Chemical Equilibrium

Definition of Chemical equilibrium refers to a state in a chemical reaction where the concentrations of reactants and products no longer change over time. In this state, the forward and reverse reactions occur at the same rate, resulting in a dynamic balance where the concentrations of all substances involved remain constant.

1.0Introduction

Chemical equilibrium like a seesaw where two kids are perfectly balanced. When the seesaw is level, it's like a chemical reaction at equilibrium. Imagine those kids as reactants and products in a chemical reaction.

At first, one side might be higher (more reactants), and the other side lower (fewer products). But as they play and move, they balance out, just like in a reaction when reactants turn into products and vice versa.

In the same way, in a chemical reaction at equilibrium, the reactants are turning into products at the same rate that products are turning back into reactants. It's a constant back-and-forth, but the overall amounts of reactants and products stay the same.

If you add more of one kid (more reactants) to the seesaw, the seesaw tilts, but then it levels out again as the kids adjust their positions. Similarly, if you change things like concentration, pressure, or temperature in a chemical reaction, the system adjusts to balance out again—just like the kids on the seesaw readjust to keep it level.

Chemical equilibrium is the state in a chemical reaction where the forward and reverse reactions occur at the same rate, resulting in a balanced condition where the concentrations of reactants and products remain constant over time. At equilibrium, there is no net change in the overall amounts of substances involved in the reaction, even though the reactions continue to proceed in both directions.

At equilibrium, the rates of the forward and reverse reactions are equal, but it's important to note that equilibrium doesn't mean that the reactions have stopped. Instead, the rates of the forward and reverse reactions have reached an equilibrium point, where there's no net change in the concentrations of reactants and products.

Which means-

the rate of the forward reaction = rate of the backward reaction

I.e.,  in a chemical reaction,     $\mathrm{aA}+\mathrm{bB}\rightleftharpoons \mathrm{cC}+\mathrm{dD}$ 

                                                           rf    =   rb 

${\mathrm{k}}_{\mathrm{f}}\times \propto \times [\mathrm{A}{]}^{\mathrm{a}}[\mathrm{B}{]}^{\mathrm{b}}={\mathrm{k}}_{\mathrm{b}}\times \propto \times [C{]}^c[\mathrm{D}{]}^{\mathrm{d}}$

2.0Equilibrium Constant  (K_eq)

The equilibrium state is described by the equilibrium constant (K_eq), which is the ratio of the concentrations of products to reactants, each raised to the power of their respective coefficients in the balanced chemical equation. 

The expression for the equilibrium constant depends on whether the reaction is in terms of concentrations (for gases and solutions) or partial pressures (for gases).

3.0Types of Chemical Equilibrium

Chemical equilibrium can manifest in different forms based on the nature of the reacting substances and the conditions under which the equilibrium occurs. Here are a few types:

Homogeneous Equilibrium: 

Involves reactants and products all in the same phase (e.g., all gases, all dissolved in a liquid). For instance, the equilibrium between nitrogen and hydrogen gases forming ammonia:

• ${\mathrm{N}}_2(\mathrm{g})+3{\mathrm{H}}_2(\mathrm{g})\leftrightarrow 2{\mathrm{NH}}_3(\mathrm{g})$       

Other common examples are-

 $2{\mathrm{SO}}_2(\mathrm{g})+{\mathrm{O}}_2(\mathrm{g})\rightleftharpoons 2{\mathrm{SO}}_3(\mathrm{g})$

${\mathrm{CH}}_3\mathrm{COOH}(\mathrm{I})+{\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{OH}(\mathrm{I})\rightleftharpoons {\mathrm{CH}}_3{\mathrm{COOCH}}_2{\mathrm{CH}}_3(\mathrm{I})+{\mathrm{H}}_2\mathrm{O}(\mathrm{I})$

Heterogeneous Equilibrium: 

Involves reactants and products in different phases (e.g., gases reacting with solids or liquids). An example is the equilibrium between a gas and a solid in a closed container. For example-

A reaction When steam reacts with red-hot carbon, it yields hydrogen gas and carbon monoxide gas.

${\mathrm{H}}_2\mathrm{O}(\mathrm{g})+\mathrm{C}(\mathrm{s})\rightleftharpoons {\mathrm{H}}_2(\mathrm{g})+\mathrm{CO}(\mathrm{g})$

4.0Factors affecting Chemical Equilibrium

Here we will discuss about Several factors, which can influence chemical equilibrium:

• Concentration: Changing the concentration of reactants or products can shift the equilibrium. According to Le Chatelier's Principle, if you increase the concentration of a substance, the system shifts to counteract that change, either favoring the forward or reverse reaction to re-establish equilibrium.
• Temperature: Altering the temperature can significantly impact equilibrium. For reactions where heat is a reactant or product, changing the temperature affects the equilibrium position. Increasing temperature can favor endothermic reactions (absorbing heat), while decreasing temperature can favor exothermic reactions (releasing heat).
• Pressure (for gases): For reactions involving gases, changing the pressure can influence equilibrium. Changing the pressure alters the concentrations of reactants and products. According to Le Chatelier's Principle, if the pressure increases, the system shifts to the side with fewer gas molecules to reduce the pressure, and vice versa.
• Catalysts: Catalysts can speed up both the forward and reverse reactions equally, enhancing the rate at which equilibrium is reached. They don't affect the position of equilibrium but help the system reach equilibrium faster.
• Volume (for gases): Changes in the volume of the container can affect the pressure and consequently influence the equilibrium position for gas-phase reactions. If the volume decreases, the pressure increases, and the system shifts to reduce the pressure, and vice versa.

5.0Importance of Chemical Equilibrium

• Reaction Optimization: Understanding equilibrium helps in optimizing reaction conditions for maximum yield of desired products. It aids in achieving the right balance between reactants and products, leading to efficient production processes in industries like pharmaceuticals, agriculture, and manufacturing.
• Environmental Chemistry: Equilibrium concepts help in understanding and predicting the behavior of pollutants and their transformation in the environment. This knowledge is crucial for environmental remediation strategies and pollution control.
• Chemical Analysis: Equilibrium plays a role in analytical techniques like chromatography, where equilibrium between mobile and stationary phases allows separation of components in a mixture.