3.1.12 Acids and Bases

Cards (36)

  • Define 'Bronsted-Lowry Acid' Proton donor
  • Define 'Bronsted-Lowry Base' Proton Acceptor
  • Formula for pH:

    pH = −log ([H+])​​
  • Acids are strong when they completely dissociate into H+ ions in water
  • Acids are weak when they only slightly dissociate when dissolved in water, giving an equilibrium mixture
  • PKA scale

    • pKA = - log KA
    • so KA 10-pka
  • Using their Ka values, how can you tell which weak acid is stronger?:
    • Higher Ka value, stronger weak acid
    • Lower Ka value, weaker weak acid
  • What is the method of working out pH of a neutralised solution?
    • Work out moles of acid and base
    • Determine which is in excess
    • If base in excess, use Kw for the excess 
  • What is an indicator? A weak acid which has a different colour to its conjugate base
  • Buffer solution: the pH does not change significantly if small amounts of acid or alkali are added to it
  • Acidic buffer: made from a weak acid and the salt of that weak acid e.g ethanoic acid and sodium ethanoate
  • Basic buffer: made from a weak base and salt of that weak base e.g ammonia and ammonium chloride
  • Experimental procedure to form a titration curve:
    • Place a fixed amount of alkali solution into a beaker
    • Use a burette to add small portions of acid
    • Stir and measure pH after each addition of acid
  • If two solutions have a pH difference of 1, what is the difference in [H+ ]? A factor of 10
  • Define a Bronsted-Lowry base: Proton acceptor
  • What ion causes a solution to be alkaline? -OH (hydroxide ion)
  • What is the value of Kw at 298K? 1.0 x 10-1
  • What physical factors affect the value of Kw ? How do they affect it?
    • Temperature only: if this is increased, the equilibrium moves to the right so Kw increases and the pH of pure water decrease
  • Lower pH = higher concentration of H+
  • Examples of strong bases: NaOH, CaCO3 , Na2CO3
  • Examples of weak acids: CH3COOH (ethanoic), any organic acid
  • Examples of weak bases: NH3
  • Write a reaction for an acidic buffer with added alkali:
    HA + OH-H2O + A
  • What products are buffers found in? Shampoos, detergents → important to keep pH right to avoid damage to skin, hair, fabrics
  • How can you calculate the new pH of a buffer solution when acid or base is added?
    • Calculate number of moles of H+ and A- and HA before acid or base is added.
    • Use equations to work out new moles of A- and HA → find [H+ ] → pH
  • What do basic buffer solutions contain in general terms? Weak base and soluble salt of that weak base
  • What ion causes a solution to be alkaline? -OH (hydroxide ion)
  • Write an equation for the ionisation of water
    (2) 2H2O (l) ⇌ H3O+ (aq) + -OH (aq)
    OR
    H2O (l) ⇌ H+ (aq) + -OH (aq)
  • What is the value of K w at 298K? 1.0 x 10-14
  • Why is pure water still neutral, even if pH does not equal 7? [H+ ] = [-OH]
  • What is the relationship between pH and concentration of H+ ? Lower pH = higher concentration of H+
  • How do you find [H+ ] from pH? [H+ ] = 10-pH
  • How do you calculate the pH of a strong alkaline solution?
    Use Kw to calculate [H+ ] from [OH- ]
    Use pH=-log[H+ ]
  • Strong acid:
    One which fully dissociates in water ( HXH+ + X - )
  • Strong base:
    One which fully dissociates in water

    (XOHX+ + -OH)
  • What is the difference between concentrated and strong?

    Concentrated means many mol per dm3 , strong refers to amount of dissociation