Equilibrium

The concept of Equilibrium is central to both physics and chemistry, explaining how systems achieve a state of balance where opposing forces or reactions act equally. Understanding equilibrium helps students connect real-life processes like chemical reactions, market balance, and even body functions with fundamental scientific laws.

In science, equilibrium represents a state of stability — where measurable properties of a system remain constant over time unless an external influence disturbs them.

1.0What is Equilibrium?

Equilibrium is defined as the condition of a system in which opposing forces or influences are balanced, and there is no net change in the system’s state.

In simple terms, equilibrium means “balance.” It can be mechanical (forces), chemical (reactions), or thermal (heat exchange).

For example:

• A book resting on a table is in mechanical equilibrium.
• A reversible chemical reaction that proceeds equally in both directions is in chemical equilibrium.

2.0Types of Equilibrium

Equilibrium can broadly be classified into two major categories based on the domain of study — mechanical equilibrium and chemical equilibrium.

1. Mechanical Equilibrium

Mechanical equilibrium occurs when the net force and net torque acting on a body are zero. In this state, the body does not accelerate and remains at rest or moves with constant velocity.

Conditions for Mechanical Equilibrium

1. ΣF = 0 → The vector sum of all external forces acting on the body is zero.
2. Στ = 0 → The sum of all torques about any point is zero.

Examples:

• A hanging object supported by a spring.
• A ladder resting against a wall without slipping.
• A car moving at constant speed on a straight road.

2. Chemical Equilibrium

Chemical equilibrium is the state of a reversible chemical reaction in which the rate of the forward reaction equals the rate of the backward reaction.

At this stage, the concentrations of reactants and products remain constant with time, even though both reactions continue at the molecular level.

Dynamic Nature of Chemical Equilibrium

Chemical equilibrium is dynamic, not static. It means the reactions continue to occur, but their effects cancel each other out.

Example: N₂ (g) + 3H₂ (g) ⇌ 2NH₃ (g)

In this Haber process, ammonia formation and its decomposition occur simultaneously at equilibrium.

3.0Characteristics of Equilibrium

• The observable properties (pressure, concentration, temperature) remain constant.
• It is dynamic — reactions or forces continue but counterbalance each other.
• Achieved only in closed systems.
• Depends on external conditions like temperature, pressure, and concentration.
• Can be shifted by altering system parameters (as explained by Le Chatelier’s Principle).

4.0Types of Mechanical Equilibrium

Mechanical equilibrium is further classified into three main types based on the stability of the object:

1. Stable Equilibrium

Static equilibrium occurs when all processes have completely stopped. Imagine a book resting on a table. The force of gravity pulls it down, and the normal force of the table pushes it up. The forces are balanced, the object is stationary, and nothing is moving on a macroscopic or microscopic scale regarding its position.
Example: A pendulum bob at rest at its lowest point.

2. Unstable Equilibrium

A body does not return to its original position but moves further away when slightly displaced.
Example: A pencil balanced on its pointed end.

3. Neutral Equilibrium

A body remains in its new position after being displaced.
Example: A ball on a horizontal table surface.

5.0Law of Equilibrium

In mechanics, a system is in equilibrium when the vector sum of all forces and moments acting on it is zero.

$\sum F_x=0,\sum F_y=0,\sum \tau =0$

In chemistry, the Law of Chemical Equilibrium states that at constant temperature, the ratio of the product of the concentrations of products to that of reactants remains constant.

$K_c=\frac{[Products]}{[Reactants]}$

6.0Understanding Chemical Equilibrium

Chemical equilibrium is the state in a reversible chemical reaction where the rate of the forward reaction equals the rate of the reverse reaction. As a result, the concentrations of reactants and products remain constant over time—they do not change, even though the reaction is still happening.

Reversible Reactions

Most introductory chemistry teaches reactions as one-way streets: reactants turn into products (e.g., baking a cake). However, many reactions are reversible.

In a reversible reaction, the products can react with each other to reform the original reactants. This is denoted by a double arrow:

• Forward Reaction: A and B react to form C and D.
• Reverse Reaction: C and D react to form A and B.

Eventually, the system reaches a point where the speed of the forward reaction matches the speed of the reverse reaction. This is Chemical Equilibrium.

Equilibrium Constant (Keq​)

How do we quantify this balance? Scientists use a value called the Equilibrium Constant, denoted as K_{eq}​ (or often K_c ​for concentration). This number tells us the ratio of products to reactants when the system is at equilibrium.

The Equilibrium Expression

For the general reaction 

, the equilibrium constant expression is:

Where:

• [] denotes the molar concentration (molarity) of the substance.
• Superscripts (a,b,c,d) are the stoichiometric coefficients from the balanced equation.

Interpreting the Value of K

The magnitude of K gives us insight into the position of the equilibrium:

• If K>1: The numerator is larger than the denominator. The equilibrium favors the products. The mixture contains mostly product at equilibrium.
• If K<1: The denominator is larger. The equilibrium favors the reactants. The mixture contains mostly reactant at equilibrium.
• If K≈1: There are roughly significant amounts of both reactants and products.

Note: Pure solids and pure liquids are excluded from the equilibrium expression because their concentrations (densities) do not change.

7.0Le Chatelier’s Principle

Le Chatelier’s Principle explains how a system at equilibrium responds to a change in conditions such as temperature, pressure, or concentration.

Statement:
“When a system at equilibrium is subjected to a change, it adjusts itself in such a way as to minimize that change and restore a new equilibrium.”

Examples:

• Increasing temperature in the Haber process shifts equilibrium to the endothermic direction (favoring reactants).
• Increasing pressure shifts equilibrium towards fewer gas molecules.

8.0Dynamic Equilibrium in Everyday Life

• Evaporation and condensation of water in a closed container.
• Dissolution of salt in water — at equilibrium, undissolved salt and dissolved ions remain constant.
• Body temperature regulation — balance between heat production and loss.

9.0Factors Affecting Chemical Equilibrium

1. Concentration: Changing the concentration of reactants or products shifts equilibrium to restore balance.

2. Temperature: Raising temperature favors the endothermic direction, while lowering it favors exothermic reactions.

3. Pressure: For gaseous reactions, increasing pressure shifts equilibrium towards fewer gas molecules.

4. Catalyst: Catalysts do not change the position of equilibrium but speed up the rate at which equilibrium is reached.

10.0Equilibrium Constant (K)

The Equilibrium Constant (K) quantifies the position of equilibrium for a reaction.

For a general reaction:
aA + bB ⇌ cC + dD

The equilibrium constant is given by:

$Kc=\frac{[C{]}^c[D{]}^d}{[A{]}^a[B{]}^b}$

• If K > 1, products dominate at equilibrium.
• If K < 1, reactants dominate.

11.0Applications of Equilibrium

• Industrial processes like ammonia synthesis, sulfuric acid, and ester formation rely on equilibrium control.
• Biological systems maintain equilibrium in respiration and digestion.
• Environmental balance such as carbon dioxide and oxygen levels in the atmosphere.