chem whatarethemechanismsofchemicalchange
Reactivity 3.1.14 - acid-base indicators are weak acids, where the components of the conjugate acid-base pair have different colours. the pH of the end point of an indicator, where it changes colour, approximately corresponds to its
Reactivity 3.1.15 - an appropriate indicator for a titration has an end point range that coincides with the pH at the equivalence point
- indicators are weak acids where the undissociated and dissociated forms have different colours
- only indicators which are weak acids will be considered here
will be a generic acidic indicator
applying 2.3.4 Le Chatelier’s principle:
- increasing
, the equilibrium will shift to - decreasing
, the equilibrium will shift to
indicators change colour when the
- the change in colour of an indicator is known as its change point or end point
the acid dissociation constant is defined as:
at the point where the equilibrium is balanced between the acid and its conjugate base, the indicator is in the middle of its colour change. so then,
here, the addition of a very small volume of acid or base will shift the equilibrium and cause the indicator to change colour.
- it follows that different indicators will have different
values, will have different end points corresponding to different values
these indicators are more correctly known as acid-base indicators to distinguish them from redox indicators
using indicators to identify the equivalence point
- when the end point of an indicator coincides with the
at the equivalence point, the indicator is effective
- determine what combination of weak and strong acid and base are reacting
- deduce the pH of the salt solution formed
- consult data tables to choose an indicator with the end point in the range of the equivalence point
example:
| reactants | example | ||
|---|---|---|---|
| strong acid + strong base | 3-11 | phenolphthalein methyl orange | 9.60, 8.3-10 colourless to pink 3.70, 3.1-4.4 red to yellow |
| weak acid + strong base | 7-11 | phenolphthalein phenol red | 9.50, 8.2-10.0 colourless to pink 7.90, 6.8-8.4 yellow to red |
| strong acid + weak base | 3-7 | methyl orange bromophenol blue | 3.46, 3.2-4.4 red to yellow 4.20, 3.0-4.6 yellow to blue |
| weak acid + weak base | n/a | n/a | n/a |
- the combination of a weak acid and base does not give a significant change in
at equivalence, so there is no suitable indicator
Important
we are only able to identify the distinct colour of one form of an indicator when the ratio of one form of indicator when the ratio to the other form is around
. for the transition to be observed from the colour of to , the ratio of the concentrations must change from to . this represents a range of 2 units. this is why there is a range of units on either side of the value of at which the eye can notice the colour change. this is given as the end point range.
a difference of 1 drop of added solution from the burette should produce the change in colour. a good indicator is one that gives a distinct or sharp colour change at the equivalence point.
challenge questions
- what structural features might indicators possess that could explain the fact that they change colour on gain or loss of
?
indicators contain conjugated delocalised electrons as a result of alternating double bonds. the structure of the indicator changes with the addition of