Energy Change Due to Equilibrium

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Definition of Equilibrium

When there is no change in the concentration of the reactants as well as the products with time, then that point is called the chemical equilibrium. When we say equilibrium, it means the reactants are going to products where the rate of forward reaction (reactant to products) is equal to the rate of backward reaction (products to reactions). We can also say the rate at which the reactants turn into products is equal to the rate at which products turn into reactants. Equilibrium is represented with a symbol ⇌.

Reactants ⇌ Products

There are many examples of chemical equilibrium around us. One example is a bottle of a fizzy drink. The bottle has the liquid of dissolved carbon dioxide into it. There is also CO2 gas present in the space between the liquid and bottle cap. There is a constant movement of CO2 from the liquid to the gas phase, and from gas to the liquid phase. However, if you look at the bottle, there does not appear to be any change. This is the point at which the system has reached chemical equilibrium (where the rate of the forward reaction is equal to the rate of backward reaction).

Equilibrium- Key Points

The State of Equilibrium is the one in which there is no net change in the concentration of reactants and products, but this doesn’t mean the reactions have stopped. At equilibrium, the forwards and backward reactions continue but at the identical rate.

  • The concentration of reactants represented as [Reactant]

  • The concentration of products represented as [Products]

At Equilibrium, the ratio of [Reactants] and [Products] are constant and this constant is known as Equilibrium Constant and represented with the symbol Keq.

For Example: consider this below chemical reaction:

A ⇌ B

Where A is the Reactant and B is a product

Forward reaction: A → B, where A goes to form B and it has some rate constant kf (the rate at which the reaction is taking place). The forward rate equation will look like this,

Ratef = kf [A], here rate of reaction is dependent on the concentration of reactant to form the product B.

Backward reaction: B → A, where B goes to form A and it has some rate constant kr (the rate at which the reaction is taking place). The backward reaction will look like this,

Rater = kr [B],

So, based on the definition of equilibrium:

The rate at which the product is formed = rate at which the reactant is formed      

kf [A] = kr [B]  (rate of the forward reaction is equal to the rate of backward reaction)

       = Constant

Equilibrium condition can be achieved from either direction, either going from reactants to the product or going back from products to reactants.

Graphical Representation

The concentration of Products & Reactants at Equilibrium

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In the above graph, prior to equilibrium, the concentration of the products is increasing, and the concentration of the reactants is decreasing. Rates of forward reactions and reverse reactions can be seen as equal in equilibrium. Therefore, there is no change in the concentration of reactants and products.

Rate of Forward & Backward reactions over time and at Equilibrium

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Over time, rates of the forward reactions and the reverse reactions become equal and due to this, the reaction system can be seen at equilibrium. 

Br₂(l) ↔ Br₂(g)

Energy Changes Due to Equilibrium:

1. Change in Concentration: 

For example, any reaction mixture is at equilibrium and you add some more reactant. Then according to Le Chatelier’s principle, the reaction will change itself in order to counteract the change. Therefore, if reactants are added, then the reaction will shift towards the products to form more products and if products are added, then the reaction will shift towards the reactant.

2. Change in Pressure: 

This can be achieved by changing the volume of the container. For example, if we mechanically decrease the volume of a container (of gases), then the pressure inside the container will increase. Since the change we made was to increase the pressure inside the container, the reaction will drift in a way to decrease the pressure. This could be attained with few gas molecules, by moving to the edge of the reaction.

3. Addition of an Inert Gas: 

Adding Inert gases like (Argon, Neon Krypton) to the reaction mixture at constant volume, it has no effect. At the constant volume with the addition of inert gas to the system, total pressure will be changed but the partial pressure of compounds will remain the same.

4. Change in Temperature: 

A temperature change occurs when the temperature is increased or decreased by the flow of heat. This shifts the chemical equilibria towards the product or the reactant, which can be determined with the study of the reaction and also deciding whether the reaction is endothermic or is exothermic.

  • For Exothermic reactions, the Equilibrium constant decreases when the temperature increases.

  • For Endothermic reactions, the Equilibrium constant increases when the temperature increases.

5. Effect of Catalyst on Equilibrium

By adding a catalyst to a reaction, the energy of activation of both forward and backward reactions are lowered. Thereby, both forward and backward reactions increase in the same amount and thus, the equilibrium remains unaffected. Catalysts are basically compounds which accelerate the rate of the reaction without being consumed.

FAQ (Frequently Asked Questions)

Q1. What Do You Mean By Equilibrium?

Ans: The rates of the forward and reverse reactions are equal and because the concentrations of the reactants and products remain constant, there is no observable change in the properties of the system. This states that the forward reaction proceeds at the same rate as the reverse reaction.

Q2. What are the Types of Equilibrium?

Ans: Following are the types of equilibrium:

Homogeneous Equilibrium- In the homogeneous equilibrium, all substances (reactants and products) are in the same state, most likely in the gas phase. In a liquid solution, the reaction between solutes belongs to only one type of homogeneous equilibria. Molecules or ions or the mixture of both can be chemical species. For example, in the below reaction, all components are in the aqueous phase.

C₂H₂(aq) + 2Br₂(aq) ↔ C₂H₂Br₄(aq)

Heterogeneous Equilibrium- In heterogeneous equilibrium, substances (reactant and products) are found in two or more phases. The phases can be of any combination like liquid and gas phase, solid, liquid, or gas phase.