Writing Equilibrium Expressions

There is an easy process to write equilibrium expressions.
  1. Find the balanced chemical equation for the reaction
  2. Write the concentrations of all products in the numerator of the fraction, and all concentrations of reactants in the denominator of the fraction.  DO NOT include liquids or solids when doing this step.
  3. The exponent on each concentration is the same as the coefficient on the chemical in the balanced chemical equation.
  4. This value is Kc.  If all of your reactants are gases, you can use PRESSURES instead of concentrations ... this value is Kp.

Take a look at the example below (step-by-step) and/or the YouTube video I made about it (at right).

Example

Write the equilibrium expression for:
Picture
  1. Find the balanced chemical equation for the reaction.  DONE.
  2. Write the concentrations of all products in the numerator of the fraction, and all concentrations of reactants in the denominator of the fraction.  DO NOT include liquids or solids when doing this step.
Picture
I've used question marks for exponents here because I have no idea what the exponents are yet.

3. The exponent on each concentration is the same as the coefficient on the chemical in the balanced chemical equation.
Picture
DONE.  These are all gases, so if I wanted to I could use pressure instead of concentration, and the constant would be Kp instead of Kc (these two are NOT necessarily the same value):
Picture
TIP: I never write the state (the little (g) symbols) when I'm doing these problems in real life.  It takes too much time, plus I can't write that tiny.
Picture