Using equilibrium constants

The size of K tells us how far a reaction goes:

• K > 10³: products dominate; the reaction goes almost to completion. H₂ + O₂ forming water at 500 K has K = 2.4 × 10⁴⁷.
• K < 10⁻³: reactants dominate; the reaction hardly goes. N₂ + O₂ ⇌ 2NO at 800 K has K = 4.8 × 10⁻³¹.
• Between 10⁻³ and 10³: both reactants and products are present in appreciable amounts.

To know which way a mixture will move, we use the reaction quotient, Q. It has the same form as K, but uses the concentrations at any moment, not only at equilibrium. Then compare Q with K:

• Q < K: too little product; the net reaction goes forward.
• Q > K: too much product; it goes backward.
• Q = K: the mixture is at equilibrium.

To calculate equilibrium amounts, follow five steps:

• 1. Write the balanced equation.
• 2. Make an ICE table (Initial, Change, Equilibrium) for each substance, writing the change as x, multiplied by each coefficient.
• 3. Substitute the equilibrium amounts into the expression for K and solve for x.
• 4. Use x to find each equilibrium concentration.
• 5. Check by putting them back into the K expression.

A shortcut: if K is very small, the reaction hardly moves, so x is tiny. Then (starting amount – x) is almost the same as the starting amount, and x can be neglected in that term, which makes the arithmetic much simpler.