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Factors Affecting Chemical Equilibrium

  • Last Updated : 29 Nov, 2021

When the concentrations of reactants and products do not change over time, they are said to be in a state of equilibrium. The stability of certain observable attributes such as pressure, density, and so on can be used to identify this state. Physical equilibrium is the equilibrium set up in physical processes. Chemical equilibrium is the state of being in equilibrium in a chemical process. 

The temperature, pressure, and concentration of the system are all factors that affect equilibrium. When one of these factors changes, the equilibrium of the system is disrupted, and the system readjusts itself until it returns to equilibrium. The following sections go through some of the most essential aspects that impact equilibria.

Chemical Equilibrium

Chemical equilibrium is the state of a system in which the concentration of the reactant and the concentration of the products do not change over time and the system’s properties do not change. When the rate of the forward reaction equals the rate of the reverse reaction, the system reaches chemical equilibrium.

In chemical equilibrium, the rate of the forward reaction is equal to the rate of the backward reaction. so it refers to the state of a system in which the reactant and product concentrations do not fluctuate over time. 

The system’s properties will no longer vary after that, and it will become constant. The system is considered to be in a state of dynamic equilibrium when the concentrations of the reactants and products do not vary due to equal rates of the forward and reverse reactions.

Factors Affecting Equilibria

French chemists Le-Chatelier and Braun proposed certain generalizations to explain the influence of changes in temperature, concentration, or pressure on the state of an equilibrium system. When one of these parameters changes, the equilibrium of the system is disrupted, and the system readjusts itself until it returns to equilibrium. Le-Chatelier Principle is the name of the generalization. 

Le-Chatelier’s principle might be described as: ”a change in any of the components that determine a system’s equilibrium conditions will move the equilibrium in such a way that the effect of the change is reduced or counteracted”.

The principle is extremely useful for qualitatively forecasting the effect of a change in concentration, pressure, or temperature on an equilibrium system. Both chemical and physical equilibrium is governed by this principle. The temperature, pressure, and concentration of the system are all factors that affect equilibrium. The following section goes through some of the most essential factors that affect equilibria.

Change in Concentration

When the concentration of any of the reactants or products in an equilibrium reaction is altered, the equilibrium mixture’s composition changes in order to minimize the effect of the concentration change. According to the Le-Chatelier principle,

  • The reaction that consumes the substance that is added, reduces the concentration of the reactants or products added.
  • The reaction in the direction that replenishes the substance that is withdrawn reduces the concentration of reactants or products removed.

It may be concluded that increasing the concentration of one or all of the reactant species causes the equilibrium to shift forward, resulting in the formation of more products. When the concentration of one or more of the product species is increased, the equilibrium moves backwards, resulting in the formation of more reactants. So, the composition of the mixture in chemical equilibrium changes when the concentration of the reactant or product is changed.

Change in Temperature

According to Le-Chatelier’s principle if the temperature of an equilibrium system is increased, i.e., when heat is supplied, the equilibrium will move in the direction of the added heat. With an increase in temperature, the equilibrium will move in the direction of an endothermic reaction. The decrease in temperature, on the other hand, will shift the equilibrium in the direction of heat production, favoring exothermic reactions. The effect of temperature on chemical equilibrium is determined by the reaction’s sign of ΔH, according to Le-Chatelier’s principle.

  • As the temperature increases, the equilibrium constant of an exothermic reaction decreases.
  • The equilibrium constant of an endothermic reaction increases as the temperature increases.

The rate of reaction is affected by temperature changes in addition to the equilibrium constant.

Change in Pressure

The volume changes, which causes a change in pressure. Since the total number of gaseous reactants and products is now different, a change in pressure can impact the gaseous reaction. The pressure has no effect on the equilibrium if the number of moles of gaseous reactants and products does not change. The change in pressure in both liquids and solids can be neglected in heterogeneous chemical equilibrium, according to Le- Chatelier’s principle, because the volume of a solution is virtually independent of pressure. As a result, the change in volume affects the equilibrium in the following way:

  • When the pressure is increased, the reaction reverses because the number of moles of gas on the reactant’s side is decreased.
  • When the pressure is reduced, the reaction proceeds forward because the number of moles of gas on the product’s side is reduced.

Change in Volume

Since an increase in pressure results in a decrease in volume, the effect of a change in volume will be exactly the opposite of that of a change in pressure. As a result, the change in volume has the following effect on the equilibrium:

  • When the volume of a gaseous mixture at equilibrium is reduced, the equilibrium moves in the direction of a smaller number of gaseous molecules.
  • When the volume of a gaseous mixture at equilibrium is increased, the equilibrium moves in the direction of a larger number of gaseous molecules.

Effect of a Catalyst

The equilibrium is unaffected by the catalyst. This is due to the fact that the catalyst favours both forward and backward reactions equally. As a result, the forward-to-reverse rate ratio remains unchanged, and there is no net change in the relative number of reactants and products present at equilibrium. Thus, a catalyst has no effect on the equilibrium position.

As a result, a catalyst has no impact on the chemical equilibrium. It simply accelerates a reaction. A catalyst, in general, speeds up both forward and reverse reactions. Thus, the reaction reaches equilibrium more quickly.

In a catalyzed or non-catalyzed reaction, the same number of reactants and products will be present at equilibrium. The presence of a catalyst simply aids the reaction by allowing it to progress through a lower-energy transition state of reactants to products.

Effect of Addition of an Inert Gas

Depending on the conditions, adding inert gas to the equilibrium has the following effects.

  • Addition of inert gas at constant volume- When a constant volume of inert gas is added to the equilibrium system, the total pressure increases. The concentrations of the reactants and products, on the other hand, will remain unchanged. As a result, there will be no effect on the equilibrium under these conditions.
  • Addition of inert gas at constant pressure- When at constant pressure, an inert gas is added to the equilibrium system, the volume increases. As a result, the number of moles per unit volume of various reactants and products will decrease. So, the equilibrium will shift in favour of an increase in the number of moles of gases.

Sample Questions

Question 1: At 27°C and 127°C, the reaction equilibrium constants are 1.6×10-3 and 7.6×10-2, respectively. Is this an endothermic or exothermic reaction?

Answer:

An endothermic reaction’s equilibrium constant increases as the temperature increases. Given that the equilibrium constants have increased with temperature from 1.6×10-3 to 7.6×10-2, so the reaction is endothermic in the forward direction.

Question 2: What is the significance of the term “Dynamic Equilibrium” for chemical equilibrium?

Answer:

An equilibrium stage is defined as the stage where the rate of the forward response equals the rate of backward reaction. The number of reactant molecules converting to products and product molecules converting to reactants is the same at this point. Chemical equilibrium is dynamic because the same equilibrium can be achieved with the same reactants in identical conditions anywhere with continuous molecule exchange.

Question 3: When the equilibrium is reached, what happens to the forward and backward reaction rates?

Answer: 

When equilibrium is reached, the rate of the forward reaction equals the rate of the backward reaction.

Question 4: Which direction will the equilibrium reaction shift in the presence of a catalyst?

Answer:

The rate of the forward and backward reactions is equally favored by a catalyst. As a result, the presence of a catalyst has no effect on the equilibrium.

Question 5: If the temperature of the following reaction is raised, which direction will the equilibrium shift?

N2O4(g) ⇌ 2NO2(g)            ΔH=+57.2

Answer:

This process is endothermic since the given heat of reaction is positive. So, increasing the temperature, according to Le Chatelier’s principle, will shift the equilibrium to the right, producing more NO2.


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