Acid / Base and pH / pOH Chemistry Unit 10

Key Topics Water is amphoteric: H2O + H2O acts as an acid and a base. Chemistry Unit 10

Key Topics Water is amphoteric: H2O + H2O H3O+ + OH- acts as an acid and a base. Donates H+ H2O + H2O H3O+ + OH- Chemistry Unit 10

Key Topics Water is amphoteric: H2O + H2O H3O+ + OH- acts as an acid and a base. Accepts H+ H2O + H2O H3O+ + OH- Chemistry Unit 10

Key Topics Water is amphoteric: H2O + H2O H3O+ + OH- BASE ACID acts as an acid and a base. Accepts H+ Donates H+ H2O + H2O H3O+ + OH- BASE ACID Chemistry Unit 10

Key Topics Water is amphoteric: H2O + H2O H3O+ + OH- acts as an acid and a base. Accepts H+ Donates H+ H2O + H2O H3O+ + OH- BASE ACID ACID BASE Chemistry Unit 10

Key Topics H2O + H2O H3O+ + OH- BASE ACID ACID BASE CONJUGATE PAIRS: Accepts H+ Donates H+ H2O + H2O H3O+ + OH- BASE ACID ACID BASE Chemistry Unit 10

Opposite sides of equation, Opposite acid vs. base, CONJUGATE PAIRS: BASE ACID ACID BASE Opposite sides of equation, Opposite acid vs. base, Opposite STRENGTHS ( weak vs strong) Chemistry Unit 10

Key Topics Water is amphoteric: Self-ionization of water acts as a WEAK acid and a WEAK base. It’s acid dissociation constant, Ka , is low. Self-ionization of water H2O + H2O H3O+ + OH- Chemistry Unit 10

Key Topics Self-ionization of water Ion Product Constant for Water, Kw H2O + H2O H3O+ + OH- Ion Product Constant for Water, Kw Like the Dissociation Constant: Measure of STRENGTH Kw = [H3O+][OH-] Concentrations of hydronium and hydroxide in molarity Chemistry Unit 10

Key Topics Ion Product Constant for Water, Kw Kw = [H3O+][OH-] Concentrations of hydronium and hydroxide in molarity Kw = 1 x 10-14 at 25 oC Chemistry Unit 10

Using Kw Calculate the concentration of hydronium and hydroxide ions in neutral water. First, we know Kw = [H3O+][OH-] AND we know Kw = 1 x 10-14 ALSO, we know the balanced equation for the self ionization of pure water: H2O + H2O H3O+ + OH- Therefore, we know [H3O+]= [OH-] according to reaction coefficients. Rest is algebra! Chemistry Unit 10

Using Kw Kw = [H3O+][OH-] = 1 x 10-14 Substitute in “x” for both [H3O+]and [OH-] x . x = 1 x 10-14 x2 = 1 x 10-14 x = 1 x 10-7 Chemistry Unit 10

Using Kw Kw = [H3O+][OH-] = 1 x 10-14 [H3O+] = 1 x 10-7 M [OH-] = 1 x 10-7 M H2O + H2O H3O+ + OH- In water, ALL three species: H2O, H3O+ and OH- exist! Chemistry Unit 10

Using Kw Kw = [H3O+][OH-] = 1 x 10-14 Substitute in “x” for both [H3O+]and [OH-] The molarity for both [H3O+]and [OH-] are: x = 1 x 10-7 [H3O+] = 1 x 10-7 M [OH-] = 1 x 10-7 M Chemistry Unit 10

Calculations with the Kw constant Calculate the concentration of hydronium, if the hydroxide ion concentration is 1 x 10-8 M. Chemistry Unit 10

Calculations with the Kw constant Calculate the concentration of hydronium, if the hydroxide ion concentration is 1 x 10-8M. First, we know Kw = [H3O+][OH-] AND we know Kw = 1 x 10-14 ALSO, we know the [OH-] = 1 x 10-8M. Rest is algebra! Chemistry Unit 10

Using Kw Kw = [H3O+][OH-] = 1 x 10-14 Substitute in “x” for [H3O+] and known value for [OH-], 1 x 10-8 x . 1 x 10-8 = 1 x 10-14 x = 1 x 10-14 1 x 10-8 x = 1 x 10-6 Chemistry Unit 10

What pH and pOH Mean pH is a numeric measurement of the hydronium ion concentration. pH = -log[H3O+] Remember the hydronium concentration must be in molarity! Hydrogen ion concentration[H+] is the same thing as the hydronium ion concentration! Chemistry Unit 10

What pH and pOH Mean pOH is a numeric measurement of the hydroxide ion concentration. pOH = -log[OH-] Remember the hydroxide concentration must be in molarity! Keep checking if the question asks or gives hydroxide, hydrogen, hydronium, pH or pOH! Chemistry Unit 10

What pH and pOH Mean pH plus pOH always equal 14. Example: If the pH = 7 then the pOH must equal 7. If the pH = 5 then pOH = 9. If the pOH = 13.2 the pH = 0.8 Chemistry Unit 10

Calculations What is the pH of a solution with a concentration of 1 x 10-12 M H3O+ Chemistry Unit 10

Calculations What is the pH of a solution with a concentration of 1 x 10-12 M OH- Chemistry Unit 10

Calculations What is the pH of a solution with a concentration of 4.5 x 10-3 M H3O+ Chemistry Unit 10

Calculations What is the pH of a solution with a concentration of 6.8x 10-5 M OH- Chemistry Unit 10

Next Steps Make sure you READ the chapter! Complete pH and pOH table Chemistry Unit 10

Neutralization The reaction of an acid with a base to produce a salt and water. HCl (aq) + NaOH (aq) NaCl (aq) + H2O (l) If we prepare a standard solution of NaOH, we can then use it to determine the concentration of HCl in a sample. This is an example of Analytical Chemistry.

Titration Method based on measurement of volume. You must have a solution of known concentration - standard solution. It is added to an unknown solution while the volume is measured. The process is continued until the end point is reached - a change that we can measure. Acids and bases are commonly measured using titrations.

Titrations Buret volumetric glassware used for titrations. It allows you to add a known amount of your titrant to the solution you are testing. An indicator will give the endpoint.

Titrations Note the color change which indicates that the ‘endpoint’ has been reached. Start End

Indicator examples Acid-base indicators are weak acids that undergo a color change at a known pH. pH phenolphthalein

Indicator examples bromothymol blue methyl red