Acids and Bases

Chemistry 12

Arrhenius Definition of an Acid

An acid has a sour taste (ex: vinegar, citric acid) and when placed in water to become an aqueous solution it will conduct electricity.

When an acid dissolves in water the acid breaks down into a hydronium ion [H3O]+ or [H]+ for short.

or HCl(aq) → Cl-(aq) + H +(aq) *

*in the presence of water.

Arrhenius Definition of a Base

A base has a slippery feel (ex: soap, cleaning products), they taste bitter and an aqueous solution with a base conducts electricity.

When a base dissolves in water it breaks down to a [OH]- ion or hydroxide ion.

KOH(aq) → K+(aq) + OH-

(aq) *

* in the presence of water.

Limitations of Arrhenius

Applies only to aqueous solutionsAllows only for one kind of base – those

with the hydroxide ion

The Brønsted-Lowry Concept

An acid is a proton (H+) donor and a base is a proton acceptor.

HCl + H2O H3O+ + Cl-

acid + base acid + baseA conjugate acid-base pair is made up of

an acid and the base that results when the acid loses one proton.Ex. HCl/Cl- and H3O+/H2O

Bronsted Lowry Theory of Acids/Bases

An acid is a substance from which a proton or H + ion can

be removed.

A base is a substance that can remove a proton or H +

ion from an acid.

This defiinition is different in the fact that the base can be any negative ion, not just the hydroxide ion. Also water is not the only solvent used.

HCl(aq) + H2O(l) → Cl-(aq) + H3O+(aq)

Which substance is the acid? Base? Conjuagte acid? Conjugate base?

Another Example

H2O + NH3 NH4+ + OH-

a) What is the conjugate base of water?b) What is the conjugate base of

ammonium?Note: Water can act as either an acid or a

base. Such substances are said to be amphoteric.

The stronger an acid, the weaker its conjugate base – HCl/Cl-

The weaker an acid, the stronger its conjugate base – HC2H3O2/C2H3O2

-

Measuring the Strength of Acids and Bases

There is a difference between vinegar and HCl(aq). It is okay to ingest vinegar while you would not want to ingest HCl(aq).

They are said to have different strengths.HCl(aq) is a strong acid, while acetic

acid(vinegar) is a weak acid.

Strong vs Weak Acids

A strong acid will dissociate completely. The HCl(aq) will break down 100% into its products. There is no reactant left.

HCl(aq) → Cl-(aq) + H +(aq)

With acetic acid, the reaction will only use up some reactants so it will reach an equilibrium mixture.

C2H4O2 (aq) + H20(aq) ↔ C2H3O2-(aq) + H30 +

(aq)

(acid) (base) (conjugate base) (conjugate acid)

Comparing Strengths