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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 Brönsted -Lowry Acid - base Theory. The protolytic reaction : Reaction of an acid with water : H Cl (g) + H 2 O H 3 O + + Cl -

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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 Brönsted-Lowry Acid - base Theory • The protolyticreaction: Reactionof an acidwithwater: HCl (g) + H2O H3O+ + Cl- Reactionofa basewithwater: NH3 (g) + H2O ↔ OH- + NH4+ Oxoniumion Hydroxideion

  3. Draw the reactionformulaswhenthesesubstancesreactwithwater as acids: HNO3 H2SO4 CH3COOH Draw the reactionformulaswhenthisesubstancesreactwithwater as bases: CH3NH2 CO32-

  4. Nitricacid HNO3+ H2O H3O+ + NO3- Sulphuricacid H2SO4 + H2O H3O+ + HSO4– HSO4 – + H2O H3O+ + SO42- Etanoicacid CH3COOH + H2O ↔H3O+ + CH3COO- Methylamine CH3NH2 + H2O ↔ OH- + NH4+ Carbonateion CO32-+ H2O ↔ OH- + HCO3-

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

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

  7. State for eachreactionwhichreactant is acid and which is base a) HSO4- + H2O ↔ SO42- + H3O+ b) NH3 + H2O ↔ NH4+ + OH- c) HCO3- + H2O ↔ CO32- + H3O+ d) HCO3-+ H2O ↔ H2CO3+ OH- e) H3O+ + OH-↔ 2 H2O

  8. State for eachreactionwhichreactant is acid and which is base a) HSO4- + H2O ↔ SO42- + H3O+ acidbase b) NH3 + H2O ↔ NH4+ + OH- baseacid c) HCO3- + H2O ↔ CO32- + H3O+ acidbase d) HCO3-+ H2O ↔ H2CO3+ OH- baseacid e) H3O+ + OH-↔ 2 H2O baseacid

  9. Whichoftheseareconjugatedacid/base-pairs? • HSO4-/SO42- • H2SO4-/SO42- c) NH3/NH4+ d) HCO3-/CO32- e) CO32-/H2CO3 f) H3O+/OH-

  10. Whichoftheseareconjugatedacid/base-pairs? • HSO4-/SO42- c) NH3/NH4+ d) HCO3-/CO32- What is the rule?

  11. The carboxylgroup

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

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

  14. 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

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

  16. 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.

  17. 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

  18. 8.2 Propertiesofacids in solution • Theyhave pH<7 • Theytastesour • Theyreactwithbasesand metals • Wherecanyoufind: Hydrochloricacid Sulphuricacid Aceticacid Carbonicacid

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

  20. 8.3 Strong acidsTotally dissociated Hydrochloric acid: HCl + H2O  Cl-+ H3O+ chloride Nitric acid: HNO3+ H2O NO3- + H3O+ nitrate Sulphuric acid: H2SO4+ H2O HSO4-+ H3O+ hydrogensulphate HSO4-+ H2O  SO42-+ H3O+ sulphate

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

  22. Weak acidsPartially dissociated • Ethanoic acid, (Acetic acid) CH3COOH + H2O  CH3COO- + H3O+ ethanoateion (acetateion) • Carbonic acid, H2CO3 +H2O HCO3-+ H3O+ hydrogen carbonate ion HCO3-+H2O CO32- + H3O+ carbonate ion

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

  24. 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

  25. http://phet.colorado.edu/en/simulation/acid-base-solutions

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

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

  28. 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

  29. Indicators

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

  31. Neutralisation- exothermic, no equilibrium Acid + base salt + water HCl+ NaOH  NaCl + H2O H2SO4 + KOH  HNO3 + Ba(OH)2

  32. Neutralisation H2SO4 + 2 KOH  K2SO4 + 2 H2O 2 HNO3 + Ba(OH)2 Ba(NO3)2 + 2 H2O http://chem-ilp.net/labTechniques/AcidBaseIdicatorSimulation.htm

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

  34. Carbonates and hydrogen carbonates Carbonates+ acids salt+ carbon dioxide+ water Na2CO3+ 2 HCl  2 NaCl + CO2 + H2O CaCO3 + HCl KHCO3 + H2SO4

  35. Carbonates and hydrogen carbonates CaCO3 + 2 HCl 2 CaCl2+ CO2 + H2O 2 KHCO3 + H2SO4 2 K2SO4+ 2 CO2+ 2 H2O

  36. With metals Acid+ metal  salt + hydrogen gas Mg + 2 HCl  MgCl2+ H2 Al + H2SO4 ? ? Na + H3PO4 ? ? 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)

  37. Acid+ metal  salt + hydrogen gas 2 Al + 3 H2SO4 Al2(SO4)3 + 3 H2 6 Na + 2 H3PO4 2 Na3PO4 + 3H2

  38. 8.4 The pH-scale pH = -log[H3O+]

  39. pH = -log[H3O+] => change in one pH unit = 10 times difference in [H3O+] pH=5  pH= 3 => 100 times more acidic. pH=8  pH= 11 => 1000 times more basic. pH-meter, pH-paper [H3O+] = 10-pH

  40. http://phet.colorado.edu/en/simulation/ph-scale

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