08AcidsandBasesP1.pptx

8 Acids and bases

8.1 Theories of acids and bases

* Define acids and bases according to the Brønsted–Lowry and Lewis theories.

* Deduce whether or not a species could act as a Brønsted–Lowry and/ or a Lewis acid or base.

* Deduce the formula of the conjugate acid (or base) of any Brønsted–Lowry base (or acid).

8.2 Properties of acids and bases

* Outline the characteristic properties of acids and bases in aqueous solution.

_{8.3 Strong and weak acids and bases}

* Distinguish between strong and weak acids and bases in terms of the extent of dissociation, reaction with water and electrical conductivity. * State whether a given acid or base is strong or weak.

8.4 The pH scale

* Distinguish between aqueous solutions that are acidic, neutral or alkaline using the pH scale.

* Identify which of two or more aqueous solutions is more acidic or alkaline using pH values.

* State that each change of one pH unit represents a 10-fold change in the hydrogen ion concentration [H+(aq)].

8.1 Theories of acids and bases

•

_{Definition #1: Arrhenius (traditional)}

- Acids – produce H+ _{ions (or hydronium ions H}

3O+),

- Bases – produce OH- ions

(problem: some bases don’t have hydroxide ions! And focus was only aqua's system which is somehow limited !)

Arrhenius acid is a substance that produces H+ _(H

3O+) in water

Arrhenius base is a substance that produces OH- _{in water}

•

_{Definition #2: Brønsted – Lowry}

- Acids – proton donor

- Bases – proton acceptor

A “proton” is really just a hydrogen atom that has lost it’s electron!

A Brønsted-Lowry acid is a proton donor A Brønsted-Lowry base is a proton acceptor

base acid conjugate

acid

conjugate base

Acids = H+

Conjugate pairs

The Brønsted definition: NH₃ is a BASE in water and water is itself an ACID

Base Acid

Acid

Base NH4

+ _{+ OH}

-NH₃ + H₂O

Base Acid

Acid

Base NH4

+ _{+ OH} -NH₃ + H₂O

Lets consider the acid base reaction between a generic acid HA and a base B

HA + B  A- _{+ BH}+

HA acts as an acid (proton donor)

B acts as a base (proton receiver)

Forward reaction



BH+ acts as an acid (proton donor)

A- acts as a base (proton receiver)

Reverse reaction



The dissociation of an acid is an equilibrium process !

HA + B



A

+ BH

The acid base pairs related to each other are called Conjugate acid-base pairs

Conjugate acid-base pair_========

 ===

=== ==



Conjugate acid-base pair=== =======

Example:

CH₃COOH(aq) + H₂O(l)  CH₃COO- _{(aq) + H}

3O+ (aq)

CH₃COOH / CH₃COO

Conjugate pair

acid base

H₂O / H₃O+

acid

base

Exercise

H₃O+ _{(aq) + NH}

3(aq)  H2O(l) + NH4+ (aq)

Identify each of the compounds below as an acid or base. Which are conjugate pairs

H₃O+ _{(aq) is the acid H}

2O(l) is the conjugate base of the acid H3O+

NH₃(aq) is the base NH₄+ _{is the conjugate acid of the base NH} 3

HCN (aq) + H₂O(l) H₃O+_{(aq) + CN}- _(aq)

HCN is the acid CN- _{is the conjugate base of the acid HCN}

H₂O is the baseH₃O+ _{is the conjugate acid of the base H} 2O

H₂SO₄ + H₂O(l)  H₃O+_{(aq) + HSO}

4(aq)

H₂SO₄ is the acid HSO₄- _{is the conjugate base of the acid H}

2SO 4

H₂O is the base H₃O+ is the conjugate acid of the base water

H₂O (l) + H₂O(l)  H₃O+_{(aq) + OH}-_(q)

H₂O (aq) is the acid OH- _{is the conjugate base of the acid H} 2O

Typical conjugate pairs:

Some species can act as acids and as a base considering the following

CH₃COOH(aq) + H₂O(l)  CH₃COO- (aq) + H

3O+ (aq)

base

acid

base

NH₃ (aq) + H₂O(l)  NH₄+ _{(aq) + OH}- _(aq)

acid

Lewis: A theory of electron pairs

 _{Some acid base reactions don’t fit the Bronsted-Lowry or Arrhenius}

classifications

 _{The Lewis acid-base concepts expands the acid class}

 _{Such reactions involve a “sharing” of electron pairs between atoms or ions}

Lewis acid - a substance that accepts an electron pair Lewis base - a substance that donates an electron pair

Formation of hydronium ion is also an excellent example.

• _{Electron pair of the new O-H bond originates on the Lewis base.}

Example

BF₃ is the Lewis acid - a substance that accepts an electron pair

NH₃ is the Lewis base - a substance that donates an electron pair

10

F

B

F _F

H N

H H +

F

B

F _F

H N

H H

An acid is an electron-pair

acceptor

A base is an electron-pair

Other examples of lewis acid base reaction are found in the chemistry of transition elements

CU 2+ (_{aq) + 6H}

2O (l)  [CU(H20)6]2+ (aq)

Cu forms a complex ion and will act as an acid

[CU(H₂0)₆]2+ _{(aq) + NH}

3 (aq)  [CU(NH3)(H20)5]2+ (aq) + H2O

Reaction Cu complex with NH₃

Comparison Bronsted-Lowry and Lewis of acids and bases

Theory Definition acid Definition of base

Bronsted Lowry Proton Donor Proton acceptor Lewis Electron pair

acceptor Electron pair donor

All Bronsted-Lowry acids are lewis acids Not all lewis acids are Bronsted Lowry acids

Lewis acids are usually reserved for those that can only be describes with lewis