CI 8.1 Acids and Bases

Acids

Turn litmus red

Neutralised by bases

pH < 7

Liberate CO2 from carbonates

BrØnsted-Lowry Theory

An acid is an H+ donor

A base is an H+ acceptor

H+Cl- + NH3 Cl- + NH4+

The oxonium ion

An acid reacts with water to form oxonium ions

HCl + H2O H3O+ + Cl-

Water is acting as a base

The oxonium ion

H

O

H

H+x

xWhat type of bond is this?

Is there a difference between an alkali and a base?

A base is a substance which accepts H+ ions

Alkali = a base which forms OH-

ions in water

How are hydroxide ions (OH-) made?

NaOH Na+ + OH-

CO32- + H2O HCO3

- + OH-

NH3 + H2O NH4+ + OH-

Acid – base pairs

CH3COOH CH3COO- + H+

CH3COO- + H+ CH3COOH

The ethanoate ion can behave as a base; it is the conjugate base of ethanoic acid.

HA H+ + A-

Conjugate acid

Conjugate base

Every acid has a conjugate base Every base has a conjugate acid

Amphoteric substances

Water can act as either an acid or a base

With a strong acid it acts as a base

With a strong base it acts as an acid

A substance which behaves in this way is said to be amphoteric

Water in the presence of a strong acid:H3O+ H+ + H2O

Water in the presence of a strong base:H2O H+ + OH-

Water as an amphoteric substance

Strength of acids and bases

A strong acid is a powerful H+ donor; it is fully dissociated

A weak acid is a moderate or weak H+ donor; it is weakly dissociated

A strong acid has a weak conjugate base and vice versa

Indicators

Coloured organic substances Weak acids Conjugate acid and conjugate base

forms are different colours e.g. litmus:

HIn H+ + In-

red blueWhich is the acid form?

Addition of acid

In- + H3O+ HIn + H2O

The blue litmus turns red

What is the equation for addition of alkali?