Equilibrium Constant Definition

A chemical reaction’s tendency to proceed to completion is described by an equilibrium constant, Keq, which is a variable. It means that all the reactants are converted to products. The point at which the conversion of reactants into products is equal to the conversion of products back into reactants is called equilibrium.

What is Equilibrium Constant?

When the reaction proceeds in the forward direction, from reactants to products it means a large equilibrium constant until almost all the reactants have been converted to products. When the chemical reaction favors the reactants and the reaction proceeds in the opposite direction, the K_eq is less than one.

When the equilibrium constant is 1, it indicates that the reactants and products will be equal at the stage of equilibrium. The equilibrium constant is used by scientists to understand how quickly the equilibrium will be reached, and it will be in favour of reactants or products.

When the equation has reached equilibrium, the constant can be calculated using the ratio of products to reactants. The variable K_eq is used to represent the equilibrium constant, and the equilibrium constant expression defines it.

Equilibrium Constant Equation for a Chemical Reaction

The concentration of products divided by the concentration of reactants describes the equilibrium constant expression when the reaction reaches equilibrium. This expression can be seen below-

K_eq = [C]^c_eq [D]^d_eq / [A]^a_eq [B]^b_eq

In this reaction= aA +bB<=> cC +dD

The concentration of a reactant or product in the reaction is described by each term when the product C and D are produced by chemicals A and B. the number of moles of each chemical is indicated by the lowercase letters.

The brackets around a letter [A], indicate the concentration of each chemical, and the equilibrium constant determined by the concentration if each molecule is denoted by the subscript.

A famous scientist, J. Willard Gibbs studied the energy present in the reaction. He described that the equilibrium constant is directly related to the amount of free energy change that occurred during the reaction.

The free energy change is denoted by ∆G. Gibbs proved that standard free-energy change or ∆G0 occurs in every reaction. The initial concentrations of chemicals govern the total free energy change of each reaction.

Using the equilibrium constant of the equation, the following equation is used to calculate the standard free energy.

∆G⁰ = -RTIn (K_eq)

According to this equation, the standard free energy change is another method to describe the driving forces of reaction, and which way they will proceed. However, we can know whether we will have more reactants or products at the end of a reaction by the equilibrium constant but it does not hint about the time taken to complete the reaction.

This is called the rate constant and it is denoted by a lowercase k. A variety of other equations related to the speed of the reaction are also related to the rate constant. A number of biological reactions need the equilibrium constant, as seen in the examples below.

Equilibrium Constant Examples

i. Ionization of water

Life on earth is based on water. The main reason behind this is, it is a good solvent and it can form hydrogen bonds with other non-water molecules and itself. Due to this ability, water can dissolve and diffuse solutes and also carry an electrical current.

A hydrogen bond is formed by water or H₂O, in which the hydrogen is pulled away from the oxygen, and the hydrogen molecule forms hydrogen ion (H⁺) and hydroxide ion (OH^–) by dissociation. In solution, individual hydrogen protons exist freely and immediately form bonds with the water molecules.

Due to this, hydronium ion, or H₃O⁺ is formed. The equilibrium constant for this reaction is-

K_eq= [H⁺]_eq [OH^–]_eq / [H₂O]_eq

The electrical conductivity of water is used to measure the equilibrium constant of this reaction, which is determined by the concentration of (H₃O⁺). An electrical signal is passed by the hydronium ion in the form of the transfer of electrons. The signal is measured by sensitive electrical equipment.

Thus, using sensitive electrical equipment, the equilibrium constant of water is measured to be 1.8 x 10^-16, which means that the water has much probability of being the reactant H2O instead of becoming the hydronium ion.

ii. Cells, free energy, and the equilibrium constant

The measurement of the equilibrium is done when a reaction is at equilibrium but it does not mean that all reactions are allowed to proceed to equilibrium. Many reactions are constantly resupplied in the cell by various chemicals due to which, the reactions of the cells are kept far from equilibrium.

However, the tendency of these reactants to form products is described by the equilibrium constant. Some reactions are exergonic, which means that when they happen, they release energy and they have high equilibrium constant that describes their tendency to become products.

It is also said that these reactions have a positive change in free energy, which means that they give off energy to reactions around them. Endergonic reactions are other important reactions that requires energy for completion.

A low equilibrium constant of these reactions describes their tendency to remain as reactants. These reactions are coupled in cells and allow the endergonic reactions to takes place.

Many cellular reactions convert ATP to ADP by using the high equilibrium constant of ATP to drive endergonic reactions. The examples include the formation of fatty acids and protein