Chemical equilibrium has been reached in a reaction when the rate of the forward reaction is equal to the rate of the reverse reaction. When a chemical reaction has reached equilibrium, collisions are still occurring: the reaction is now happening in each direction at the same rate. This means that reactants are being formed at the same rate as products are being formed, and this is indicated by double arrows, 
. At equilibrium, the reaction can lie far to the right, meaning that there are more products in existence at equilibrium, or far to the left, meaning that at equilibrium there are more reactants. The concentration of the reactants and products in a reaction at equilibrium can be expressed by an equilibrium constant, symbolized K or Keq:
For the general reaction
aA + bB 
cC + dD
In the above expression, the brackets, as always, symbolize the concentration of the reactants and products in molarity. However, while in the above expression we used the plain symbol K to symbolize the equilibrium constant, there are several types of equilibrium constants. For example, Kc symbolizes the equilibrium constant in an aqueous solution, Kp symbolizes the partial pressures of gases in equilibrium, and Ksp symbolizes the solubility product of solids classified as insoluble. K values have no units, and a K > 1 means that the reaction favors the products at equilibrium, while a K < 1 means that the reaction favors the reactants at equilibrium.
Here are a couple of rules to follow when using equilibrium constant expressions on the exam:
- Pure solids do not appear in the equilibrium expression.
- Pure liquids do not appear in the equilibrium expression.
- Water, either as a liquid or solid, does not appear in the equilibrium expression.
- When a reactant or product is preceded by a coefficient, its concentration is raised to the power of that coefficient in the Keq expression.
- When the Keq of a reaction has been multiplied by a number, the K is raised to the power of the multiplication factor (Kn), so if it has been multiplied by 2, K is squared, if it has been multiplied by 3, K is cubed, and so on.
- The Keq of a reaction occurring in the reverse direction is simply the inverse of the Keq of the reaction occurring in the forward direction (1/Keq).
- The Keq of a net reaction that has two or more steps is found by the product of the Keq s for each of the steps: Ks = (K1
K2
K3 . . .).
Let’s work through an example now of an equilibrium question.
Example
Write the equilibrium expression for the following equation:
H2(g) + I2(g) 
2HI(g)
If K is calculated to have a value of 2.5 for the reaction above, what is the value of the equilibrium constant for the following reaction?
4HI(g) 
2H2(g) + 2I2(g)
Explanation
The equilibrium constant expression for the reaction is
The reaction has been doubled and reversed, so the new K is the reciprocal of the old K squared (since the reaction coefficients are doubled):
