Equilibrium Constant
Definition and meaning of Equilibrium Constant in chemistry.
Equilibrium constant (K) is a number describing the final balance of a reversible chemical reaction. It tells us how much product forms before the forward and backward rates become equal. We measure this specific value at a constant temperature.
In more detail
For a general reaction aA + bB ⇌ cC + dD, we find K using a simple fraction. The formula is K = [C]^c[D]^d / [A]^a[B]^b. You can plug in molar concentrations for Kc or gas pressures for Kp.
A very large K value means the chemical reaction mostly makes products. A tiny K means the mixture stays mostly as starting reactants. This important number depends only on the temperature of the system.
Changing the starting amounts or adding a catalyst will never change K. It connects to standard Gibbs free energy through the equation ΔG° = -RT ln K. Many chemical reactions look like they have stopped completely. In reality, they keep running in both directions at the exact same speed.
The equilibrium constant describes this hidden, busy, but balanced state. Students often mistakenly think a catalyst increases the final product yield. A catalyst just helps the reaction reach its equilibrium state much faster.
When calculating K, you must always ignore pure solids and pure liquids. Their active concentrations do not change during the reaction.
Key facts
| Symbol | K (Kc, Kp, Ka, Kb, Ksp, etc.) |
|---|---|
| Formula | K = [products]^coeff / [reactants]^coeff |
| Depends on | Temperature only |
| Excludes | Pure solids and pure liquids |
| Related math | ΔG° = -RT ln K |
| Field | Physical Chemistry |
Consider the Haber process used to manufacture ammonia for fertilizer. The balanced equation is N2(g) + 3H2(g) ⇌ 2NH3(g). The math for this equilibrium constant is Kc = [NH3]^2 / ([N2][H2]^3). At room temperature (25°C), this Kc value is extremely large. This huge number shows the natural balance strongly favors making ammonia. However, this specific chemical reaction releases a lot of heat. If you raise the temperature, the Kc value drops sharply. A smaller K at high heat means less ammonia forms at equilibrium. Engineers must carefully balance temperature and pressure to get enough ammonia quickly.
Frequently asked questions
Does a catalyst change the equilibrium constant?
No. A catalyst speeds up both the forward and reverse reactions equally. It helps the system reach equilibrium faster, but it never changes the final value of K.
What does it mean if K equals exactly 1?
A value of exactly 1 means the products and reactants have roughly equal activities. The final mixture will contain similar amounts of both sides, so neither side is strongly favored.
Why are solids and liquids left out of the K expression?
The concentration of a pure solid or liquid stays constant throughout the reaction. Since their values do not change, they are built into the constant K itself.