Topic 8: Acids and Bases• Theories of acids and bases• Properties of acids and bases• Strong and weak acids and bases• The pH-scale

8.1 Brönsted-Lowry Acid - base Theory

• The protolytic reaction:Reaction of an acid with water:HCl (g) + H2O ↔ H3O+ + Cl-

Reaction of a base with water:NH3 (g) + H2O ↔ OH- + NH4

+

• Draw the reaction formulas when these substances react as acids:

HNO3

H2SO4

• Draw the reaction formulas when these substances react as bases:

Brönsted-Lowry Acid - base Theory

• Acid: proton donor• Base: proton acceptor

Conjugate acid-base pair

CH3COOH + H2O CH3COO- + H3O+

Acid1 Base2 Base1 Acid2

State for each reaction which reactant is acid and which is base

a) HSO4- + H2O ↔ SO4

2- + H3O+

b) NH3 + H2O ↔ NH4+ + OH-

c) HCO3- + H2O ↔ CO3

2- + H3O+

d) HCO3- + H2O ↔ H2CO3 + OH-

e) H3O+ + OH- ↔ 2 H2O

State for each reaction which reactant is acid and which is base

a) HSO4- + H2O ↔ SO4

2- + H3O+

acid baseb) NH3 + H2O ↔ NH4

+ + OH-

base acid c) HCO3

- + H2O ↔ CO32- + H3O+

acid based) HCO3

- + H2O ↔ H2CO3 + OH-

base acide) H3O+ + OH- ↔ 2 H2O base acid

Which of these are conjugated acid/base-pairs?

a) HSO4-/SO4

2- b) H2SO4

-/SO42-

c) NH3 /NH4+

d) HCO3-/CO3

2- e) CO3

2-/H2CO3

f) H3O+/OH-

Which of these are conjugated acid/base-pairs?

a) HSO4-/SO4

2- c) NH3 /NH4

+ d) HCO3

-/CO32-

What is the rule?

The carboxyl group

Amphiprotic• Water can act both as an acid and as a base;

H3O+ H2O OH-

• Such compounds are said to be amphiprotic (ampholytic).

Monoprotic Polyprotic

• Monoprotic: CH3COOH CH3COO-

Acetic acid

• Diprotic: HOOC-COOH -OOC-COO-

Oxalic acid

• Triprotic: H3PO4 PO43-

Phosphoric acid

• Polyprotic

Arrhenius Acid-Base Theory

• Acid: H+ Hydrogen ion / Proton– Acidic solutions contain H+ / H3O+

(oxonium, hydroxonium or hydronium ion)

• Base: OH- Hydroxide ion– Alkaline solutions contain OH-

Alkaline = Water soluble base

Lewis Acid-Base Theory

• Lewis acid: electron pair acceptor, – e.g. H+, AlCl3, BF3

• Lewis base: electron pair donor, – e.g. OH-, NH3

• A Lewis acid-base reaction involves the formation of a covalent bond. The Lewis base provides the electrons in that bond. This kind of covalent bond is called dative covalent bonds (see topic 13) or co-ordinate covalent bond.

• Its no difference between a normal covalent bond and a dative covalent bond except the origin of the electrons. Sometimes an arrow is used instead of a line to show that it's a dative bond e.g. H3NBF3.

• The term Lewis acid is often just used for acids that aren’t Brönstedt acids

• The formation of complex ions, topic 13, is usually Lewis acid-base reactions

8.2 Properties of acids in solution

• They have pH<7• They taste sour• They react with bases and metals• Where can you find:

Hydrochloric acidSulphuric acidAcetic acidCarbonic acid

Properties of bases in solution

• They have pH>7• They feel ”slippery”• They react with acids• Where can you find:

AmmoniaSodium bicarbonateCalcium carbonate

8.3 Strong acidsTotally dissociated

• Hydrochloric acid: HCl + H2O Cl- + H3O+

chloride

• Nitric acid: HNO3+ H2O NO3- + H3O+

nitrate

• Sulphuric acid: H2SO4 + H2O SO42- + H3O+

sulphate

HCl + H2O H3O+ +Cl-

Start 100% 0% End 0% 100%

Weak acidsPartially dissociated

• Ethanoic acid, (Acetic acid)CH3COOH + H2O CH3COO- + H3O+

ethanoate ion(acetate ion)

• Carbonic acid, H2CO3 H2O CO3

2- + H3O+

carbonate ion

(HCO3- hydrogencarbonate)

CH3CH2COOH + H2O H3O+ + CH3CH2COO-

start 100% 0%end 99% 1%

If the concentration is the same for the strong and the weak acid:

– The strong acid is more acidic than the weak acid– The strong acid has a higher concentration of

hydroxonium ions than the weak acid– The strong acid has higher conductivity

Strong basesContaining the OH- ion

• All group I hydroxides: NaOH(s) + H2O Na+ + OH-

• Group II hydroxides Ba(OH)2 + H2O Ba2+ + 2 OH-

Weak basesPartially dissociated

• Ammonia NH3 + H2O NH4

+ + OH-

• Ethylamine CH3CH2-NH2 + H2O CH3CH2-NH3

+ + OH-

The anions from carbonic acid; CO3

2- and HCO3-

• Alkaline properties• Often water soluble salts

• H2CO3 + H2O HCO3- + H3O+

Conjugated acid and base pair

Acid Base Base Acid

• HCO3- + H2O CO3

2- + H3O+Conjugated acid and base pair

Acid Base Base Acid

IndicatorsAcidic Neutral Basic

Litmus red blue

BTB (red) yellow green Blue

Phenolphthalein colourless colourless cerise

Universal paper (red) (green) (blue)

Some typical reactions of acids- salt formations

• Neutralisation• Reactions with metals or metal oxides

Neutralisation

Acid + base salt + waterHCl + NaOH NaCl + H2OH2SO4 + KOH HNO3 + Ba(OH)2

Neutralisation

H2SO4 + 2 KOH K2SO4 + 2 H2O 2 HNO3 + Ba(OH)2 Ba(NO3)2 + 2 H2O

With metals

Acid + metal salt + hydrogen gas

Mg + 2 HCl MgCl2 + H2

Al + H2SO4 ? ?

More noble metals (Cu, Ag, Au) doesn’t react with HCl or H2SO4 . They demand more oxidative acids (HNO3) and will then give other gases than H2 (N2O)

With metal oxides

Acid + metal oxide salt + water

CuO + 2 HCl CuCl2 + H2O

To synthesise a salt from a noble metal you can’t start with metal + acid (Why?)

Carbonates and hydrogen carbonates

Carbonates+ acids salt+ carbondioxide+ waterNa2CO3 + 2 HCl 2 NaCl + CO2 + H2O

CaCO3 + HCl KHCO3 + H2SO4

Carbonates and hydrogen carbonates

CaCO3 + 2 HCl 2 CaCl2 + CO2 + H2O

2 KHCO3 + H2SO4 2 K2SO4 + 2 CO2 + 2 H2O

8.4 The pH-scale

pH = -log[H+] [H+] = [H3O+]

• pH = -log[H+] => change in one pH unit = 10 times difference in [H+]

• pH=5 pH= 3 => 100 times more acidic.• pH=8 pH= 11 => 1000 times more basic.

• pH-meter, pH-paper• [H+] = 10-pH