7.6 Properties of the Equilibrium Constant

Manipulating the reaction changes K

If you reverse a reaction, the new K is the reciprocal (1/K) of the original. If you multiply coefficients by a factor n, the new K is raised to that power (K^n). These rules come from how the equilibrium expression's exponents relate to stoichiometric coefficients in the law of mass action.

Adding reactions multiplies K values

When two or more reactions are added together to give an overall reaction, the overall K equals the product of the individual K values (K_overall = K1 × K2 × ...). This works because multiplying the equilibrium expressions together causes intermediate species to cancel, just like adding ΔG values for reaction steps.

K depends only on temperature

The equilibrium constant K is constant at a given temperature regardless of the initial concentrations or pressures used. Changing temperature shifts equilibrium and changes the value of K, but changing concentration, pressure, or adding a catalyst does not change K itself — only the position of equilibrium (for concentration/pressure) or the rate (for catalyst).

Key terms

Effect of reversing a reaction on K

The new equilibrium constant is the reciprocal of the original: K_reverse = 1/K_forward.

Effect of multiplying coefficients by n on K

The new K is raised to the power n: K_new = K^n.

K for a sum of reactions

K_overall = K1 × K2 × ... (product of the individual equilibrium constants).

What determines the value of K?

Only temperature. K is constant at a given temperature regardless of starting concentrations or pressures.

Does a catalyst change K?

No. A catalyst speeds up the rate of reaching equilibrium but does not change K or the equilibrium position.

Try an AP-style question

K = 4 for a reaction. What is K for the reverse reaction?

1. 0.25
2. 16
3. 4
4. −4