Equilibrium Constants and their Applications - SS3 Chemistry Lesson Note
Chemical equilibrium constants are numerical values that quantify the position of a chemical equilibrium for a given reaction at a specific temperature. These constants play a crucial role in understanding the behaviour of chemical systems and have diverse applications across various areas of chemistry.
Equilibrium Constant Expression:
For a reversible chemical reaction represented as:
aA + bB ⇌ cC + dD
The equilibrium constant, denoted as K, is defined by the following expression:
K = [C]c [D]d / [A]a [B]b
where [A], [B], [C], and [D] represent the molar concentrations of the species A, B, C, and D at equilibrium, respectively. The exponents (a, b, c, d) in the expression are the stoichiometric coefficients of the reactants and products in the balanced chemical equation.
Homogeneous and Heterogeneous Equilibria:
Equilibrium constants apply to both homogeneous and heterogeneous equilibria. Homogeneous equilibria involve reactants and products in the same physical state (e.g., all gases or all in solution), whereas heterogeneous equilibria involve species in different physical states (e.g., gas and solid or liquid and gas).
Relationship between K and Reaction Quotient (Q):
The reaction quotient (Q) is calculated using the same expression as the equilibrium constant but with concentrations at any given point during the reaction. The relationship between K and Q provides valuable information about the direction in which the reaction will proceed to reach equilibrium:
● If Q < K, the reaction proceeds forward (towards the products) to reach equilibrium.
● If Q > K, the reaction proceeds in the reverse direction (towards the reactants) to reach equilibrium.
● If Q = K, the reaction is at equilibrium, and the concentrations of reactants and products remain constant.
Applications of Equilibrium Constants:
a. Predicting the Extent of Reaction:
The magnitude of the equilibrium constant (K) indicates whether the reaction predominantly favours reactants or products. A large value of K suggests that the reaction favours products, while a small value of K suggests that the reaction favours reactants. This information helps predict the extent to which a reaction proceeds to reach equilibrium under given conditions.
b. Optimization of Reaction Conditions:
Equilibrium constants are essential for optimising reaction conditions in industrial processes. By adjusting the temperature, pressure, or concentrations of reactants, chemists can manipulate the value of K to favour the desired products and achieve higher yields.
c. Calculating Concentrations at Equilibrium:
Equilibrium constants allow the calculation of equilibrium concentrations of reactants and products, given initial concentrations and K. This is valuable in understanding the composition of a system at equilibrium.
d. Understanding Acid-Base Equilibria:
In acid-base chemistry, equilibrium constants (Ka and Kb) are used to quantify the strengths of acids and bases. Strong acids have high Ka values, while weak acids have low Ka values. Similarly, strong bases have high Kb values, while weak bases have low Kb values.
e. Solubility Equilibria:
Solubility product constants (Ksp) are equilibrium constants that describe the solubility of ionic compounds in water. They are used to predict the solubility of sparingly soluble salts and to analyse precipitation reactions.
f. Environmental Studies:
Equilibrium constants are used in environmental studies to understand chemical reactions involved in natural processes, such as the dissolution of minerals in water, gas exchange between the atmosphere and the oceans, and acid-base equilibria in natural waters.
In conclusion, chemical equilibrium constants provide valuable insights into the position of chemical equilibrium and have a wide range of applications in chemistry, including predicting reaction extents, optimising reaction conditions, understanding acid-base and solubility equilibria, and studying natural processes in environmental chemistry. Understanding equilibrium constants is fundamental to gaining a deeper comprehension of chemical systems and their behaviour under different conditions.