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

The protolytic reaction: Reaction of an acid with water: HCl (g) + H 2 O H 3 O + + Cl - Reaction of a base with water: NH 3 (g) + H 2 O ↔ OH - + NH Brönsted-Lowry Acid - base Theory Oxonium ion Hydroxide ion

Draw the reaction formulas when these substances react with water as acids: HNO 3 H 2 SO 4 CH 3 COOH Draw the reaction formulas when thise substances react with water as bases: CH 3 NH 2 CO 3 2-

Nitric acid HNO 3 + H 2 O H 3 O + + NO 3 - Sulphuric acid H 2 SO 4 + H 2 O H 3 O + + HSO 4 – HSO 4 – + H 2 O H 3 O + + SO 4 2- Etanoic acid CH 3 COOH + H 2 O ↔ H 3 O + + CH 3 COO - Methylamine CH 3 NH 2 + H 2 O ↔ OH - + NH 4 + Carbonate ion CO H 2 O ↔ OH - + HCO 3 -

Brönsted-Lowry Acid - base Theory Acid: proton donor Base: proton acceptor

Conjugate acid-base pair CH 3 COOH + H 2 O  CH 3 COO - + H 3 O + Acid 1 Base 2 Base 1 Acid 2

State for each reaction which reactant is acid and which is base a) HSO H 2 O ↔ SO H 3 O + b) NH 3 + H 2 O ↔ NH OH - c) HCO H 2 O ↔ CO H 3 O + d) HCO H 2 O ↔ H 2 CO 3 + OH - e) H 3 O + + OH - ↔ 2 H 2 O

State for each reaction which reactant is acid and which is base a) HSO H 2 O ↔ SO H 3 O + acid base b) NH 3 + H 2 O ↔ NH OH - base acid c) HCO H 2 O ↔ CO H 3 O + acid base d) HCO H 2 O ↔ H 2 CO 3 + OH - base acid e) H 3 O + + OH - ↔ 2 H 2 O base acid

Which of these are conjugated acid/base-pairs? a)HSO 4 - /SO 4 2- b)H 2 SO 4 - /SO 4 2- c) NH 3 /NH 4 + d) HCO 3 - /CO 3 2- e) CO 3 2- /H 2 CO 3 f) H 3 O + /OH -

Which of these are conjugated acid/base-pairs? a)HSO 4 - /SO 4 2- c) NH 3 /NH 4 + d) HCO 3 - /CO 3 2- What is the rule?

The carboxyl group

Amphiprotic Water can act both as an acid and as a base; H 3 O +  H 2 O  OH - Such compounds are said to be amphiprotic (ampholytic).

Monoprotic  Polyprotic Monoprotic: CH 3 COOH  CH 3 COO - Acetic acid Diprotic: HOOC-COOH  - OOC-COO - Oxalic acid Triprotic: H 3 PO 4  PO 4 3- Phosphoric acid Polyprotic

Arrhenius Acid-Base Theory Acid: H + Hydrogen ion / Proton – Acidic solutions contain H + / H 3 O + (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 +, AlCl 3, BF 3 Lewis base: electron pair donor, – e.g. OH -, NH 3

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. H 3 N  BF 3.

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 acid Sulphuric acid Acetic acid Carbonic acid

Properties of bases in solution They have pH>7 They feel ”slippery” They react with acids Where can you find: Ammonia Sodium bicarbonate Calcium carbonate

8.3 Strong acids Totally dissociated Hydrochloric acid: HCl + H 2 O  Cl - + H 3 O + chloride Nitric acid: HNO 3 + H 2 O  NO H 3 O + nitrate Sulphuric acid: H 2 SO 4 + H 2 O  HSO H 3 O + hydrogensulphate HSO H 2 O  SO H 3 O + sulphate

HCl + H 2 O  H 3 O + +Cl - Start 100% 0% End 0% 100%

Weak acids Partially dissociated Ethanoic acid, (Acetic acid) CH 3 COOH + H 2 O  CH 3 COO - + H 3 O + ethanoate ion (acetate ion) Carbonic acid, H 2 CO 3 +H 2 O  HCO H 3 O + hydrogen carbonate ion HCO 3 - +H 2 O  CO H 3 O + carbonate ion

CH 3 CH 2 COOH + H 2 O  H 3 O + + CH 3 CH 2 COO - 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 bases Containing the OH - ion All group I hydroxides: NaOH (s) + H 2 O  Na + + OH - Group II hydroxides Ba(OH) 2 + H 2 O  Ba OH -

Weak bases Partially dissociated Ammonia NH 3 + H 2 O  NH OH - Ethylamine CH 3 CH 2 -NH 2 + H 2 O  CH 3 CH 2 -NH OH -

The anions from carbonic acid; CO 3 2- and HCO 3 - Alkaline properties Often water soluble salts H 2 CO 3 + H 2 O  HCO H 3 O + Conjugated acid and base pair Acid Base Base Acid HCO H 2 O  CO H 3 O + Conjugated acid and base pair Acid Base Base Acid

Indicators AcidicNeutralBasic Litmusredblue BTB(red) yellowgreenBlue Phenolphthaleincolourless cerise Universal paper(red)(green)(blue)

Some typical reactions of acids- salt formations Neutralisation Reactions with metals or metal oxides

Neutralisation- exothermic, no equilibrium Acid + base  salt + water HCl + NaOH  NaCl + H 2 O H 2 SO 4 + KOH  HNO 3 + Ba(OH) 2 

Neutralisation H 2 SO KOH  K 2 SO H 2 O 2 HNO 3 + Ba(OH) 2  Ba(NO 3 ) H 2 O

With basic metal oxides Acid + metal oxide  salt + water CuO + 2 HCl  CuCl 2 + H 2 O To synthesise a salt from a noble metal you can’t start with metal + acid (Why?)

Carbonates and hydrogen carbonates Carbonates+ acids  salt+ carbon dioxide+ water Na 2 CO HCl  2 NaCl + CO 2 + H 2 O CaCO 3 + HCl  KHCO 3 + H 2 SO 4 

Carbonates and hydrogen carbonates CaCO HCl  2 CaCl 2 + CO 2 + H 2 O 2 KHCO 3 + H 2 SO 4  2 K 2 SO CO H 2 O

With metals Acid + metal  salt + hydrogen gas Mg + 2 HCl  MgCl 2 + H 2 Al + H 2 SO 4  ? ? Na + H 3 PO 4  ? ? More noble metals (Cu, Ag, Au) doesn’t react with HCl or H 2 SO 4. They demand more oxidative acids (HNO 3 ) and will then give other gases than H 2 (N 2 O)

Acid + metal  salt + hydrogen gas 2 Al + 3 H 2 SO 4  Al 2 (SO 4 ) H 2 6 Na + 2 H 3 PO 4  2 Na 3 PO H 2

8.4 The pH-scale pH = -log[H 3 O + ]

pH = -log[H 3 O + ] => change in one pH unit = 10 times difference in [H 3 O + ] pH=5  pH= 3 => 100 times more acidic. pH=8  pH= 11 => 1000 times more basic. pH-meter, pH-paper [H 3 O + ] = 10 -pH