Dynamic Equilibrium . Definition & Examples .Chemistry

Dynamic equilibrium refers to a state of balance or stability in a system where two opposing processes occur at equal rates, resulting in a relatively constant concentration of substances or properties within the system over time. This concept is commonly encountered in various fields of science, such as chemistry, physics, and biology.

In a dynamic equilibrium, even though the individual processes are ongoing, there is no net change in the overall state of the system. This balance is achieved because the rate of the forward process is equal to the rate of the reverse process.

One of the most well-known examples of dynamic equilibrium is the equilibrium between the liquid and vapor phases of a substance in a closed container. When a liquid evaporates into a vapor, molecules are leaving the liquid phase and entering the vapor phase. Conversely, vapor molecules can condense back into the liquid phase. Eventually, the rates of evaporation and condensation become equal, and the system reaches a dynamic equilibrium where the concentration of molecules in both phases remains constant over time.

Another example is the equilibrium involving chemical reactions. Consider a reversible reaction where reactants are constantly being converted into products and vice versa. When the rates of the forward and reverse reactions become equal, the system reaches a dynamic equilibrium where the concentrations of reactants and products remain relatively stable.

The Concept Of Dynamic Equilibrium

Dynamic equilibrium is a concept from chemistry and physics that describes a state in which opposing processes are occurring at equal rates, resulting in a stable overall condition. In dynamic equilibrium, even though individual processes continue to take place, there is no net change in the macroscopic properties of the system.

This concept is often encountered in chemical reactions, particularly reversible reactions, where reactants are continuously being converted into products, and products are being converted back into reactants. However, the rates of the forward and reverse reactions are equal, leading to a steady state where the concentrations of reactants and products remain constant over time.

Key characteristics of dynamic equilibrium include:

Reversible Reactions: Dynamic equilibrium typically arises in reversible reactions, where the conversion of reactants into products and products into reactants can occur in both directions. These reactions are denoted using a double arrow (↔) to signify the reversibility.

Constant Concentrations: In a dynamic equilibrium, the concentrations of reactants and products remain relatively stable over time. However, it's important to note that the individual molecules are still undergoing changes; it's just that the rates of change in both directions are balanced.

Equilibrium Constant: For a given chemical reaction, the equilibrium constant (K) expresses the ratio of the concentrations of products to reactants at equilibrium. It is derived from the law of mass action and provides a quantitative measure of the position of equilibrium.

Temperature and Pressure Dependence: The establishment of dynamic equilibrium is influenced by factors such as temperature and pressure. Changes in these conditions can shift the equilibrium position either towards the products or the reactants, altering the concentrations at equilibrium.

A classic example of dynamic equilibrium is the reaction between nitrogen dioxide (NO2) and dinitrogen tetroxide (N2O4):

2 NO2 ⇌ N2O4

At higher temperatures, the reddish-brown nitrogen dioxide predominates due to the endothermic nature of the reaction. At lower temperatures, the colorless dinitrogen tetroxide becomes more prevalent. At a specific temperature, the two forms will coexist in a dynamic equilibrium.

Examples

Here are some examples of dynamic equilibrium, along with the reactions and graphs associated with each:

H2O(g) ⇌ H2O(l) (Water Vapor and Liquid Water Equilibrium):

Reaction: H2O(g) ⇌ H2O(l)

Graph: At equilibrium, the vapor pressure of water above the liquid remains constant, as shown by a flat pressure vs. temperature graph.

N2(g) + 3H2(g) ⇌ 2NH3(g) (Haber-Bosch Process for Ammonia Synthesis):

Reaction: N2(g) + 3H2(g) ⇌ 2NH3(g)

Graph: The concentration of ammonia reaches a constant value when the forward and reverse reaction rates are balanced.

CO(g) + H2O(g) ⇌ CO2(g) + H2(g) (Water-Gas Shift Reaction):

Reaction: CO(g) + H2O(g) ⇌ CO2(g) + H2(g)

Graph: Concentrations of CO, H2O, CO2, and H2 stabilize over time, indicating equilibrium.

CH4(g) + 2H2O(g) ⇌ CO2(g) + 4H2(g) (Steam Methane Reforming):

Reaction: CH4(g) + 2H2O(g) ⇌ CO2(g) + 4H2(g)

Graph: Concentrations of reactants and products reach steady-state values, showing equilibrium.

N2O4(g) ⇌ 2NO2(g) (Dinitrogen Tetroxide and Nitrogen Dioxide Equilibrium):

Reaction: N2O4(g) ⇌ 2NO2(g)

Graph: Equilibrium is reached when the color of the gas changes from colorless (N2O4) to reddish-brown (NO2) and remains constant.

