Le Chatelier's Principle: Videos & Practice Problems

Chemical equilibria involve understanding both the equilibrium constant and thermodynamics. While kinetics focuses on the rate of reactions, thermodynamics determines the direction of reaction shifts at equilibrium. According to Le Châtelier's principle, a disturbed system will adjust to restore equilibrium. Key concepts include the equilibrium constant (K), Gibbs free energy (ΔG), and entropy (ΔS). These principles are essential for predicting how changes in concentration, temperature, or pressure affect chemical reactions, enhancing comprehension of dynamic equilibrium in chemical systems.

Thermodynamics

In the study of chemical equilibria, understanding the equilibrium constant is essential as it relates closely to thermodynamics. While chemical kinetics focuses on the rate at which reactants transform into products, thermodynamics examines the direction in which a reaction will shift when at equilibrium. This distinction is crucial; kinetics provides insight into how fast a reaction occurs, whereas thermodynamics reveals the stability and favorability of the products versus the reactants.

Le Chatelier's principle plays a pivotal role in this context. It states that if a system at equilibrium is disturbed by changes in concentration, temperature, or pressure, the system will respond by shifting in a direction that counteracts the disturbance, thereby striving to restore equilibrium. For instance, if the concentration of reactants is increased, the equilibrium will shift to the right, favoring the formation of products. Conversely, if products are added, the equilibrium may shift to the left, favoring the reactants.

To fully grasp the dynamics of chemical reactions, it is important to consider both the rate of reaction and the equilibrium position. This comprehensive approach allows for a deeper understanding of how various factors influence the behavior of chemical systems at equilibrium.

The following is an endothermic reaction where Kc = 6.73 x 103.For each of the choices below predict in which direction the reaction will proceed

Le Chatelier's Principle is a fundamental concept in chemistry that helps predict how a system at equilibrium responds to changes in concentration, pressure, and temperature. Understanding this principle is crucial for analyzing chemical reactions, particularly in the context of endothermic and exothermic processes.

In an endothermic reaction, heat is absorbed, and thus, it is treated as a reactant. This means that if the temperature increases, the reaction will shift to the right, favoring the formation of products, as the system attempts to counteract the added heat. Conversely, decreasing the temperature will shift the reaction to the left, favoring the reactants.

When considering changes in volume and pressure, it is essential to remember that decreasing the volume increases the pressure. According to Le Chatelier's Principle, the reaction will shift toward the side with fewer moles of gas. For example, if a reaction has 7 moles of gas on one side and 8 on the other, decreasing the volume will cause the reaction to shift to the left, where there are fewer moles of gas.

Removing a product from the reaction will also shift the equilibrium to the right, as the system seeks to replace the lost product. Similarly, adding a reactant will push the reaction forward, producing more products to restore balance. The opposite is true when reactants are removed or products are added; the reaction will shift in the reverse direction to re-establish equilibrium.

It is important to note that catalysts do not affect the position of equilibrium; they only speed up the rate at which equilibrium is reached. Additionally, changes involving solids and liquids do not influence the equilibrium position, as they are not included in the equilibrium constant expression.

When an inert gas is added at constant volume, it does not affect the equilibrium position either, as it does not participate in the reaction. Thus, the addition of a catalyst, solid, liquid, or inert gas at constant volume results in no shift in the reaction.

In summary, mastering Le Chatelier's Principle involves understanding how changes in temperature, pressure, and concentration affect the direction of a reaction. By applying these concepts, students can effectively predict the behavior of chemical systems under various conditions.

Consider the reaction below:
CH₄ (g)  +  F₂ (g)  ⇌  CF₄ (g)  + HF (g)   H = + 38.1 KJ/mol
The following changes will shift the equilibrium to the left except one. Which one would not cause a shift to the left?

• A. Add some CF₄.
• B. Remove some F₂.
• C. Decrease the Temperature.
• D. Decrease the container volume.
• E. Increase the partial pressure of HF.

Answer: (D)

The following data was collected for the following reaction at equilibrium 
2 A (g) + 3 B (g)  ⇌ C (g)     
At 25 ^oC K is 5.2 x 10^-4 and at 50 ^oC K is 1.7 x 10^-7. Which of the following statements is true?
a) The reaction is exothermic.
b) The reaction is endothermic.
c) The enthalpy change, ΔH, is equal to zero.
d) Not enough information is given. 

• A. The reaction is exothermic.
• B. The reaction is endothermic.
• C. The enthalpy change, ΔH, is equal to zero.
• D. Not enough information is given.

Answer: (A)

Which direction will the following reaction (in a 10.0 L flask) proceed if a catalyst is added to the system? 
CaCO₃ (s)  ⇌ CaO (s) + CO₂ (g)            K_p = 3.2 x 10^-28

• A. To the right.
• B. To the left.
• C. The equilibrium position will not change but the rate will increase.
• D. The equilibrium position will not change but the concentrations of everything will increase.

Answer: (C)

Consider the following gas reaction of A₂ ( shaded spheres) and B₂ ( unshaded spheres)
A₂ (g)  +  B₂ (g)  ⇌  2 AB (g) 
Which container proceeds more to completion? 

• A. A
• B. B

Answer: (B)

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 system at equilibrium is disturbed, the system will adjust itself to counteract the disturbance and restore a new equilibrium. This principle applies to chemical equilibrium by predicting how changes in concentration, temperature, or pressure will affect the position of the equilibrium. For example, if the concentration of a reactant is increased, the system will shift towards the products to reduce the effect of the added reactant. Similarly, if the temperature is increased for an exothermic reaction, the system will shift towards the reactants to absorb the added heat.

How does a change in concentration affect a system at equilibrium according to Le Chatelier's Principle?

According to Le Chatelier's Principle, changing the concentration of reactants or products in a system at equilibrium will cause the system to shift in a direction that counteracts the change. If the concentration of a reactant is increased, the equilibrium will shift towards the products to consume the added reactant. Conversely, if the concentration of a product is increased, the equilibrium will shift towards the reactants to reduce the concentration of the product. This shift helps the system to re-establish equilibrium.

How does temperature affect chemical equilibrium according to Le Chatelier's Principle?

Temperature changes affect chemical equilibrium based on whether the reaction is exothermic or endothermic. According to Le Chatelier's Principle, increasing the temperature of an exothermic reaction (which releases heat) will shift the equilibrium towards the reactants to absorb the excess heat. Conversely, increasing the temperature of an endothermic reaction (which absorbs heat) will shift the equilibrium towards the products to absorb the added heat. Decreasing the temperature will have the opposite effects, shifting the equilibrium towards the side that releases heat.

What role does pressure play in shifting the equilibrium of a gaseous reaction according to Le Chatelier's Principle?

For gaseous reactions, pressure changes can shift the equilibrium according to Le Chatelier's Principle. If the pressure is increased by decreasing the volume, the equilibrium will shift towards the side with fewer moles of gas to reduce the pressure. Conversely, if the pressure is decreased by increasing the volume, the equilibrium will shift towards the side with more moles of gas. This shift helps the system to counteract the change in pressure and re-establish equilibrium.

How does Le Chatelier's Principle explain the effect of a catalyst on a chemical equilibrium?

Le Chatelier's Principle states that a system at equilibrium will adjust to counteract disturbances. However, the addition of a catalyst does not shift the position of the equilibrium. Instead, a catalyst speeds up the rate at which equilibrium is reached by lowering the activation energy for both the forward and reverse reactions. This means that while a catalyst helps the system reach equilibrium faster, it does not change the concentrations of reactants and products at equilibrium.