XI Equilibrium
UNIT 6: EQUILIBRIUM
Reversible Reactions
A reversible reaction proceeds in both forward and reverse directions simultaneously. In these reactions, reactants interact to produce products, and subsequently, the products can react again to yield reactants. This cyclic process establishes a situation known as the State of Equilibrium, where the rates of the forward and reverse reactions are equal, resulting in constant concentrations of reactants and products. This concept is applicable across various physical processes and is represented by a double arrow (⇌) between the reactants and products.
Types of Equilibrium
Static Equilibrium
In static equilibrium, all particles are at rest, leading to the cessation of motion between reactants and products. Both forward and backward reactions are halted, indicating a stable state. This type of equilibrium is commonly discussed within mechanical contexts rather than in chemical processes.
Dynamic Equilibrium
In contrast, dynamic equilibrium occurs when changes happen within the mixture while the total composition remains unchanged. Here, the forward and backward reactions continue to occur at the same rate, which is a concept predominantly addressed in chemical contexts. It's crucial to note that despite constant concentrations, the reactions themselves are ongoing.
Equilibrium in Physical Processes
Phase Transitions
Common examples of phase transitions demonstrating equilibrium include:
Solid ⇌ Liquid (Melting and Freezing)
Liquid ⇌ Gas (Evaporation and Condensation)
Solid ⇌ Gas (Sublimation and Deposition)
Solid-Liquid Equilibrium
This specific equilibrium occurs at 273 K (0 °C) under atmospheric pressure, where ice and water reach a state of balance: [\text{H}_2\text{O}(s) \leftrightarrow \text{H}_2\text{O}(l)]. At this equilibrium point, the rates at which ice melts and water freezes are identical, maintaining a constant mass and temperature. It is critical to note that this equilibrium is specific to certain temperature and pressure conditions, often referred to as the normal melting/freezing point.
Liquid-Vapor Equilibrium
Liquid-vapor equilibrium can be illustrated with a transparent box containing a U-tube filled with mercury and a dish of water. When water is introduced, vapor pressure builds until an equilibrium is reached: [\text{H}_2\text{O}(l) \leftrightarrow \text{H}_2\text{O}(vapor)]. At this equilibrium, the vapor pressure remains constant, influenced directly by temperature. The vapor pressure varies among different liquids; a higher vapor pressure indicates a greater volatility and a lower boiling point. The time needed for complete evaporation of a liquid is dependent on the nature of the liquid, the volume present, and the existing temperature.
Solid-Vapor Equilibrium
Equilibrium states can also be observed with solids transitioning to vapor, such as iodine sublimating in a closed vessel, where the solid converts to vapor and returns. Similar behaviors are exhibited by compounds like camphor and ammonium chloride.
Equilibrium in Solutions
Solids in Liquids
Saturation occurs in solutions when no additional solute can dissolve at a particular temperature. A dynamic equilibrium exists where dissolution and crystallization occur concurrently: [\text{Sugar (solution)}