# Chemical Equilibrium - ICE Tables

### Equilibrium Review

• When a reaction is allowed to ‘run’ for some time, at some point it will no longer be favorable to continue forward or reverse and it will be at equilibrium

• It is important to remember that equilibrium does not mean that the reactant concentrations and the product concentrations are equal, but that the rate of the forward reaction is equal to the rate of the reverse reaction until something were to change (add more reactants/products, temperature, etc.)

-The equilibrium constant, K, tells us about the extent of a chemical reaction

### ICE Tables

Initial Change Equilibrium

• Can be used to determine K

• Can be used to determine concentrations at equilibrium

• 5% rule is necessary to check if approximating x is appropriate, the 5% rule is determines if it is appropriate to ignore the “-x” when calculating

• 5% > (x / [initial reactant concentration]) * 100

-When the 5% rule does not work, you must use the quadratic formula to solve for x

• Can be used to determine final pH for weak acids or weak bases

-If using ICE tables for a weak base, remember to use $$\mathrm{K}_{\mathrm{b}}=\mathrm{K}_{\mathrm{a}} / \mathrm{K}_{\mathrm{W}}$$ instead of $$\mathrm{K}_{\mathrm{a}}$$

-Water can be considered negligible in ICE tables because it is the solution of the reaction

**also beneficial to know that: $$\mathrm{pK}_{\mathrm{a}}+\mathrm{pK}_{\mathrm{b}}=\mathrm{pK}_{\mathrm{W}}=14$$

### Concept Question (Buffers)

-this problem is considered a buffer because at equilibrium both the acid and its conjugate base are still present

-with buffer problems, you can use the Henderson-Hasselbalch Equation

** ICE tables need everything to either be in moles or molarity

Rachel H.Comment