Acid-base titrations

Question 1

What must be known to do a titration?

Answer 1

• equation for the reaction
• conc of second solution being added

Question 2

What happens in an acid-base titration?

Answer 2

• acid of known conc added from burette to a measured amount of a solution of a base(alkali) until an indicatir shows the base has been neutralised
• alternatively, alkali added to acid
• or pH meter can be used

Question 3

How is the pH meter calibrated?

Answer 3

• placing probe in a buffer solution of a known pH

Question 4

Draw the titration curves for :

• strong acid/ strong base
• strong acid / weak base
• weak acid/ weak base
• weak acid / strong base

Answer 4

Question 5

What is stange about the curve?

Answer 5

• pH does not change in linear manner as a base is added,
• horizontal sections, where a lot of base can be added without changing pH
• and a steep portions of each curve, exscept weak acid-weak base where a single drop changes the pH by several units

Question 6

What is the equivalence point in titration?

Answer 6

• the point at which sufficient base has been added to just neutralise the acid (vice-versa)
• usually a rapid change (except weak acid - weak base)
• may not be centred on 7
• volume at which exactly the same number of moles of hydrogen ions (or hydroxide ions) has been added as there are moles of hydroxide ions (or hydrogen ions)

Question 7

What is the end point? Why is the right indicator imporant?

Answer 7

• volume of alkali or acid added when the indicator just changes colour
• unless you choose the right indicator, the equivalence point and the end point may not give the same answer

Question 8

What properties does a suitable indicator for a particular titration need?

Answer 8

• colour change needs to be sharp, rather than gradual at the end point
  
  • no more than one drop of acid or alkali needed to give complete colour change
• end point given by the indicator must be the same as the equivalence point, otherwise the titration will give the wrong answer
• distinct colour change

Question 9

Why is universal indicator not suitable for any titration?

Answer 9

gradual colour changes

Question 10

What is the half-neutralisation point?

Answer 10

• half way between the zero and the equivalence points

Question 11

Why is the half-nbeutralisation poiint imporatnt?

Answer 11

• knowledge that you can add acid or base up to this point with the certainity that pH will change very little is relevant to the theory of buffers
• aloows you to find pKa of the weak acid
  
  • halft neutralisation point, half the HA has been converted to A- and half remains
  • therefore Ka = [H+] [A-] / [HA]
    
    • A- and HA cancel