8.1 Theories of acids and bases

Transcription

1 8. Acids and bases

2 8.1 Theories of acids and bases Sour-tasting substances (acids) have been known for thousands of years. Lavoisiers early theory: Acid= a compound of oxygen and a nonmetal.

3 Arrhenius ionic theory Acid = a substance that produces hydrogen ions (H+) in aqueous solution Base = a substance that produces hydroxide ions (OH-) in aqueous solution e.g HCl(aq) H+(aq) + Cl-(aq) NH3(g) + H2O(l) NH4+ (aq) + OH-(aq)

4 Bronstedt-Lowry theory An acid is a molecule or ion that can donate a proton (H+) = proton donor e.g HCl (g) + H2O(l) H3O+(aq) + Cl- (aq) A base is a molecule or ion that can accept a proton = proton acceptor NH3 (g) + H2O (l) NH4+ (aq) + OH- (aq)

5 Substances such as water, which can act both as an acids and as a bases depending on the species they are reacting with are termed amphiprotic (amphoteric).

6 Conjugate acid-base pairs differs by just one proton. A strong acid (HCl) has a weak conjugate base (Cl-) A weak acid (H2O) has a strong conjugate base.

7 Monoprotic acids: have a single proton to donate e.g HCl, HNO3, CH3COOH Diprotic acids: have two protons to donate e.g H2CO3, H2SO4, H2SO3 Triprotic acid: H3PO4

8 Lewis theory Acid: an electron pair acceptor Base: an electron pair donor A dative (coordinate) covalent bond is formed. Note that this definition does not involve protons or hydrogen!

9 A positively charged metal ion can acts as a Lewis acid, and form a complex with a ligand that acts as a base. 4 NH3 + Zn2+ [Zn(NH3)4]2+ Lewis bonding is found in the (coloured) complex ions formed by the transition metals. 6 H2O + Fe3+ [Fe(H2O)6]3+ Typical ligands: H2O, CN-, NH3

10 8.2 Properties of acids and bases

11 1) Neutralization= acid + base salt + water

12 Acid-base titration In a titration a solution of known concentration (=standard solution) is added in small measured quantities, from a burette, to a fixed volume of another solution, measured with pipette. The addition of the solution is continued until the indicator that is present changes colour. At the end-point (=eqvivalence point) the two substances are present in stoichiometric (equal) quantities.

13 A 50,0 cm3 sample of concentrated sulfuric acid was diluted to 1,00 dm3. A sample of the diluted sulfuric acid was analyzed by titrating with aqueous sodium hydroxide. In the titration, 25,00 cm3 of 1,00 mol dm-3 aqueous sodium hydroxide required 20,0 cm3 of the diluted sulfuric acid for neutralization. Determine the concentration of the original concentrated sulfuric acid solution.

14 2) Metal oxide + acid salt + water

15 3) Acid + carbonate/hydrogencarbonate salt + water + carbon dioxide

16 4) Acid + metal salt + hydrogen

17 5) Acid/base + indicator

18 6) Ammonium salts ammonia

19 8.3.1 Strong acids and bases Strong acids and bases are completely dissociated into their ions in aqueous solution: HCl (aq) H+(aq) + Cl- (aq) Monoprotic acids: 1 mol of acid gives 1 mol of hydrogen ions Diprotic acids: 1 mol of acid gives 2 mol of hydrogen ions

20 8.3.1 Weak acids and bases Weak acids and bases are only slightly dissociated into their ions in aqueous solution. e.g. CH3COOH(aq) CH3COO-(aq) + H+(aq)

21 Acidic strength vs concentration

22 8.3.3 Distinguishing between strong and weak acids and bases 1. Electrical conductivity: When solutions of the same concentration are compared at the same temperature, strong acids and bases have a high amount of ions present in their solutions and therefore a much higher electrical conductivity than weak acids and bases.

23 2. Rate of reaction: Strong acids can be distinguished from weak acids of the same concenentration by comparing the reactions with for example magnesium or calcium carbonate. The strong acid will react more rapidly, leading to quicker evolution of gas.

24 3. ph: The higher the H+ concentration, the lower the ph value. The ph value can be measured using a ph meter or a universal indicator.

25 8.4 The ph-scale

26 For every billion of water molecules only two are ionized:

27 ph = power of Hydrogen

28

29 Acid rain

30 Rain is naturally acidic, because the water molecules react with the CO2 in the air and form the weak acid H2CO3. ph for rain containing this weak acid is max 5.6. Acid rain is therefore: precipitation (rain, snow) with ph lower than 5.6.