What is Equilibrium?

If a body is said to be at rest or moving with a uniform velocity, the body is in equilibrium condition. This means that all the forces are balanced in the body. It can be understood with the help of Newton's first law of motion which states that the resultant force on a system is null, where the system remains to be at rest or moves at uniform motion. It is when the rate of the forward reaction is equal to the rate of the backward reaction. 

There are two types of equilibrium: Static equilibrium and Dynamic equilibrium. The static equilibrium is defined as the forces that act on a body horizontally or vertically but the body continues to stay at rest. The dynamic equilibrium is defined as the forces that when the forces acting on a body, the body continues to remain in motion.

Chemical Equation Equilibrium

The chemical reaction is directly proportional to the product of molar concentrations of the products and inversely proportional to the molar concentrations of the reactant at a constant temperature. This chemical reaction is called the equilibrium chemical equation when the concentration of the reactants and products are the same. Chemical equilibrium is a dynamic equilibrium since the rate of forward and backward reactions are the same. At equilibrium, the two opposing reactions occur at equal velocities so there is no net change in the amount of substance involved. 

General Equilibrium Chemical Equation

Let's say a reaction, $\text{aA+bB}\iff \text{cC+dD}$, the equilibrium constant is denoted by Kc where its corresponding formula is defined as ${\text{K}}_{\text{c}}\text{ =}\frac{{\left[\text{C}\right]}^{\text{c}}{\left[\text{D}\right]}^{\text{d}}}{{\left[\text{A}\right]}^{\text{a}}{\left[\text{B}\right]}^{\text{b}}}$

For reactions that are not equilibrium, there is reaction quotient Q which is equal to Kcat equilibrium. The equation mainly depends on the equilibrium constant and equilibrium quotient.

The reversible reactions occur in a closed system which can be considered to be irreversible depending on the reactants and products. An example of a reversible reaction is a solution of dinitrogen tetroxide${\text{N}}_{\text{2}}{\text{O}}_{\text{4}}{}^{}\iff {\text{2NO}}_{\text{2}\left(\text{g}\right)}$

Some Properties of Chemical Equilibrium Equation

1) Pure solids and liquids for a specific reaction at a specific temperature, the equilibrium constant K_c is constant and it only varies with the temperature.

2) The equilibrium constant is unit less. 

3) The reaction should be balanced with coefficients since the lowest possible integer values in order to get the correct value of K

4) The numerator of the equilibrium constant expression is the product of the concentration of the products of the reaction raised to a power equal to the coefficient for this component in the balanced equation of the reaction. 

Now factors that decide the reaction type is when:

K=Q, then the reaction is in equilibrium, K>Q, then the reaction goes in the backward direction, and if K<Q, the reaction goes in a forward direction.

The SI unit of the equilibrium constant is mol/L (Molarity).

Let us take an example of the chemical equation:

CH₄ + H20⇔CO+3H₂. In this given solution, the number of molecules on the reactant side is 2 and the product side is 4. So Δn=4-2. 

The conditions to understand the equilibrium chemical equation with respect to the equilibrium constant is:

1) In the given equation of the chemical equilibrium, if the equilibrium constant value is high (K>1000), then the molar concentrations of the products are high compared to the reactant.

2) In the given equation of the chemical equilibrium, if the equilibrium constant value is low (K<1000), then the molar concentrations of the products are low compared to the reactant.

Other adjustments of the equilibrium constant in the equation of equilibrium Chemical formula for a given solution are reversed, then the value of K is inverted. While performing the calculation of the value of the equilibrium constant, if the coefficients in the balanced equation of a chemical solution are multiplied by a common factor, then the equilibrium constant is raised to the power of the corresponding factor. If the coefficients of the balanced equation are divided by a common factor, then the calculation of the corresponding root of the equilibrium constant is done. If the individual equations are combined, then to solve the equilibrium constant, the equilibrium constants are multiplied for the overall reaction.

Le-Chatelier's Principle

The Le-Chatelier's principle is also known as the Equilibrium law that is defined when a system experiences a disturbance like a change in temperature, pressure, or concentration, the systems tend to react in such a way that it tries to come back to its equilibrium state. 

If there is a change in concentration of the reactants, an increase or decrease, if the increase in the concentration of the reactants then the equilibrium state shift to forward direction and has a forward reaction, if there is an increase in the concentration of the products then there is a shift in the backward direction. If there is a decrease in temperature, then there is a forward reaction and the system becomes exothermic by releasing the heat. Similarly, if there is an increase in temperature, then there is a backward reaction and the system becomes endothermic by absorbing the heat.

Equilibrium Chemical Equations for Gases

The equilibrium constant for the gas-phase equilibria, the equation is a function of the reactants and products of partial pressures. The mathematical equation for this constant is ${\text{K}}_{\text{p}}{\text{=P}}_{\text{C}}{}^{\text{c}}{\text{P}}_{\text{D}}{}^{\text{d}}{\text{/P}}_{\text{A}}{}^{\text{a}}{\text{PB}}^{\text{b}}$ Kp where K represents the equilibrium constant and P represents the partial pressure in the atmosphere.

The relationship between Kp and Kc is ${\text{K}}_{\text{p}}{\text{ =K}}_{\text{c}}{\left(\text{RT}\right)}^{\text{Δn}}$ where $\text{Δn=}\left(\text{c-d}\right)\text{-}\left(\text{a-b}\right)$$\text{Δn=}\left(\text{c-d}\right)\text{-}\left(\text{a-b}\right)$

Application of Chemical Equilibrium Equations

1) Fundamental part of the study in chemical equations, this chemical equilibrium in real life is used to calculate and analyze the hemoglobin and oxygen rate of the body.

2) These chemical equilibrium equations are used to analyze, predict and calculate the rate of hemoglobin and carbon monoxide, also the cooling of food and preservative addition required to add to the food, refrigeration cooling required to keep those food preserved.

3) This is also seen in the bottle of carbonated drink or fizzy cold drink like beverages where when the bottle is shaken the equilibrium gets disturbed and there is a reaction.

Practice Problems

1) Calculate and solve the increase in the ${\text{PCl}}_{\text{3}}{\text{ and Cl}}_{\text{2}}$ concentrations that occur as the following reaction comes to equilibrium if the concentration of PCl5 decreases by 0.042 moles per liter.

${\text{PCl}}_{\text{5}}\iff {\text{PCl}}_{\text{3}}{\text{+Cl}}_{\text{2}}$

2) Assume that the concentrations of H2, I2, and HI can be measured for the following reactions at any moment in time

${\text{H}}_{\text{2 }}{\text{+I}}_{\text{2}}\iff \text{2HI}$

Here the K_c=60. For each of the following given sets of concentrations, verify whether the reaction is at equilibrium. If it is not, predict the direction in which the reaction occurs.

Context and Applications 

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for

• Bachelors in Science (Chemistry)
• Masters in Science (Chemistry)