Equilibrium
Definition and meaning of Equilibrium in chemistry.
Equilibrium is the state of a reversible chemical reaction where the forward and reverse reactions occur at equal rates. Once this balance is reached, the concentrations of reactants and products stop changing over time. The reaction itself does not stop, only the net change does.
In more detail
Chemical equilibrium is a dynamic state, not a static one. Molecules keep converting between reactants and products in both directions. Since both reactions happen at the same rate, there is no observable change in overall composition.
Chemists describe how far a reaction proceeds using the equilibrium constant, K. This value is calculated by dividing the concentrations, or partial pressures, of the products by those of the reactants. Each term is raised to the power of its coefficient in the balanced equation.
A large K value means products dominate at equilibrium, while a small K value means reactants dominate. Le Chatelier's principle predicts how a system at equilibrium responds when disturbed. If concentration, pressure, volume, or temperature changes, the equilibrium shifts in the direction that partially counteracts the disturbance.
Catalysts speed up how quickly a reaction reaches equilibrium. They do not change the value of the equilibrium constant or shift the position of equilibrium itself. Equilibrium concepts extend beyond simple gas-phase reactions to acid-base chemistry and solubility.
Biological systems also rely on dynamic equilibria, such as enzyme activity and hormone levels regulated within the body. Industrial processes like the Haber process are run under carefully chosen conditions to balance yield against reaction speed.
Key facts
| Field | Physical Chemistry |
|---|---|
| Notation | ⇌ (double half-arrow) |
| Governing quantity | Equilibrium constant, K |
| Related principle | Le Chatelier's principle |
| Nature | Dynamic, not static |
In the Haber process, N2(g) + 3H2(g) ⇌ 2NH3(g) reaches equilibrium in a sealed vessel at a fixed temperature. Ammonia forms at exactly the same rate it decomposes back into nitrogen and hydrogen, so the concentrations of all three gases stop changing.
Frequently asked questions
Does equilibrium mean the reaction has stopped?
No. Equilibrium is dynamic: both the forward and reverse reactions continue, but since they occur at equal rates, the measurable concentrations remain constant.
What is the difference between Kc and Kp?
Kc is the equilibrium constant written in terms of molar concentrations, while Kp uses partial pressures of gaseous species. For a reaction they are related by Kp = Kc(RT)^Δn, where Δn is the change in moles of gas.
What does a very large equilibrium constant mean?
A large K value, much greater than 1, means the reaction favors products at equilibrium. Most of the reactants convert to products before the forward and reverse rates become equal.