How to write an Equilibrium Constant Expression?

To write the equilibrium expression for a system in equilibrium, you’ll need to know the following:

• the balanced equation of the reaction
• each species in the reaction’s phases (liquid, solid, gas)

Allowing a single reaction to reach equilibrium and then measuring the concentrations of all of the substances participating in that reaction yields the numerical value of an equilibrium constant. The product-to-reactant-concentration ratio is determined. The equilibrium constant for a given reaction remains constant regardless of initial concentrations since concentrations are measured at equilibrium. Scientists were able to create a model expression that may be used as a “template” for any response using this information.

In 1864 Cato Guldberg (1836–1902) and Peter Waage (1833–1900), two Norwegian chemists meticulously analysed the components of several reaction systems at equilibrium. They discovered that any reversible reaction of the general form –

aA + bB ⇌ cC + dD                                         ……1

Here A and B are reactants, C and D are products, while a, b, c, and d are the stoichiometric coefficients of the balanced chemical equation for the reaction.  Under a given set of conditions, the product of the equilibrium of the product concentrations (raised to their coefficients in the balanced chemical equation) and the product of the equilibrium reactant concentrations (raised to their coefficients in the balanced chemical equation) is always a constant. This relationship is described by the law of mass action, which is stated as follows:

K = [C]c[D]d/ [A]a[B]b                                            ……2

Where K is the reaction’s equilibrium constant. Equation 1. is known as the equilibrium equation, while Equation 2 on the right side is known as the equilibrium constant expression. Regardless of the mechanism of the reaction or the number of steps in the mechanism, the relationship stated in Equation 2 holds for any pair of opposing reactions.

THE EQUILIBRIUM CONSTANT

The value of a chemical reaction’s reaction quotient at chemical equilibrium, a state reached by a dynamic chemical system after a period of time in which its composition exhibits no visible tendency to change, is known as the equilibrium constant. The equilibrium constant is independent of the initial measured concentrations of the reactant and product entities in the mixture for a given set of reaction conditions. As a result, given the initial composition of a system, known equilibrium constant values can be utilised to determine the composition of the system at equilibrium. However, reaction factors like temperature, solvent, and ionic strength can affect the value of the equilibrium constant.

Characteristics of the equilibrium constant

• It is reaction specific, and it is fixed at a constant temperature.
• The rate of forward and backward reactions is changed equally by a catalyst; hence the value of the equilibrium constant is not affected.
• Changes in concentration, temperature, pressure, and the presence of inert gases can lead to modification of the equilibrium, which favours either forward or backward reaction but not the equilibrium constant.
• △G0 = -RT ln Keq is related to the standard free energy.
• Kequ has varying values at different temperatures for the same reversible reaction.
• The reverse equilibrium constant is the inverse of the original equilibrium, i.e. Krev = 1/Kequ.
• When the stoichiometry of an equilibrium reaction is changed, the power of the equilibrium constant is likewise changed by the same amount.
• If K is the equilibrium constant for the reaction A + B ⇌C + D, then K3 is the equilibrium constant for the reaction 3A = 3B ⇒ 3C + 3D.
• The equilibrium constant of the net equilibrium = product of each stepwise equilibrium constant in the situation of stepwise numerous equilibria leading to the final products. As a result, K = K1* K2 *K3 is the net equilibrium constant.
• Equilibrium reactions occur at the same time which produce the same product. The reaction’s equilibrium constant does not change. The product concentrations are reduced because of the larger concentration of the product.

Application of equilibrium constant

1. Determining the severity of the reaction

• If Kc is greater than 103, products receive priority over reactants. The reaction almost completes itself if Kc is quite large.
• When Kc is greater than 103, reactants receive priority over products. The reaction does not continue at all if Kc is very tiny.
• If Kc is between 103 and 103, there is a reasonable concentration of both reactants and products.

   2. The concentration ratio, which is the ratio of the product of product concentrations to that of reactants, is also known as the concentration quotient and is indicated by Q.

Concentration quotient, Q=[X] [Y]/ [A] [B]

It’s worth noting that when the reaction reaches equilibrium, Q equals the equilibrium constant (K). Q K = Kc = Kp. Thus, at equilibrium, Q K = Kc = Kp

• The reaction will proceed in the direction of the reactants if Q > K. (reverse reaction).
• If Q < K, the reaction will proceed in the product’s direction (forward reaction).
• The reaction mixture is already at equilibrium if Q = K.

As a result, if Q < K, a reaction has a tendency to generate products, and if Q > K, it has a tendency to form reactants.

CONCLUSION

By studying the equilibrium constant and expression, we can tell whether the reaction has a higher concentration of products or reactants at equilibrium condition.