Glossary
Closed System
A system that does not allow for the exchange of matter or energy with its surroundings, which is essential for a chemical reaction to reach equilibrium.
Example:
Performing a reaction in a sealed flask ensures it's a closed system, allowing the concentrations to stabilize at equilibrium without external interference.
Double Arrow (⇌)
The symbol used in chemical equations to indicate that a reaction is reversible and can reach a state of equilibrium.
Example:
When writing the dissociation of a weak acid like acetic acid, CH₃COOH(aq) ⇌ H⁺(aq) + CH₃COO⁻(aq), the double arrow signifies its reversible nature.
Dynamic Equilibrium
A state of balance in a reversible reaction where the forward and reverse reactions continue to occur at equal rates, resulting in no net change in reactant or product concentrations.
Example:
The constant movement of water molecules between liquid and vapor phases in a sealed container, even when the overall amount of liquid and vapor remains constant, illustrates dynamic equilibrium.
Equilibrium
A state in a reversible reaction where the rate of the forward reaction is equal to the rate of the reverse reaction, leading to constant concentrations of reactants and products.
Example:
In a sealed bottle of soda, CO₂(g) ⇌ CO₂(aq), the rate at which CO₂ dissolves into the liquid equals the rate at which it escapes, maintaining equilibrium.
Equilibrium Constant (Kc)
A numerical value that expresses the ratio of product concentrations to reactant concentrations at equilibrium, each raised to the power of their stoichiometric coefficients.
Example:
For the reaction 2NO₂(g) ⇌ N₂O₄(g), a large Kc value indicates that at equilibrium, there will be a significantly higher concentration of N₂O₄ compared to NO₂.
Equilibrium Constant (Kp)
A numerical value that expresses the ratio of the partial pressures of gaseous products to gaseous reactants at equilibrium, each raised to the power of their stoichiometric coefficients.
Example:
When dealing with gas-phase reactions like the Haber process, N₂(g) + 3H₂(g) ⇌ 2NH₃(g), Kp is often used to describe the equilibrium in terms of pressures.
Reversible Reactions
Reactions that can proceed in both the forward (reactants to products) and reverse (products to reactants) directions simultaneously.
Example:
The formation of ammonia from nitrogen and hydrogen, N₂(g) + 3H₂(g) ⇌ 2NH₃(g), is a classic example of a reversible reaction used in industrial processes.