. Conversely, increasing pressure favors the side with fewer moles of gas. Temperature changes affect equilibrium based on the reaction's enthalpy; increasing temperature shifts away from heat, while decreasing temperature shifts towards it. Catalysts do not shift equilibrium but speed up reactions. Understanding these concepts is crucial for predicting reaction behavior in chemical systems.

Le Chatelier's Principle

Le Chatelier's principle states that when a chemical reaction at equilibrium is disturbed, the system will adjust to counteract the disturbance and restore equilibrium. This adjustment can occur through shifts in the reaction direction, either to the left (toward reactants) or to the right (toward products), depending on the nature of the disturbance. The principle applies under constant temperature conditions, focusing on factors such as concentration changes, pressure and volume adjustments, and the introduction of inert gases.

When considering the concentrations of gaseous or aqueous compounds, increasing the concentration of reactants will drive the reaction forward, converting excess reactants into products. Conversely, decreasing the concentration of products will also shift the reaction forward to replenish the lost products. This balancing act illustrates how the system seeks to maintain equilibrium by adjusting the amounts of reactants and products.

In terms of pressure and volume, there is an inverse relationship: increasing pressure decreases volume. To determine the direction of the shift, one must analyze the number of moles of gas on each side of the reaction. For example, if a reaction has 2 moles of gas on the reactant side and 5 moles on the product side, increasing pressure will shift the equilibrium toward the side with fewer moles of gas, which in this case is the reactants.

The introduction of inert gases, typically noble gases, can also affect equilibrium. If an inert gas is added at constant volume, there is no shift in equilibrium because the partial pressures of the reacting gases remain unchanged. However, if the inert gas is added at constant pressure, the volume must expand to accommodate the additional gas, leading to a shift toward the side with more moles of gas, which would be the products in the previous example.

It is important to note that adding a catalyst does not shift the equilibrium position; instead, it increases the rate of both the forward and reverse reactions equally by lowering the activation energy. Thus, the catalyst accelerates the approach to equilibrium without affecting the concentrations of reactants and products at equilibrium.

In summary, understanding how changes in concentration, pressure, volume, and the addition of inert gases influence chemical equilibrium is crucial. The system's response to these changes is aimed at reestablishing equilibrium, and recognizing these patterns will aid in predicting the behavior of chemical reactions under various conditions.

Disturbances and Equilibrium Shifts Example

In the context of an endothermic reaction at equilibrium, understanding how to predict shifts in the reaction is crucial for maintaining balance. According to Le Chatelier's Principle, when a system at equilibrium experiences a disturbance, it will adjust to counteract that disturbance and restore equilibrium.

For instance, if oxygen gas (O₂) is removed from the reaction, which is a product, the system will respond by shifting to the right (forward direction) to produce more O₂. This shift helps to reestablish the equilibrium by compensating for the loss of product.

Conversely, if glucose, another product, is added to the system, the equilibrium will shift to the left (reverse direction) to reduce the concentration of glucose. This shift converts some of the product back into reactants, thereby restoring balance.

When the volume of the container decreases, the pressure increases. In this scenario, the system will shift towards the side with fewer moles of gas to alleviate the pressure. If the reaction has 12 moles of gas on the reactant side and 6 moles on the product side, the equilibrium will shift to the right, favoring the formation of products.

Lastly, if xenon gas is added to the reaction mixture under constant volume, there will be no shift in the equilibrium. This is because the addition of an inert gas does not affect the concentrations of the reactants or products, thus maintaining the equilibrium status.

By consistently applying the principles of Le Chatelier, one can effectively predict the direction of shifts in response to various disturbances in chemical reactions.

Temperature and Equilibrium Shifts

Temperature plays a crucial role in determining the equilibrium status of a chemical reaction, as described by Le Chatelier's principle. When the temperature of a reaction at equilibrium is altered, it disrupts the equilibrium and causes a shift in the reaction's direction. This shift is influenced significantly by the enthalpy change (ΔH) of the reaction, which indicates whether the reaction is exothermic or endothermic.

The equilibrium constant (K) is also temperature-dependent, meaning that changes in temperature can affect both the direction of the shift and the value of K. In an exothermic reaction, heat is released, and thus can be considered a product. Conversely, in an endothermic reaction, heat is absorbed and treated as a reactant.

When the temperature increases, the system shifts away from the heat source. For an exothermic reaction, this means the equilibrium will shift to the left, towards the reactants, resulting in an increase in their concentration and a decrease in the concentration of the products. This can be summarized as:

For exothermic reactions:

Increase in temperature → Shift left (towards reactants)

On the other hand, when the temperature decreases, the system shifts towards the heat. In the case of an endothermic reaction, where heat is a reactant, a decrease in temperature will also cause the equilibrium to shift left, towards the reactants. This results in an increase in the reactants and a decrease in the products. This can be summarized as:

For endothermic reactions:

Decrease in temperature → Shift left (towards reactants)

In summary, understanding whether a reaction is endothermic or exothermic is essential for predicting how temperature changes will affect equilibrium. If the temperature increases, the reaction shifts away from heat, while a decrease in temperature causes the reaction to shift towards heat. This principle is vital for solving problems related to temperature changes in chemical equilibria.

