3.1.12 Acids and Bases

• 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 ( HX → H+ + X - )
• Strong base:

  One which fully dissociates in water

  
  

  (XOH → X+ + -OH)
• What is the difference between concentrated and strong?

  
  

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