SURVEY OF CHEMISTRY I

CHEM 1151

CHAPTER 9

DR. AUGUSTINE OFORI AGYEMANAssistant professor of chemistryDepartment of natural sciences

Clayton state university

CHAPTER 9

ACIDS, BASES, AND SALTS

ARRHENIUS ACIDS- Acids are substances that ionize in aqueous solutions to

produce hydrogen ions (proton, H+)

HCl, HNO3, H2SO4

HCl(aq) → H+(aq) + Cl-(aq)HNO3(aq) → H+(aq) + NO3

-(aq)

Ionize: dissolving in solution (water) to form ions

- Arrhenius acids are covalent compounds in the pure state

Propertiessour taste, change blue litmus paper to red, corrosive

ARRHENIUS BASES

- Bases are substances that ionize in aqueous solutions to produce hydroxide ions (OH-)

NaOH, KOH, Ca(OH)2

NaOH → Na+(aq) + OH-(aq)Ca(OH)2 → Ca2+(aq) + 2OH-(aq)

- Arrhenius bases are ionic compounds in the pure state

Propertiesbitter taste, change red litmus paper to blue, slippery to touch

BRONSTED-LOWRY ACIDS

- Acids are proton (H+) donors

- Not restricted to aqueous solutions

HCl, HNO3, H2SO4

- Bases are proton acceptors

- Not restricted to aqueous solutions

NH3, dimethyl sulfoxide (DMSO)

- Proton donation cannot occur unless an acceptor is present

BRONSTED-LOWRY BASES

LEWIS ACIDS

- Acids are electron pair acceptors

- Not restricted to protons or aqueous solutions

BF3, B2H6, Al2Cl6, AlF3, PCl5

- Bases are electron pair donors

- Not restricted to protons or aqueous solutions

NH3, ethers, ketones, carbon monoxide, sulfoxides

LEWIS BASES

ACIDS

Monoprotic Acid- Donates one proton per molecule (HNO3, HCl)

Diprotic Acid- Donates two protons per molecule (H2SO4, H2CO3)

Triprotic Acid- Donates three proton per molecule (H3PO4, H3AsO4)

Polyprotic Acid- Donates two or more protons per molecule

CONJUGATE ACID BASE PAIRS

- Most Bronsted-Lowry acid-base reactions do not undergo 100% conversion

- Acid-base equilibrium is established

- Every acid has a conjugate base associated with it (by removing H+)

- Every base has a conjugate acid associated with it (by adding H+)

CONJUGATE ACID BASE PAIRS

HX(aq) + H2O(l) X-(aq) + H3O+(aq)

- HX donates a proton to H2O to form X-

HX is the acid and X- is its conjugate base

- H2O accepts a proton from HX H2O acts as a base and H3O+ is its conjugate acid

CONJUGATE ACID BASE PAIRS

NH3(aq) + H2O(l) NH4+(aq) + OH-(aq)

HF(aq) + H2O(l) H3O+(aq) + F-(aq)

HNO3(aq) + H2O(l) H3O+(aq) + NO3-(aq)

AMPHOTERIC SUBSTANCES

- A substance that can lose or accept a proton

- A substance that can function as either Bronsted-Lowry acid or Bronsted-Lowry base

- H2O is the most common

(refer to previous slide for examples)

AUTOPROTOLYSIS OF WATER

H2O(l) + H2O(l) H3O+(aq) + OH-(aq)Kw

- Self ionization- Pure water molecules (small percentage) interact with one

another to form equal amounts of H3O+ and OH- ions

reduces to

H+(aq) + OH-(aq)H2O(l)Kw

- The number of H3O+ and OH- ions present in a sample of pure water at any given time is small

- At equilibrium (25oC)

[H3O+] = [OH-] = 1.00 x 10-7 M

- [H3O+] = hydronium ion concentration

- [OH-] = hydroxide ion concentration

AUTOPROTOLYSIS OF WATER

- The ion product constant of water = [H3O+] x [OH-]

