Acids, Bases and pH (Chapter 20)

Acids, Bases and pH (Chapter 20)

1. BRONSTED - LOWRY ACIDS AND BASES (20.1)

   • an alkali is a soluble base
   • A Bronsted Lowry acid is a proton donor and a Bronsted - Lowry base is a proton acceptor
   • a conjugate acid and base pair contains two species that can be interconverted by the transfer of a proton
     • I.e. HCl releases a proton to form Cl- and Cl- accepts a proton to form HCl
   • monobasic acid = releases one proton dibasic acid = releases two protons tribasic acid = releases three protons
   • in the forward reaction, HA acts as an acid and donates a proton
     • In the reverse reaction, A- acts as a base and accepts a proton to form HA
   • when writing an ionic equation, cancel spectator ions (ions that do not change on either side of the equation)
   • reactions of acids with different substances
     • acid + carbonate --> salt + water + carbon dioxide
     • acid + base (ie metal oxide) --> salt + water
     • acid + metal --> salt + hydrogen
     • acid + alkali = salt + water

2. THE PH SCALE AND STRONG ACIDS (20.2)

   • A low [H+] means a high pH ad a high [H+] means a low concentration
   • pH = -log [H+]
     • [H+] = 10 ^ (-pH)
   • strong monobasic acids such as HCl and HNO3 ionise fully in solution so [H+] = [Acid]
     • this means that the pH can be worked out directly from the concentration of the acids for strong, monobasic acids

3. THE ACID DISSOCIATION CONSTANT Ka (20.3)

   • a strong acid completely dissociates in aqueous solutions whereas a weak acid only partially dissociates
     • The dissociation of any weak acid, HA, can be shown with the general form
       • HA (aq) ⇌ H+ (aq) + A- (aq)
   • Ka is calculated as:
     • [H+] [A-] ------------ [HA]
   • the greater the value of Ka, the greater the strength of the acid
     • the stronger the acid, the smaller the pKa value
   • pKa = -log Ka
     • Ka = 10 ^ (-pKa)

4. THE pH OF WEAK ACIDS (20.4)

   • Ka can be calculated using equilibrium concentrations and by making two approximations:
     • approximation one: [H+] at eqm = [A-] at eqm
     • approximation two: [HA] at eqm = [HA] at start
       • the dissociation is small in weak acids so we assume the concentration stays the same
     • Ka = [H+]^2 / [HA]
   • approximations do not work when the acid is extremely weak or dilute or also when they are too strong

5. pH AND STRONG BASES (20.5)

   • water ionises very slightly : H2o +H20 ⇌ H3O+ + OH-
   • Kw = [H+] [0H-]
     • Kw varies with temperature
     • a solution is acidic when [H+] > [OH-]
     • a solution is neutral when [H+] = [OH-]
     • a solution is alkaline when [H+] < [OH-]
   • the pH of a strong base can be calculated from the concentration of the base and Kw
     • find conc of OH- ions and use this to find conc of H+ ions
   • the pH weak bases are calculated in the same way as thatr for weak acids