Equilibrium

Chemical Equilibrium Review

• Most chemical reactions do not go to completion. They appear to stop.

• These reactions are reversible• A chemical reaction that can occur in both

the forward and reverse direction

Chemical Equilibrium

• A state in which the forward and reverse reactions balance each other because they take place at equal rates.

Rate forward reaction = Rate reverse reaction

Conditions for Chemical Equilibria

• A closed system• Constant temperature• Reversibility• Rates of opposing changes are equal

Law of Chemical Equilibrium

aA + bB ↔ cC +dD

Le Châtelier’s Principle

• You can use Le Châtelier’s Principle to predict how changes in concentration, volume (pressure), and temperature affect equilibrium

Solubility Product Constant

• The solubility product constant represents the concentrations of the products of a dissociation reaction (a in reaction in which a solid dissociates into aqueous ions).

• The solubility product is found by multiplying the concentration of the solid by the equilibrium constant.

• Equilibrium is reached in a dissociation reaction when the rate of the dissociation is the same as the rate of recrystalization.

Last topic in Chemistry 40s!!

Starting now...

Ion Product of Acids and Bases

• Pure water undergoes a small degree of ionization

• Only two molecules out of one billion will ionize

2H2O(l) ↔ H3O+ (aq) + OH- (aq)

• Recall the definition of an acid and a base.

Dissociation Constant of Water (Kw)

• In pure water, the [H3O+] and [OH-] at 25oC are experimentally measured as 1x10-7

mol/L• K w = [H3O+][OH-]

• K w = (1x 10-7)(1 x 10-7)

• K w = (1 x 10-14)

Every water solution is neutral, acidic, or basic

• A neutral solution occurs when the hydronium ion concentration is equal to the hydroxide ion concentration

• An acidic solution occurs when the hydronium ion concentration is greater than the hydroxide ion concentration

• A basic solution occurs when the hydronium ion concentration is less than the hydroxide ion concentration

pH

• Most concentrations of hydronium ions are very small (around 4x10-8 mol/L), so a scientist named Soren P. Sorenson proposed the idea of the potency of hydrogen

• Or simply the pH scale of a solution

Calculating pH

• pH is calculated as follows: • pH= log[H3O+]

• Similarly, we can calculate a potency of hydroxide (pOH):

• pOH= log[OH-]• Together: pH + pOH = 14

Strengths of Acids and Bases

Recall:• Strong Acid

– Completely dissociates into ions• Strong Base

– Completely dissociates into ion

Weak Acids

• Dissociate only slightly into ions

HA(aq) + H2O(l) ↔ H3O+ (aq) + A- (aq)

• Ka is called the acid dissociation constant

Weak Bases

• Dissociate only slightly into ions• HB(aq) + H2O(l) ↔ BH2

+ (aq) + OH- (aq)

• Kb is called the base dissociation constant

Questions?

Review for tomorrow

• Pg. 458– Questions: 3, 10-18