unit twelve acids and bases

StrengthsWhat is an electrolyte?

A solution that contains ions and will conduct electrical current

Acids and bases ionize (molecular) or dissociate (ionic) into ions

Dissociation – when ionic compounds break apart to form ions Examples: NaCl, NaOH, Na2SO4

Ionization – when molecular compounds break apart to form ions (acids) Examples: HCl, H2SO4

Dissolving – when solute molecules are surrounded by solvent molecules and go into solution

StrengthsStrengths depend on how much

ionization/dissociation takes place

Strong acids and bases completely ionize/dissociate in solution 100% dissociated (lots of H+ and OH-) Strong electrolytes

Weak acids and bases do not completely ionize/dissociate in solution < 100% dissociated (only a few H+ and OH-) Weak electrolytes

Strong versus weakFor strong acids and bases we use a single

arrow to indicate the forward reaction is favored

For weak acids and bases we use a double arrow to indicate the partial ionization (forward and reverse reactions take place)

Strong Acids and BasesThere are six strong

acids

HClO4

H2SO4

HNO3

HCl

HBr

HI

There are eight strong basesLiOHNaOHKOHRbOHCsOHCa(OH)2

Sr(OH)2

Ba(OH)2

WaterAmphoteric – water can act as an acid or a

base

Self-ionizations – water can also act as an acid and a base with itself! (Even neutral, pure, distilled water)

KwIn pure water at 25°C, the preceding reaction occurs

only to a very small extent, resulting in equal, small concentrations of H+ and OH–.[H+] = [OH–] = 1.0 x 10-7 M

Ion product constant for water (Kw) – the product of the concentration of H+ and OH– in aqueous solutionsKw = [H+][OH–] Kw = (1.0 x 10-7 M)(1.0 x 10-7 M)Kw = 1.0 x 10-14 M

Can be used for aqueous solutions at 25°C (↑temp, ↑ movement, ↑ dissociation)

Kw will not change when the concentrations change because strengths are based on the amount of ionization.

KwSince Kw is constant, and Kw = [H+] [OH‾], it

follows that: If [H+] increases, then [OH‾] decreases, andIf [H+] decreases, then [OH‾] increases.

In a neutral solution: [H+] = [OH-]In an acidic solution: [H+] > [OH-]In a basic solution: [H+] < [OH-]

pH and pOH ScaleThe pH scale relates to the strengths

pH scale measures the hydrogen ion concentration and the pOH measures the hydroxide ion concentration

Logarithmic scale – a change in 1 pH unit corresponds to a tenfold change in [H+] (lime (pH=2) versus plum(pH=3))

pH < 7 : acidic solutionpH = 7 : neutralpH > 7 : basic solution

pOH 13 12 11 10 9 8 7 6 5 4 3 2 1

CalculationsKw = [H+] [OH‾]

pH = -log[H+]

pOH = -log[OH‾]

14 = pH + pOH

[H+] = 10-pH

[OH‾] = 10-pOH

Practice ProblemsWhat are the [H+] and [OH‾] concentrations in

a 0.01 M HCl solution?

What are the [H+] and [OH‾] concentrations in a 0.0001 M NaOH solution?

What are the [H+] and [OH‾] concentrations in a 0.00001 M HNO3 solution?

What is the pH of a 0.0001 M HNO3 solution?

What is the pOH of a 0.001 M KOH solution?

Practice ProblemsCalculate the [H+] of a solution with a pH of

8.37.

What is the concentration of OH- in a solution with a pOH of 4.80?

Calculate the pH of a solution with [OH-] = 1.3 x 10-2 M. Is the solution acidic or basic? Hint: Start with KW first to find [H+].

Calculate the OH- concentration for a solution with a pH of 3.66.

Worksheet Two will be due Thursday