Applications of Equilibrium Constants

When a chemical process reaches equilibrium, the equilibrium constant (usually represented by the symbol K) provides information on the relationship between the products and reactants. For example, the equilibrium constant of concentration (denoted by K_c) of a chemical reaction at equilibrium can be defined as the ratio of product concentration to reactant concentration increased to their respective stoichiometric coefficients. It is crucial to remember that there are numerous types of equilibrium constants that give relationships in terms of different units between the products and reactants of equilibrium reactions.

What is an Equilibrium Constant?

The equilibrium constant for a chemical reaction can be described as the ratio between the amount of reactant and the amount of product that is used to determine chemical behaviour. The rate constants are constant at a given temperature. The ratio of the forward reaction rate constant to the backward reaction rate constants should be constant, and this is referred to as the equilibrium constant (K_equ).

Equilibrium Constant and Gibbs Free Energy

K denotes the proportional number of products to reactants at equilibrium, whereas Q denotes the ratio at any point in time during the reaction. The direction of the reaction can be determined by comparing the Q value to K. The process’s spontaneity is related to the free energy change. The following is the relationship between G (Gibbs Free Energy), K (Equilibrium Constant), and Q (Reaction Quotient).

• △G < 0 and Qc ˂ Kc or Kp at the start of the reaction: The reaction will continue to produce products.
• △G = 0 and Qc = Kc or Kp at equilibrium and there will be no additional variations in the concentration of the combination.
• △G > 0 and Qc > Kc or Kp after equilibrium: The reaction will proceed in the direction of reactant formation.

K_c is the equilibrium constant measured in moles per litre, while K_p denotes the equilibrium constant determined from partial pressures.

Characteristics of Equilibrium Constant

1. It is reaction-specific and fixed at a steady temperature.
2. A catalyst modifies the rate of forwarding and backward reactions in such a way that the value of the equilibrium constant is unaffected.
3. Changes in concentration, pressure, temperature and inert gases can all have an effect on the equilibrium, favouring either forward or backward reaction but not the equilibrium constant.
4. G0 = -RT ln K_equ is related to the standard free energy.
5. K_equ has varying values at different temperatures for the same reversible reaction.
6. The reverse equilibrium’s equilibrium constant is the reciprocal of the original equilibrium, i.e. K_rev = 1/K_equ.
7. When the stoichiometry of the equilibrium reaction changes, the power of the equilibrium constant changes by the same amount.
8. In the scenario of stepwise multiple equilibria leading to the final products, the net equilibrium constant = product of each stepwise equilibrium constant. As a result, the net equilibrium constant K = K₁ * K₂ * K₃.
9. Equilibrium reactions occur at the same time and produce the same product. The equilibrium constant of the reactions remains constant. Product concentrations will be reduced due to the larger concentration of the common product.

Applications of Equilibrium Constant

Equilibrium Constant For Predicting the Extent of Reaction

The equilibrium constant (Kc) can be used to forecast the extent of a reaction, i.e. the degree to which the reactants disappear. The value of the equilibrium constant indicates the relative quantities of the reactants and products.

• The higher the value of the equilibrium constant (>103), the more forward reaction is preferred, indicating that the concentration of products is substantially higher than that of reactants at equilibrium. This demonstrates that at equilibrium, the concentration of the products is very high, indicating that the reaction is nearly complete. As an example:

H₂(g) + Br₂(g)⇌ 2HBr(g) ⇒ K_c = 5.4×10¹⁸

• The intermediate values of the equilibrium constant (10^-3 to 10³) indicate that the reactant and product concentrations are equivalent. As an example:

Fe₃ (aq) + SCN (aq) ⇌ [Fe(SCN)]₂ (aq) ⇒ K_c = 138 at 298 K

• A low value of the equilibrium constant (<10^-3) indicates that the backward reaction is favoured, implying that the concentration of reactants is significantly greater than that of products, implying that the reaction progresses to a very minor extent in the forward direction. As an example:

N₂ (g) + O₂ (g) ⇌ 2NO (g) ⇒ K_c =4.8 × 10^-31 at 298K

Equilibrium Constant for Predicting the Direction of a Reaction

The equilibrium constant can be used to forecast the reaction’s direction. We require a term called the reaction quotient (Q_c in terms of concentrations or Q_p in terms of partial pressures), which is identical to the equilibrium constant except that the conditions are not in equilibrium. aA + bB ⇌ cC + dD for a balanced reply In comparison to K_c, and in terms of reaction direction:

• The reaction is in equilibrium if Q = K_c. [Where K_c is the equilibrium constant]
• If Q > K_c, Q will tend to decline until it equals K. As a result, the reaction will reverse itself.
• If Q<  K_c, Q will tend to increase until it equals K. As a result, the reaction will go onward.

Other Applications

1. The expression for the equilibrium constant is only valid when the concentrations of the reactants and products have reached a constant value at equilibrium.
2. Its value is independent of the original reactant and product concentrations.
3. It is affected by temperature. It displays a one-of-a-kind value for a balanced equation at a specific temperature.
4. It is equal to the inverse of the equilibrium constant for the forward reaction for the reverse reaction.
5. The equilibrium constant K of a reaction is connected to the equilibrium constant of the corresponding reaction, whose equation is generated by multiplying or dividing the original reaction’s equation by a small integer.
6. It is used to forecast the size of a reaction based on its magnitude.
7. It is used to forecast the reaction’s direction.
8. It is employed in the calculation of equilibrium concentrations.

Sample Questions 

Question 1: Write two factors on which equilibrium constant (Kc) depends.

Answer:

The equilibrium constant is affected by the following factors:

1. The temperature at which the reaction occurs.
2. The amount of reactants present.
3. The nature of the reactants refers to whether the reactants are acids, bases, or other chemical substances.

Question 2: When the equilibrium is reached, what happens to the forward and backward reaction rates?

Answer:

When equilibrium is established, the forward reaction rate equals the backward reaction rate.

Question 3: Which direction will the equilibrium reaction shift in the presence of a catalyst?

Answer:

A catalyst favours the pace of both forward and backward reactions equally. As a result, the existence of a catalyst has no influence on the balance.

Question 4: What is the significance of the term “Dynamic Equilibrium” for chemical equilibrium?

Answer:

An equilibrium stage is defined as the point at which the forward response rate equals the backward reaction rate. At this moment, the number of reactant molecules changing to products and product molecules converting to reactants is the same. Chemical equilibrium is dynamic because it can be attained with the same reactants in identical conditions anywhere with ongoing molecule exchange.