Glossary
Equilibrium
A dynamic state in a reversible reaction where the rates of the forward and reverse reactions are equal, resulting in no net change in the concentrations of reactants and products.
Example:
When a saturated solution of salt is formed, it reaches equilibrium where the rate of salt dissolving equals the rate of salt crystallizing out of solution.
Equilibrium Constant (K)
A ratio of product concentrations to reactant concentrations at equilibrium, each raised to the power of their stoichiometric coefficients. It quantifies the extent to which a reaction proceeds towards products.
Example:
For the synthesis of ammonia, N₂(g) + 3H₂(g) ⇌ 2NH₃(g), the Equilibrium Constant K tells us the ratio of [NH₃]² to [N₂][H₂]³ at equilibrium.
Extent of Reaction
A qualitative measure of how far a chemical reaction proceeds towards products before reaching equilibrium. It is directly indicated by the magnitude of the equilibrium constant (K).
Example:
A reaction with a K value of 100 has a greater extent of reaction than one with a K value of 0.1, meaning more products are formed in the first case.
Kc
The equilibrium constant expressed in terms of molar concentrations of reactants and products in a solution or gas phase.
Example:
When calculating the Kc for the dissociation of a weak acid like acetic acid, you would use the molarities of the acid, its conjugate base, and H⁺ ions.
Kinetics
The study of reaction rates and the mechanisms by which chemical reactions occur, distinct from equilibrium which describes the state of a reaction at its endpoint.
Example:
Even if a reaction is highly product-favored (large K), its kinetics might be very slow, requiring a catalyst to speed up the process to reach equilibrium in a reasonable time.
Kp
The equilibrium constant expressed in terms of partial pressures of gaseous reactants and products.
Example:
For a gas-phase reaction like the decomposition of N₂O₄ into NO₂, we often use Kp to relate the partial pressures of the gases at equilibrium.
Product-Favored
A reaction where the equilibrium constant (K) is significantly greater than 1, indicating that at equilibrium, there are more products than reactants.
Example:
A combustion reaction with a K value of 10¹⁰ is highly product-favored, meaning it will essentially go to completion, forming a vast amount of products.
Reactant-Favored
A reaction where the equilibrium constant (K) is significantly less than 1, indicating that at equilibrium, there are more reactants than products.
Example:
If a reaction has a K value of 10⁻⁵, it is reactant-favored, implying that very little product forms, and the starting materials largely remain unreacted.
Strong Acid/Base
An acid or base that dissociates almost completely in solution, characterized by a very large equilibrium constant (K >> 1) for its dissociation reaction.
Example:
Sulfuric acid (H₂SO₄) is a strong acid because its dissociation into H⁺ and HSO₄⁻ has an extremely large K value, meaning it essentially fully ionizes in water.
Weak Acid/Base
An acid or base that only partially dissociates in solution, characterized by a small equilibrium constant (K < 1) for its dissociation reaction.
Example:
Ammonia (NH₃) is a weak base because its reaction with water to form NH₄⁺ and OH⁻ has a small Kb value, meaning only a fraction of its molecules react.