Acids and Bases

Acids and Bases

• Acid: a substance that, when dissolved in water, increases the concentration of H+ (protons)• HCl H+ + Cl-

• HCl + H2O H3O+ + Cl-

• Strong acid: an acid that completely ionizes/dissociates in water• HNO3 + H2O H3O+ + NO3

-

• Strong electrolyte

H2O

• Strong Acids:• Hydrochloric Acid HCl

• Nitric Acid HNO3

• Sulfuric Acid H2SO4

• Perchloric Acid HClO4

• Hydrobromic Acid HBr• Hydroiodic Acid HI

• Weak Acid: an acid that only partially ionizes/dissociates in water• CH3COOH(aq) CH3COO-(aq) + H3O+(aq)

• Weak electrolytes• Weak Acids:

• Phosphoric acid H3PO4

• Acetic Acid CH3COOH

• Carbonic Acid H2CO3

• Hydrocyanic Acid HCN

• Benzoic Acid C6H5COOH

Polyprotic Acids

• Polyprotic acids: acids that can release more than one H+

• Sulfuric Acid• H2SO4(aq) HSO4

-(aq) + H3O+(aq)

• HSO4-(aq) SO4

2-(aq) + H3O+(aq)

Problem

• Write out the equation(s) for the ionization of phosphoric acid, H3PO4, in water

Bases

• Base: a substance that, when put in water, increases the concentration of OH- ions or a substance that accepts H+ ions• NaOH(aq) Na+(aq) + OH-(aq)

• Strong Bases: bases that completely ionize in water

• Weak Bases: bases that only partially ionize in water• Ammonia: NH3

• Pyridine: C5H5N

Each acid has a conjugate base and every base has a conjugate acid

conjugate acid-base pair 1

HA + B A− + BH+

conjugate acid-base pair 2

Congugate Acid-Base Pairs

Problems

Identify the conjugate bases for the following:

1) HBr

2) H2S

3) H2CO3

Identify the conjugate acids for the following

1) NO2-

2) NH3

3) OH-

In the following equations, identify the conjugate acid and base pairs:

1) HCl + H2O H3O+ + Cl-

1) C5H5N + H2O C5H6N+ + OH-

Neutralization Reactions

• When strong acids and bases in aqueous solution react with each other, they form water and a salt

• HX(aq) + MOH(aq) HOH(l) + MX(aq)

• HCl(aq) + NaCl(aq) H2O(l) + NaCl(aq)

Water Salt

Problems

Write out the molecular, total ionic, and net ionic equations for the reactions of the following acids and bases

1) HBr and KOH

2) HCl and Mg(OH)2

3) H2SO3 and NaOH

Acid Ionization Constant

• Acid Ionization Constant (Ka): the equilibrium constant for the ionization reaction of an acid with water• HA + H2O A- + H3O+

• Large Ka = Strong acid

• Small Ka = Weak acid

Questions

1) Write the equilibrium constant expression for the dissociation of HCN (hydrocyanic acid) in water

2) Write the equilibrium expression for the dissociation of HF in water

3) If the Ka for HCN is 4.9 x 10-10 and the Ka for HF is 7.2 x 10-4, which acid is stronger?

4) Predict whether the equilibrium for each of the following reactions favors the reactants or products.

a) H3PO4(aq) + H2O(l) H3O+(aq) + H2PO4(aq)

b) NH4+(aq) + H2O(l) H3O+(aq) + NH3(aq)

Base Ionization Constant

• Base Ionization Constant (Kb): the equilibrium constant for the ionization reaction of a base with water• B + H2O OH- + BH+

• Large Kb = Strong base

• Small Kb = Weak base

Autoionization of Water

Autoionization of Water• Also called “Self Ionization”• About 1 out of every 10 million water

molecules form ions through self ionization

H2O H+ + OH–

H2O + H2O H3O+ + OH–

• All aqueous solutions contain both H3O+ and OH–

Ion Product Constant for Water

• Ion Product Constant for Water (Kw): the numerical value obtained by multiplying the molar concentrations for hydronium and hydroxide ions present in pure water at 25°C• KKww = [H = [H33OO++][OH][OH--] = 1.00 x 10] = 1.00 x 10-14-14 at 25 at 25 ooCC• the concentration of H3O+ and OH– are equal

in pure water• [H3O+] = [OH–] = 10-7M @ 25°C

Ion Product of Water• the product of the H3O+ and OH–

concentrations is always the same number

• Kw =[H3O+][OH–] = 1.00 x 10-14 @ 25°C– if you measure one of the concentrations,

you can calculate the other

• as [H3O+] increases the [OH–] must decrease so the product stays constant– inversely proportional

[H+] vs. [OH-]

