Acids & Bases have distinctive properties…

Properties of Acids and BasesACIDS● Have a sour taste

● Change the color of many indicators

● Are corrosive (react with metals)

● Neutralize bases

● Conduct an electric current

BASES● Have a bitter taste

● Change the color of many indicators

● Have a slippery feeling

● Neutralize acids

● Conduct an electric current

● An acid is a compound that contains an H+, and produces hydrogen ion (H+) when dissolved in water.

Example: HCl H+ + Cl-

HCl + H2O H3O+(aq) + Cl-

(aq)

Hydronium Ion

(can be used interchangeably with H+)

● A base is a compound that contains an OH- group and produces hydroxide ions (OH-) when dissolved in water.

Example: NaOH Na+ + OH-

Neutralization● Neutralization: the combination of H+ with

OH- to form water.

● H+(aq) + OH-

(aq) H2O(l)

One problem with the Arrhenius theory is that it’s not comprehensive enough. Some compounds act like acids and bases that don’t fit the standard definition.

Bronsted-Lowry Theory of Acids & Bases:● An ACID is a proton (H+) donor

● A BASE is a proton (H+) acceptor

HCl(aq) + H2O(l) H3O+(aq) + Cl-(aq)

Proton transfer

Acid

Base

NH3(aq) + H2O(l) NH4+

(aq) + OH-(aq)

BASE

ACID

CONJUGATE ACID

CONJUGATE BASE

Ammonia is a proton acceptor, and thus a

base

Water is a proton donor, and thus an

acid.

Conjugate acid-base pairs

● Conjugate acid-base pairs differ by one proton (H+)

**A conjugate acid is the particle formed when a base gains a proton.

**A conjugate base is the particle that remains when an acid gives off a proton.

Examples: In the following reactions, label the conjugate acid-base pairs:

H3PO4 + NO2- HNO2 + H2PO4

-

CN- + HCO3- HCN + CO3

2-

acid base c. acid c. base

acidbase c. acid c. base

Examples: In the following reactions, label the conjugate acid-base pairs:

HCN + SO32- HSO3

- + CN-

H2O + HF F- + H3O+

acid base c. acid c. base

acidbase c. acidc. base

● A substance that can act as both an acid and a base (depending on what it is reacting with) is termed AMPHOTERIC.

**Water is a prime example.

The Lewis Theory of Acids & Bases

● Lewis acid: a substance that can accept an electron pair to form a covalent bond (electron pair acceptor).

● Lewis base: a substance that can donate an electron pair to form a covalent bond (electron pair donor).

Example 1:Ionization of NH3:

NH3 + H2O NH4+ + OH-

N NH

H

H O HH+ H

H

H

H H+ O.. .....

.

.

.

+-

acidbase

Example 2:Auto-ionization of water:

H2O + H2O H3O+ + OH-

O OH H O HH+ H

HH H+ O.. .

.

.

...

.

.

+-

acidbase

.

.

.

.

Example 3:Reaction of NH3 with HBr (a Lewis AND

a Bronsted-Lowry acid-base reaction):

NH3 + HBr NH4+ + Br-

N NH

H

H BrH+ H

H

H

H + Br..

.

.

.

...

.

.

+-

acidbase

Lewis

Bronsted-Lowry

Arrhenius

SUMMARY OF ACID-BASE THEORIES:

Theory Acid Definition Base Definition

Arrhenius Theory

Any substance which releases H+ ions in water solution.

Any substance which releases OH- ions in water solution

Brǿnsted-Lowry Theory

Any substance which donates a proton (H+).

Any substance which accepts a proton (H+).

Lewis Theory

Any substance which can accept an electron pair.

Any substance which can donate an electron pair.