Acids and bases

• pH of some selected crops

  • Acidic: pH 2.00- 2.60
  • Acidic; pH- 4.65- 5.45
  • Acidic: 3.70-4.60
  • Acidic: 5.88-6.40
  • Acidic: pH 5.00- 6.00
  • Acidic: pH 3.90
  • Acidic: pH 6.10- 6.40
• Acids


  • Sour to taste
  • Cause color changes in plant dyes (blue to red in the case of litmus)
  • Aqueous solution of acids are electrically conductive
  • Reacts with carbonates & bicarbonates to produce CO2 gas
  • Reacts with certain metals to produce H2 gas
• Bases


  • Taste bitter
  • Cause color changes in plant dyes (red to blue in the case of litmus)
  • Aqueous solutions of bases conduct electricity
  • Feel slippery
• When gaseous hydrogen chloride meets gaseous ammonia, a smoke composed of ammonium chloride is formed. HCl(g) + NH3(g) → NH4Cl(s)
• This is an acid–base reaction
• H+


  Proton donor
• H+


  Proton acceptor
• Arrhenius acid


  Substance that, when dissolved in water, increases the concentration of hydronium ion, H3O+(aq)
• Arrhenius base


  Substance that, when dissolved in water, increases the concentration of hydroxide ion, OH-(aq)
• Neutralization


  1. Reaction of an H+ (H3O+) ion from the acid and the OH- ion from the base to form water, H2O

  2. Exothermic and releases approximately 56 kJ per mole of acid and base
• The Arrhenius concept limits bases to compounds that contain a hydroxide ion
• Importance of Arrhenius Theory

  • Allows quantitative description of acid and base strengths
  • Strong acids such as HCl & HNO3 are very good conductors of electricity and good catalysts for hydrolysis of methyl acetate with water
  • Weak acids are poor conductors of electricity and poor catalysts for hydrolysis of esters
• Shortfall of Arrhenius Theory
• Brønsted–Lowry acid


  Proton donor
• Brønsted–Lowry base


  Proton acceptor
• Brønsted-Lowry acid must contain H in its formula; HNO3 and H2PO4- are two examples
• All Arrhenius acids are Brønsted-Lowry acids
• Brønsted-Lowry bases are not Arrhenius bases, but all Arrhenius bases contain the Brønsted-Lowry base OH-
• In the Brønsted-Lowry perspective, one species donates a proton and another species accepts it: an acid-base reaction is a proton transfer process
• Acids donate a proton to water, bases accept a proton from water
• Amphoteric


  Can act as either an acid or a base
• Substances that are not Brønsted-Lowry acids

  • HBr
  • CCl4
• Substances that are not Brønsted-Lowry bases

  • H3C-CH3-O
• Conjugate


  Joined together as a pair
• Acid-base reaction


  Yields their conjugate bases and acids
• Acid strength


  Conjugate base strength
• Strong acids are completely dissociated in water, their conjugate bases are quite weak
• Weak acids only dissociate partially in water, their conjugate bases are strong bases
• Substances with negligible acidity do not dissociate in water, their conjugate bases are exceedingly strong
• In every acid–base reaction, equilibrium favors transfer of the proton from the stronger acid to the stronger base to form the weaker acid and the weaker base
• Acetate is a stronger base than H2O


  Equilibrium favors the left side (K < 1)
• Lewis acids are electron pair acceptors, Lewis bases are electron pair donors
• All Brønsted–Lowry acids and bases are also called Lewis acids and bases
• There are compounds which do not meet the Brønsted–Lowry definition which meet the Lewis definition
• Lewis acid


  Species that accepts a pair of electron to form a covalent bond
• Lewis base


  Species that donates a pair of electron to form a covalent bond
• Shortfall of Lewis Concept
• Water is amphoteric
• Ion product constant for water (Kw)


  Equilibrium constant for the autoionization of water
• At 25 °C, Kw = 1.0 × 10-14