Equilibrium Shift Example

In the given reaction, 1 mole of nitrogen gas (N₂) reacts with 1 mole of oxygen gas (O₂) and 1 mole of bromine gas (Br₂) to produce 2 moles of nitrogen monobromide (N₂Br). The enthalpy change for this reaction is -32.5 kJ, indicating that it is an exothermic process, meaning heat is released as a product.

When considering how various changes affect the equilibrium of this reaction, it is essential to apply Le Chatelier's principle, which states that if a system at equilibrium is subjected to a change, the system will adjust to counteract that change.

Removing N₂ from the reaction will shift the equilibrium to the left to replenish the lost reactant. Similarly, increasing the partial pressure of N₂Br, which means adding more of the product, will also cause the equilibrium to shift left to reduce the excess product.

However, increasing the overall pressure in the container does not directly increase the partial pressures of the individual gases but instead decreases the volume. In this case, the equilibrium will shift toward the side with fewer moles of gas. Since the products side has 2 moles of gas compared to 3 moles on the reactants side, the equilibrium will shift to the right.

Adding more N₂Br is similar to increasing its partial pressure, which will also shift the equilibrium to the left. Conversely, decreasing the temperature will shift the equilibrium toward the side that produces heat, which is the right side in this exothermic reaction. Lastly, decreasing the container volume, akin to increasing the pressure, will again shift the equilibrium to the right due to the fewer moles of gas on that side.

In summary, the changes that do not shift the equilibrium to the left include increasing the overall pressure, adding N₂Br, decreasing the temperature, and decreasing the container volume. These changes will all result in a shift to the right, favoring the formation of products.

Select correct answer(s) that would yield more products in the following reaction.
CH₄(g) + 4 F₂(g) ⇌ CF₄(g) + 4 HF(g)     ΔH = 38.2 kJ/mol
a) increase pressure                          c) add 0.31 moles of F₂                   e) cool down reaction vessel
b) increase temperature                    d) add some Xenon gas                  f) decrease volume

• A. a) and d)
• B. a) and e)
• C. b) and c)
• D. b) and e)
• E. a), d) and e)
• F. b), c), and f)

Answer: (C)

At 26°C K_p = 2.3 x 10^–12 and at 56°C K_p = 3.7 x 10^–5 for the hypothetical reaction. Determine if the reaction is endothermic or exothermic.
AB(s) + 2 B(g) ⇌ 3 BB (g) + C (g)

• A. endothermic
• B. exothermic
• C. enthalpy change equal to zero

Answer: (A)

Here’s what students ask on this topic:

What is Le Chatelier's Principle and how does it apply to chemical equilibrium?

Le Chatelier's Principle states that if a chemical reaction at equilibrium is disturbed, the system will adjust itself to counteract the disturbance and reestablish equilibrium. This principle applies to changes in concentration, pressure, volume, and temperature. For example, if the concentration of reactants is increased, the system will shift towards the products to reduce the added reactants. Similarly, if the pressure is increased by decreasing the volume, the system will shift towards the side with fewer moles of gas. Temperature changes also affect equilibrium; increasing temperature shifts the equilibrium away from the heat source in exothermic reactions and towards the heat source in endothermic reactions.

How does changing the concentration of reactants or products affect the equilibrium position according to Le Chatelier's Principle?

According to Le Chatelier's Principle, changing the concentration of reactants or products will cause the equilibrium to shift in a direction that counteracts the change. If the concentration of reactants is increased, the system will shift towards the products to reduce the excess reactants. Conversely, if the concentration of products is increased, the system will shift towards the reactants to reduce the excess products. This shift helps to reestablish equilibrium by balancing the concentrations of reactants and products.

What is the effect of pressure and volume changes on chemical equilibrium?

Pressure and volume changes affect chemical equilibrium in systems involving gases. According to Le Chatelier's Principle, increasing the pressure (by decreasing the volume) will shift the equilibrium towards the side with fewer moles of gas. Conversely, decreasing the pressure (by increasing the volume) will shift the equilibrium towards the side with more moles of gas. This shift occurs because the system adjusts to minimize the disturbance caused by the change in pressure or volume.

How does the addition of an inert gas affect the equilibrium position of a reaction?

The addition of an inert gas, such as a noble gas, does not affect the equilibrium position of a reaction if it is added at constant volume. This is because inert gases do not react with the components of the reaction and do not change the partial pressures of the reactants or products. However, if an inert gas is added at constant pressure, the volume of the system increases, causing the equilibrium to shift towards the side with more moles of gas to accommodate the increased volume.

How does temperature change affect the equilibrium constant and the direction of the shift in a chemical reaction?

Temperature changes affect both the equilibrium constant (K) and the direction of the shift in a chemical reaction. For exothermic reactions (ΔH < 0), increasing the temperature shifts the equilibrium towards the reactants, decreasing the value of K. For endothermic reactions (ΔH > 0), increasing the temperature shifts the equilibrium towards the products, increasing the value of K. Conversely, decreasing the temperature shifts the equilibrium towards the heat source: towards the products in exothermic reactions and towards the reactants in endothermic reactions. This shift helps to reestablish equilibrium by balancing the heat absorbed or released.