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Topic 8: Acids and Bases

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 Arrhenius Acid-Base Theory. Acid: H + Hydrogen ion / Proton Acidic solutions contain H + / H 3 O +

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Topic 8: Acids and Bases

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

  2. 8.1 Arrhenius Acid-Base Theory • Acid: H+Hydrogen ion / Proton • Acidic solutions contain H+ / H3O+ (oxonium, hydroxonium or hydroniumion) • Base: OH- Hydroxide ion • Alkaline solutions containOH- Alkaline = Water soluble base

  3. Brönsted-LowryAcid - base Theory • Acid: proton donor • Base: proton acceptor

  4. Conjugate acid-base pair CH3COOH + H2O  CH3COO- + H3O+ Acid1 Base2 Base1Acid2

  5. The carboxylgroup

  6. Amphiprotic • Water can act both as an acid and as a base; H3O+ H2O OH- • Such compounds are said to be amphiprotic (ampholytic).

  7. Monoprotic Polyprotic • Monoprotic: CH3COOH  CH3COO- Acetic acid • Diprotic: HOOC-COOH -OOC-COO- Oxalic acid • Triprotic: H3PO4  PO43- Phosphoric acid • Polyprotic

  8. Lewis Acid-Base Theory • Lewis acid: electron pair acceptor, • e.g. H+, AlCl3, BF3 • Lewis base: electron pair donor, • e.g. OH-, NH3

  9. 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. H3NBF3.

  10. 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

  11. 8.2 Propertiesofacids in solution • Theyhave pH<7 • Theytastesour • Theyreactwithbasesand metals • Wherecanyoufind: Hydrochloricacid Sulphuricacid Aceticacid Carbonicacid

  12. Propertiesofbases in solution • Theyhave pH>7 • Theyfeel ”slippery” • Theyreactwithacids • Wherecanyoufind: Ammonia Sodium bicarbonate Calcium carbonate

  13. 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

  14. HCl + H2O  H3O+ +Cl- Start 100% 0% End 0% 100%

  15. Weak acidsPartially dissociated • Ethanoic acid, (Acetic acid) CH3COOH + H2O  CH3COO- + H3O+ ethanoateion (acetateion) • Carbonic acid, H2CO3H2O  CO32- + H3O+ carbonate ion (HCO3-hydrogencarbonate)

  16. CH3CH2COOH + H2O  H3O+ + CH3CH2COO- start 100% 0% end 99% 1%

  17. 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

  18. Strong basesContaining the OH- ion • All group I hydroxides: NaOH(s) + H2O Na+ + OH- • Group II hydroxides Ba(OH)2 + H2O Ba2+ + 2 OH-

  19. Weak basesPartially dissociated • Ammonia NH3+ H2O  NH4+ + OH- • Ethylamine CH3CH2-NH2 + H2O  CH3CH2-NH3+ + OH-

  20. The anions from carbonic acid; CO32- and HCO3- • Alkaline properties • Often water soluble salts • H2CO3 + H2O HCO3- + H3O+ Conjugated acid and base pair Acid Base Base Acid • HCO3- + H2O  CO32- + H3O+ Conjugated acid and base pair AcidBase BaseAcid

  21. Indicators

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

  23. Neutralisation Acid + base salt + water HCl+ NaOH  NaCl + H2O H2SO4 + KOH  ? ?

  24. 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)

  25. With metal oxides Acid+ metaloxide salt + water CuO+ 2 HCl CuCl2+ H2O To synthesise a salt from a noble metalyoucan’tstart with metal + acid (Why?)

  26. Carbonates and hydrogen carbonates Carbonates+acidssalt+carbondioxide+water Na2CO3+ 2 HCl  2 NaCl + CO2 + H2O

  27. 8.4 The pH-scale pH = -log[H+] [H+] = [H3O+]

  28. 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

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