Understanding Chemical Equilibrium: Factors and Principles
Chemical Equilibrium: Definition
Chemical Equilibrium: The condition where the forward and reverse reaction rates are equal.
Homogeneous Equilibrium
Homogeneous equilibrium: A state in which reactants and products are in the same phase and physical state.
Law of Mass Action
Law of mass action: This law states that the equilibrium constant is the ratio of the product of the concentrations of the products raised to their respective stoichiometric coefficients, divided by the product of the concentrations of the reactants raised to their stoichiometric coefficients.
Equilibrium Constant (Kc)
The value of Kc is specific for each chemical reaction and independent of the initial concentrations of reactants and products. It depends only on temperature.
Degree of Dissociation
The degree of dissociation indicates the amount of reagent that has not reacted. It is the ratio between the quantity (in moles) of reagent that has reacted and the initial amount of reactant (in moles).
Kc and Kp: Temperature Dependence
The value of Kc or Kp is independent of the initial concentrations of reactants and products and depends only on temperature.
Factors Affecting Equilibrium
Changing pressure, adding or removing a species in a gaseous state (which is equivalent to changing the concentration of one of the reacting species), will shift the reaction. Adding an inert gas, keeping the volume constant, increases the total pressure but not the partial pressures, so the addition has no effect on the equilibrium. However, adding an inert gas at constant pressure can modify the equilibrium because it would increase the volume of the mixture to make room. Changing the system volume at a constant temperature means pressure and volume are inversely proportional; an increase in volume produces a decrease in pressure and vice versa.
Changing the volume: If we increase the volume, pressure decreases, and concentration is inversely proportional to the volume. This decrease is offset by a shift towards the side with a greater number of moles of gas. If we decrease the volume, pressure increases, and therefore concentrations increase. The system compensates for this change by evolving towards the side with fewer moles of gas.
Le Chatelier’s Principle
Le Chatelier’s principle: When a system at equilibrium undergoes a change in concentration of reacting species, pressure, or temperature, the system responds by attaining a new equilibrium that partially offsets the effect of the modification.
Initially, the system is in equilibrium, and therefore the reaction quotient (Qc) coincides with the equilibrium constant (Kc).
If we increase the concentration of reactants A and B or decrease the concentration of the products C and D, Qc will be less than Kc.
To restore equilibrium, the system will evolve from left to right, towards the formation of products.
Conversely, if we decrease the concentration of reactants or increase the concentration of products, Qc will be greater than Kc.
To restore equilibrium, the system will evolve from right to left, towards the formation of reactants. From this perspective, if we increase the concentration of one species, the system will offset this change by promoting the reaction in which this species is consumed. For example, if we increase the concentration of a reactant, the system evolves forward to form products.