= (1.00 x 10-7) x (1.00 x 10-7)

= 1.00 x 10-14

- Valid in all solutions (pure water and water with solutes)

AUTOPROTOLYSIS OF WATER

Addition of Acidic Solute

- increases [H3O+] - [OH-] decreases by the same factor to make product 1.00 x 10-14

Addition of Basic Solute

- increases [OH-] - [H3O+] decreases by the same factor to make product 1.00 x 10-14

AUTOPROTOLYSIS OF WATER

STRENGTH OF ACIDS

Strong Acids - Transfer 100% (or very nearly 100%) of their protons

to H2O in aqueous solution- Completely or nearly completely ionize in aqueous solution

- Strong electrolytes HCl, HNO3, H2SO4

Weak Acids - Transfer only a small percentage (< 5%) of their protons

to H2O in aqueous solution Organic acids: acetic acid, citric acid

Strong Bases- Completely or nearly completely ionize in aqueous solution

- Strong electrolytes

Hydroxides of Groups IA and IIA are strong bases LiOH, CsOH, Ba(OH)2, Ca(OH)2

most common in lab: NaOH and KOH

Weak bases- produce small amounts of OH- ions in aqueous solution

methylamine, cocaine, morphinemost common: NH3

STRENGTH OF BASES

THE pH CONCEPT

pH

- Negative logarithm of the hydronium ion concentration [H3O+] in an aqueous solution

- [H3O+] and [H+] are used interchangeably

pH = - log[H3O+]

Or

pH = - log[H+]

Acidic Solution- An aqueous solution in which [H3O+] is higher than [OH-]

(pH is less than 7.0)

Basic Solution- An aqueous solution in which [OH-] is higher than [H3O+]

(pH is greater than 7.0)

Neutral Solution- An aqueous solution in which [H3O+] is equal to [OH-]

(pH is equal to 7.0)

- Increasing [H3O+] lowers the pH

THE pH CONCEPT

THE pH CONCEPT

- For [H3O+] coefficient of 1.0 - Expressed in exponential notation

- The pH is the negative of the exponent value

[H3O+] = 1.0 x 10-5 M, then pH = 5.0

[H3O+] = 1.0 x 10-3 M, then pH = 3.0

[H3O+] = 1.0 x 10-11 M, then pH = 11.0

- A change of 1 unit in pH corresponds to a tenfold change in [H3O+]

pH = 3.0 implies [H3O+] = 1.0 x 10-3 M = 0.0010 M pH = 2.0 implies [H3O+] = 1.0 x 10-2 M = 0.010 M

which is tenfold

- The pH meter and the litmus paper are used to determine pH values of solutions

THE pH CONCEPT

ACID-BASE REACTIONS

- Neutralization reactions- Occurs when a solution of an acid is mixed with a solution of a base- The products are salt and water when the base is a metal hydroxide

HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)

HNO3 (aq) + KOH(aq) → KNO3(aq) + H2O(l)

- These reactions can be viewed as proton transfer reactions or double-replacement reactions

- H+ and OH- ions always react in a one-to-one ratio to form water

SALTS

- A cation from a base combines with an anion from an acid to form a salt

(A reaction between an acid and a hydroxide base)

- Salts are ionic compounds

- The positive ion is a metal or polyatomic ion

- The negative ion is a nonmetal or polyatomic ion [exception is the hydroxide ion (OH-)]

- Salts dissociate completely into ions in solution (strong electrolytes)

BUFFER SOLUTION

- A mixture of a conjugate acid-base pair

- Tends to resist changes in pH upon addition of an acid or a base

- The resistive action is the result of equilibrium between the weak acid (HA) and its conjugate base (A-)

HA(aq) + H2O(l) → H3O+(aq) + A-(aq)

- Commonly used in biological systems

- Enzyme-catalyzed reactions depend on pH

BUFFER SOLUTION

Examples

HC2H3O2/C2H3O2-

HF/F-

NH3/NH4+

H2CO3/HCO3-