Fe3+(aq) + SCN-(aq) ⇌ FeSCN2+(aq) (Formation of Iron Thiocyanate Complex):

Reaction: Fe3+(aq) + SCN-(aq) ⇌ FeSCN2+(aq)

Graph: Absorbance of the red FeSCN2+ complex stabilizes, indicating equilibrium between reactants and products.

C6H12O6(aq) ⇌ 2C2H5OH(aq) + 2CO2(g) (Fermentation of Glucose to Ethanol):

Reaction: C6H12O6(aq) ⇌ 2C2H5OH(aq) + 2CO2(g)

Graph: Concentrations of glucose, ethanol, and carbon dioxide reach steady levels, showing equilibrium in the fermentation process.

Dynamic Equilibrium vs Static Equilibrium

Dynamic equilibrium and static equilibrium are concepts from physics that describe the balance of forces and motion in different situations. Let's explore the differences between the two:

Static Equilibrium

Static equilibrium refers to a situation in which an object is at rest and remains at rest due to the balanced forces acting on it. In other words, the net force and net torque (rotational force) acting on the object are both zero. In static equilibrium, there is no change in position, and all forces and torques cancel each other out.

For example, consider a book resting on a flat table. The gravitational force pulling the book downward is balanced by the normal force exerted by the table upward. As long as the book doesn't move or rotate, it remains in static equilibrium.

Dynamic Equilibrium

Dynamic equilibrium, on the other hand, refers to a situation in which an object is in motion at a constant velocity. In dynamic equilibrium, the net force and net torque are still zero, but unlike static equilibrium, the object is in motion and continues to move with a constant speed and direction.

An example of dynamic equilibrium is a car traveling at a constant speed on a straight road. The engine provides the necessary force to overcome friction and maintain a steady velocity. At this constant velocity, the net force is zero because the forward force from the engine is balanced by the frictional and air resistance forces.

In summary, the key difference between static equilibrium and dynamic equilibrium lies in the state of motion. Static equilibrium involves objects at rest, where all forces and torques balance out, while dynamic equilibrium involves objects in motion with balanced forces and torques, resulting in a constant velocity.

Frequently Asked Questions:

1. What is dynamic equilibrium?

Dynamic equilibrium is a state in a chemical reaction where the rates of the forward and reverse reactions are equal, resulting in no net change in the concentrations of reactants and products over time.

2. How does dynamic equilibrium differ from static equilibrium?

Static equilibrium occurs when a system is at rest, while dynamic equilibrium involves a continuous exchange of reactants and products without any change in overall concentrations.

3. What are the key characteristics of dynamic equilibrium?

In dynamic equilibrium, the rates of the forward and reverse reactions are equal, concentrations remain constant, and macroscopic properties like color, pressure, and temperature stay consistent.

4. Can dynamic equilibrium be disturbed?

Yes, dynamic equilibrium can be disturbed by changes in temperature, pressure, or concentration. When this happens, the system will temporarily shift to restore equilibrium.

5. How is dynamic equilibrium represented in a chemical equation?

In a chemical equation, dynamic equilibrium is indicated by a double arrow (⇌) between the reactants and products, indicating that both the forward and reverse reactions are occurring simultaneously.

6. What is an example of dynamic equilibrium?

The dissociation of a weak acid, like acetic acid (CH3COOH), in water is an example of dynamic equilibrium. The acid molecules both dissociate into ions and recombine into molecules at equal rates.

7. How is dynamic equilibrium achieved in reversible reactions?

Dynamic equilibrium is achieved in reversible reactions when the rates of the forward and reverse reactions become equal. This can happen when the concentration of reactants and products stabilize.

8. Does dynamic equilibrium mean that the reactions have stopped?

No, dynamic equilibrium doesn't mean the reactions have stopped. It indicates a balance between the forward and reverse reactions, with constant conversion between reactants and products.

9. Can dynamic equilibrium be achieved in an open system?

No, dynamic equilibrium cannot be achieved in an open system because an open system constantly exchanges matter and energy with its surroundings, preventing a stable balance between reactants and products.

10. How is Le Chatelier's principle related to dynamic equilibrium?

Le Chatelier's principle states that when a system at equilibrium is subjected to a change in temperature, pressure, or concentration, it will respond by shifting the equilibrium to counteract the change. This principle helps predict how dynamic equilibrium will be affected by external changes.