Buffers and neutrilisation

• What is a buffer solution?

  A system which minimises pH changes when a small amount of acid or alkali is added, buffers contain two components to remove acid or alkali, (weak acid and conjugate base)
• How can a buffer solution be prepared from a weak acid and its salt?


  CH3COOH -> H+ + CH3COO-

  CH3COO- + Na+ -> CH3COONa
• Preparation of buffer by neutrilisation of the weak acid?


  add aqueous solution of alkali to a weak acid to form, some of the weal acid is left over unreacted, the mixture left over is salt and
• Two reservoirs used to remove acid/alkali?


  In ethanoic acid equilibrium, the equilibrium lies towards the ethanoic acid however when - ions are added the equilibrium moves left meaning that the H+ decreases but both components which make it acidic and basic still remain, this enables the buffer to counteract changes to pH
• How can the pH calculations of buffer solutions be written?


  [H+]= Ka x HA(aq)/A-(aq)
• When can you equate pKa and pH for buffer solutions?


  Provided that the concentrations of A- and Ha are the same you can equate them
• What are the steps for calculating pH for buffer solution made from acid and salt?


  Find the amounts in moles of the buffer solution

  Calculate the concentrations of HA and A-

  Calculate the H+ from Ka, HA and A-

  Use -log[H+]
• What is a natural buffer solution?

  Blood, this is due to blood having to be the correct pH for enzymes to work correctly
• What pH is your blood?


  7.4, if it slips below 7.35 it can cause acidosis which can cause fatigue and potentially death, if it goes above 7.45 alkalosis happens and it causes spasms and nausea
• What is the carbonic acid-hydrogencarbonate buffer solution?


  When H+ increases

  H+ ions react with conjugate base

  The equilibrium shifts to the left removing H+ ions

  (Balances out the addition of acid by making it more basic)

  Vice versa
• What is the henderson-hasselbalch equation?


  pH= pKa + log ([A-]/[HA])
• How can you measure neutrilisation reaction accurately?


  Using a pH curve and monitoring using a pH probe
• What are the key features of a typical pH curve?


  Excess of acid at the beginning (the pH begins gradually increasing as it is added)

  It then reaches the point where pH increases rapidly, when little amounts of alkali are added

  It then reaches the equivalence point which is the centre of the vertical point on the curve

  It then reaches an excess of base where the pH gradually increases
• What is the equivalence point?


  It is where the volume of one solution that exactly reacts with the volume of the other solution
• What is an acid base indicator?


  A weak acid, that has been distinctively different colour from its conjugate base

  HA is red

  A- is yellow

  Endpoint is where the indicator has equal concentrations of HA and A- and the colour will be orange
• Which way does equilibrium shift when when you add a basic solution?


  It will shift right to make more H+ ions and therefore change the pH of the solution back
• Which way does equilibrium shift when you add an acidic solution?


  It will shift left and remove H+ ions from the solution and will form more of the conjugate base
• What happens at the end point?

  H+= Ka and pH=pKa
• What indicators are appropriate for Strong acids and bases?


  Phenolthalein and Methyl orange
• What indicators are appropriate for weak acids and strong bases?

  Phenolthalein
• What indicator is appropriate for a strong acid weak base?


  Methyl orange
• What indicator is appropriate for weak acid weak base?


  No indicator required