10. Acids, bases and salts, acid-base theories
10. Acids, bases and salts, acid-base theories
Arrhenius, Brönsted, acidity scale, Lewis, definition of pH, types of acids and salts, denominations
Arrhenius theory:
· Acid:
o releases H+ in water
o e.g. HCl, H2SO4, H2CO3, CH3COOH
· Base:
o releases OH- in water
o e.g. NaOH, KOH, Ca(OH)2, NH3
· acid + base = salt + H2O
· HCl + NaOH= NaCl + H2O
· limitations
o restricted to aqueous solutions
o doesn’t explain basicity of substances like ammonia (with lack of OH)
Brönsted theory:
· Acid:
o releases an H+ in water, proton donors
o e.g. HCl, H2SO4, H2CO3, CH3COOH
· Base:
o takes up a H+ in water, proton acceptors
o e.g. NaOH, KOH, Ca(OH)2, NH3
· This means that in an acid base reaction acid-base pairs form, the remainder of an acid
in the product will act as a base and vice versa
acid + base = base’ + acid’
HCl + Na+ + OH- = Cl- + Na+ + H2O
HCl + H2O = Cl- + H3O+
- Conjugate Acid-Base Pairs:
· Acid becomes its conjugate base after donating a proton.
· Base becomes its conjugate acid after accepting a proton.
· from these two reactions it is clear that according to Brönsted water can be base and acid as well
· Strong acid:
o the corresponding base is weak
o fully ionize in water
o HCl, H2SO4
· Strong base:
o the corresponding acid is weak
o completely dissociate into ions in water
o NaOH, KOH
· Weak acid:
o the corresponding base is strong
o partially ionize in water
o H2SO4
· Weak base
o the corresponding acid is strong
o partially dissociate into ions in water
o NH3
· Similarity to redox reactions:
o the acid releases the proton(s) (H+ is basically a proton) (similar to oxidation)
o the base picks up the released proton (s) (similar to reduction)
· According to Brönsted nearly everything can be an acid and a base
o e.g. CH3COOH
o in practice only H is released but in theory H could also be released
o e.g. CH4 could be an acid in theory, but even K is not basic enough to get CH4
to release a proton, if it could happen CH3- would be a very very strong base, since it wants its proton back
o Similarly CH4 can theoretically be a base and turn into CH5+ *
Lewis theory
· Acid:
o has an electron pair-deficit, electron-pair acceptor
o BF3
· Base:
o has an excess electron pair, electro-pair donor
o NH3
· Acid + Base =complex (salt)
o (dative bond)
o this reaction is not redox, there is no electron transfer
* HOSO2F (very strong acid) + SbF5 (Lewis acid) = H+ + OSO2F(SbF5) (superacid)
protonates CH4 → George Oláh got a Nobel-prize for this discovery
pH
· H+/H3O+ concentration is fundamental in 2 of the acid-base theories
· the hydrogen ion-concentration (mol/l) is characteristic for acidity, but it changes in a wide range → logarithm
· since the H+ concentration is often less than 1 mol/l we use -logarithm
· thus pH= -log10[H+]
· in case of bases it is easier to calculate with the pOH and then convert it to pH
· pOH = -log10[OH-]
· pOH + pH=14
· for strong bases and acids we can calculate directly from the concentration of the base/acid
· for weak acids/bases we can calculate using Ka/Kb which are the dissociation constants of the bases/acids (basically equilibrium constants) and the initial concentration of the acid/base
o pH>7 → basic
o pH=7 → neutral
o pH<7 → acidic
Types of acids and salts
· Acids:
o monoprotic acid:
§ a single proton dissociates
§ e.g. HCl, HBr, HClO4, HCOOH
o biprotic acid:
§ two protons dissociate
§ e.g. H2SO4, H2CO3
o triprotic acid:
§ three protons dissociate
§ e.g. H3PO4
· Salts:
o normal salts
§ formed by metals completely replacing hydrogen ions in an acid
§ e.g. NaCl, K2SO4
o acid salts
§ Formed when not all hydrogen ions are replaced
§ e.g. NaHSO4
o basic salts
§ formed from the incomplete neutralization of a base
§ e.g. Zn(OH)Cl
o double salts
§ contains more than one cation or anion
§ e.g. Kal(SO4)2
· Denominations
o Acid-anhydride
§ dehydration of an acid (- H2O)
§ e.g. H2SO4 → SO3 (reversible, since SO3 can be dissolved in water)
§ e.g. HCOOH → CO (irreversible, since CO can’t be dissolved in water)
o Base-anhydride
§ dehydration of a base (- H2O)
§ e.g. Ca(OH)2 →CaO
o Anions of an acid:
§ deprotonation of acids
§ e.g. HCl → Cl- , H2SO4 → SO4-
o Acid anions and acid-anhydrides are important to not mix up