Chapter 15 · Chemical Equilibrium
15.4The Equilibrium Constant: A Measure of How Far a Reaction Goes
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Predict
For aA + bB ⇌ cC + dD, what is K?
The idea
- Write K from a balanced equation.
- Interpret the size of K.
The equilibrium constant K is a ratio of equilibrium concentrations with each concentration raised to its coefficient. Products go in the numerator, reactants in the denominator. Pure liquids and solids will be left out in the next section; for reactions of solutes and gases, include every species the equation shows, using molarity for solutes. A large K, well above 1, means the equilibrium mixture is mostly products. A small K means mostly reactants. K near 1 means significant amounts of both.
K depends on temperature and on how you wrote the equation. Double the coefficients and you square K. Flip the equation and you take 1/K. You cannot compare two K values if they belong to different writings of the reaction. Units are often dropped in intro work and K is treated as a number; follow the convention your problem uses, and always use equilibrium concentrations, not the initial ones, when you compute K.
Keep these
- K = products over reactants, each to its coefficient power.
- Large K favors products. Small K favors reactants.
- K belongs to a specific equation at a specific temperature.
Worked path
Write K for N₂ + 3 H₂ ⇌ 2 NH₃, and say what happens to K if the equation is reversed.
Observe
Products in the numerator, coefficients as exponents.