topic 8: acids and bases
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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 - - PowerPoint PPT PresentationTRANSCRIPT
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Topic 8: Acids and Bases• Theories of acids and bases• Properties of acids and bases• Strong and weak acids and bases• The pH-scale
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• 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
+
8.1 Brönsted-Lowry Acid - base Theory
Oxonium ion
Hydroxide ion
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Draw the reaction formulas when these substances react with water as acids:HNO3
H2SO4
CH3COOH
Draw the reaction formulas when thise substances react with water as bases:CH3NH2
CO32-
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Nitric acidHNO3+ H2O H3O+ + NO3
-
Sulphuric acidH2SO4 + H2O H3O+ + HSO4
–
HSO4 –
+ H2O H3O+ + SO42-
Etanoic acidCH3COOH + H2O ↔ H3O+ + CH3COO-
MethylamineCH3NH2 + H2O ↔ OH- + NH4
+
Carbonate ionCO3
2- + H2O ↔ OH- + HCO3
-
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Brönsted-Lowry Acid - base Theory
• Acid: proton donor• Base: proton acceptor
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Conjugate acid-base pair
CH3COOH + H2O CH3COO- + H3O+
Acid1 Base2 Base1 Acid2
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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
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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
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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-
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Which of these are conjugated acid/base-pairs?
a) HSO4-/SO4
2- c) NH3 /NH4
+ d) HCO3
-/CO32-
What is the rule?
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The carboxyl group
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Amphiprotic• Water can act both as an acid and as a base;
H3O+ H2O OH-
• Such compounds are said to be amphiprotic (ampholytic).
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Monoprotic Polyprotic
• Monoprotic: CH3COOH CH3COO-
Acetic acid
• Diprotic: HOOC-COOH -OOC-COO-
Oxalic acid
• Triprotic: H3PO4 PO43-
Phosphoric acid
• Polyprotic
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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
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Lewis Acid-Base Theory
• Lewis acid: electron pair acceptor, – e.g. H+, AlCl3, BF3
• Lewis base: electron pair donor, – e.g. OH-, NH3
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• 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.
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• 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
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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
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Properties of bases in solution
• They have pH>7• They feel ”slippery”• They react with acids• Where can you find:
AmmoniaSodium bicarbonateCalcium carbonate
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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 SO4
2-+ H3O+
sulphate
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HCl + H2O H3O+ +Cl-
Start 100% 0% End 0% 100%
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Weak acidsPartially dissociated
• Ethanoic acid, (Acetic acid)CH3COOH + H2O CH3COO- + H3O+
ethanoate ion(acetate ion)
• Carbonic acid, H2CO3 +H2O HCO3
- + H3O+
hydrogen carbonate ion
HCO3- +H2O CO3
2- + H3O+
carbonate ion
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CH3CH2COOH + H2O H3O+ + CH3CH2COO-
start 100% 0%end 99% 1%
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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
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http://phet.colorado.edu/en/simulation/acid-base-solutions
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Strong basesContaining the OH- ion
• All group I hydroxides: NaOH(s) + H2O Na+ + OH-
• Group II hydroxides Ba(OH)2 + H2O Ba2+ + 2 OH-
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Weak basesPartially dissociated
• Ammonia NH3 + H2O NH4
+ + OH-
• Ethylamine CH3CH2-NH2 + H2O CH3CH2-NH3
+ + OH-
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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
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IndicatorsAcidic Neutral Basic
Litmus red blue
BTB (red) yellow green Blue
Phenolphthalein colourless colourless cerise
Universal paper (red) (green) (blue)
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Some typical reactions of acids- salt formations
• Neutralisation• Reactions with metals or metal oxides
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Neutralisation- exothermic, no equilibrium
Acid + base salt + waterHCl + NaOH NaCl + H2OH2SO4 + KOH HNO3 + Ba(OH)2
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Neutralisation
H2SO4 + 2 KOH K2SO4 + 2 H2O 2 HNO3 + Ba(OH)2 Ba(NO3)2 + 2 H2O
http://chem-ilp.net/labTechniques/AcidBaseIdicatorSimulation.htm
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With basic 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?)
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Carbonates and hydrogen carbonates
Carbonates+ acids salt+ carbon dioxide+ waterNa2CO3 + 2 HCl 2 NaCl + CO2 + H2O
CaCO3 + HCl KHCO3 + H2SO4
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Carbonates and hydrogen carbonates
CaCO3 + 2 HCl 2 CaCl2 + CO2 + H2O
2 KHCO3 + H2SO4 2 K2SO4 + 2 CO2 + 2 H2O
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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)
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Acid + metal salt + hydrogen gas2 Al + 3 H2SO4 Al2(SO4)3 + 3 H2
6 Na + 2 H3PO4 2 Na3PO4 + 3 H2
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8.4 The pH-scale
pH = -log[H3O+]
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• 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
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http://phet.colorado.edu/en/simulation/ph-scale