E - Le Chatelier's Principle (Lesson)
Le Chatelier's Principle
In a previous lesson, the concept of chemical equilibrium was described as a dynamic equilibrium, meaning that a chemical system can be described by both forward and reverse reactions. Once equilibrium has been achieved, however, the reactions do not come to a stop; rather, there is no change in the concentrations of all substances involved. Given such a system, it is possible for this condition to be disturbed by several factors. However, a system at equilibrium that has been disturbed will always re-achieve equilibrium. More details on this phenomenon are provided below.
Le Chatelier's Principle
The ideas stated above are essentially a summary of an idea called Le Chatelier's Principle which is a fundamental concept in chemistry that describes how a chemical system at equilibrium responds to changes in conditions. The principle states that if a system at equilibrium is subjected to a change in concentration, temperature, or pressure, the system will shift its position to counteract the change and establish a new equilibrium.
Changes in Concentration
The first type of change discussed is a change in concentration or a change in the amount of a substance present in a system at equilibrium. If the concentration of reactants, is increased, the system will shift the equilibrium position to the right (towards the products) to consume the excess reactants and form more products. Conversely, if the concentration of products is increased, the equilibrium position will shift to the left (towards the reactants) to consume the excess products and form more reactants. It must also be noted that the same effect can be made by removing a substance from a system at equilibrium. For example, if a product is selectively removed from a system at equilibrium, the equilibrium will shift towards the side of the products to reestablish the equilibrium position.
The video linked below illustrates this concept using the equilibrium shown below:
[Co(H2O)6]+2 + 4 Cl- ⇌ [CoCl4]-2 + 6 H2O
Changes in Temperature
In addition to changes in concentration, changes in temperature also disturb a system at equilibrium. In this case, the manner in which a system responds to a change in temperature is dependent upon whether the reaction is exothermic or endothermic in nature. For an endothermic reaction (where heat is absorbed and acts as a reactant), an increase in temperature will favor the endothermic direction meaning the equilibrium position will shift to the right, promoting the formation of more products. In an exothermic reaction (where heat is released and acts as a product), an increase in temperature will favor the exothermic direction meaning the equilibrium position will shift to the left, favoring the formation of more reactants.
An example of this effect can also be seen in the video above as well.
Changes in Pressure
For equilibrium situations that involve gases, a response to changes in pressure can also be observed. If the pressure is increased, the system will shift towards the side with fewer moles of gas to reduce the pressure. Conversely, if the pressure is decreased, the system will shift towards the side with more moles of gas to increase the pressure.
You Try It!
In the following self-assessment activity, determine which direction the equilibrium will shift in various situations. Click on the plus sign to check your answer!
Effects of Catalysts
As discussed in the kinetics module, catalysts have the effect of speeding up a reaction. So, do catalysts also affect systems that are at equilibrium? Shown below is an energy diagram for both a catalyzed and an uncatalyzed chemical reaction:
The activation energies for both the forward and reverse reactions are also indicated in this diagram. It is clearly evident that the activation barrier has been lowered after the addition of a catalyst (red vs. blue line). Furthermore, it can be inferred that the activation barrier for both the forward and reverse reactions has been lowered. What implication does this have in the context of Le Chatelier's principle? The addition of a catalyst does not alter the equilibrium position (i.e. no shift occurs), but rather the addition of a catalyst simply allows equilibrium to be achieved faster.
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