OH-H+ H+ H+ H+ H+

OH-OH-OH-OH-

[OH-]10-14 10-13 10-11 10-9 10-7 10-5 10-3 10-1 100

[H+] 100 10-1 10-3 10-5 10-7 10-9 10-11 10-13 10-14

Even though it may look like it, neither H+ nor OH- will ever be 0

Acid Base

Acidic and Basic Solutions• Neutral solutions have equal [H3O+] and [OH–]

• [H3O+] = [OH–] = 1 x 10-7

• acidic solutions have a larger [H3O+] than [OH–]• [H3O+] > [OH–]• [H3O+] > 1 x 10-7; [OH–] < 1 x 10-7

• basic solutions have a larger [OH–] than [H3O+]• [H3O+] < [OH–]• [H3O+] < 1 x 10-7; [OH–] > 1 x 10-7

Questions

1) Calculate the [OH] at 25°C when the [H3O+] = 1.5 x 10-9 M, and determine if the solution is acidic, basic, or neutral

2) Calculate the [H3O+] at 25°C in a solution that has a [OH] 1.3 x 10-10 M and determine if the solution is acidic, basic, or neutral

3) Calculate the [H3O+] at 25°C in a solution that has a [OH] 1.0 x 10-7 M and determine if the solution is acidic, basic, or neutral

pH and pOH

• Acidic solutions• pH < 7

• Basic solutions• pH > 7

• Neutral solutions• pH = 7

• pH is a measure of the concentration of H+ or H3O+ in solution

• pH = -log [H+] OR pH = -log [H3O+]

• [H3O+] = 10-pH

• pOH is a measure of the concentration of OH- in solution• pOH = -log [OH-]• [OH-] = 10-pOH

• pH + pOH = 14

Problems

1) What is the pH for an HCl solution with an [H3O+] = 2.3 x 10-2?

2) What is the pH of an NaOH solution with an [H3O+] = 3.39 x 10-10?

3) What is the pH of a KOH solution with a [OH-] = 1.1 x 10-2?

4) What is the [H3O+] for a solution with a pH of 8.45. What is the [OH-]?

• Electrolytes: solutes that separate into ions when dissolved in water (they’re soluble)– Have the ability to conduct electricity– 2 types

• Strong electrolytes• Weak electrolytes

Classification of Water Soluble Substances

• Strong electrolytes: solutes that completely dissociates into ions when dissolved in water– Examples: NaCl,

MgBr2, HCl

– Strong electrical conductors

– Strong electrolyte(aq or s) → Cation+(aq) + Anion-(aq) – Example: NaCl(s) → Na+(aq) + Cl-(aq)

• Weak Electrolytes: solutes that, when dissolved in water, only partially dissociates into ions– Examples: HF, NH3,

acetic acid – Weak electrical

conductors

– Weak electrolyte(aq) ↔ Cation+(aq) + Anion-(aq)

– Example: HF(aq) ↔ H+(aq) + F-(aq)

• Nonelectrolytes: solutes that dissolve in water without separating into ions– Examples: sucrose,

ethanol– Do not conduct electricity

Nonelectrolyte (s or l) → Nonelectrolyte(aq)Example: C12H22O11(s) → C12H22O11(aq)

Problems

1) The following salts are strong electrolytes. Write a balanced equation for their dissociation in water

a) LiBr

b) FeCl32) HCN is a weak acid. Write a balanced

equation for its dissociation in water

3) Do you expect Fructose (C6H12O6) to be a strong electrolyte, weak electrolyte, or nonelectrolyte? Write a balanced equation for its solvation in water

Titration

• Titration: a procedure for the quantitative analysis of a substance of unknown concentration whereby a measured quantity of another substance, of know concentration, is completely reacted with the with the original substance.– Often used to determine the

concentration of acids and bases

• Equivalence point: the point in a titration at which one reactant has been exactly consumed by the by the addition of another reactant– Midpoint of vertical rise– Occurs at pH = 7 in a strong acid-strong base titration– [H3O+] = [OH-]

Indicators

• Acid-Base Indicator: a chemical that changes color with a change in pH– Added to solutions in small amounts in order

to determine to solution’s pH visually

– Usually organic compounds

– Weak acid or base

– establishes an equilibrium with the H2O and H3O+ in the solution

HInd(aq) + H2O(l) Ind(aq) + H3O+

(aq)

53

Phenolphthalein

Bromocresol Green

Yellow Green Blue

Methyl Red

Questions

1) You have 25.0 mL of a HCl solution of unknown concentration. If you titrate your acid with 17.3 mL of a 0.877 M NaOH solution, what is the concentration of your acid?

2) You also have 25.0 mL of a sulfuric acid solution. If you titrate this solution with 32.1 mL of a 1.50 M KOH solution, what is the concentration of your acid?