21.5 Choice of indicator for titration Flashcards
What is equivalence point
What is end point
. The equivalence point is volume where exactly the same number of moles of hydrogen ions has been added as there are moles of hydroxide ions
. The end point is the volume of acid or alkali added when indicator changes colour
These values will be different unless you use the correct indicator
Why does colour change need to be sharp when using an indicator
. The colour change must be sharp rather than gradual at the end point, so no more than one drop of acid is needed to give a complete colour change.
An indicator that changes gradually over several cm3 would be unsuitable
What other properties does a good indicator have
. The end point of the titration given by the indicator must be the same as the equivalence point
Otherwise titration will give the wrong answer
. The indicator should give a distinct colour change eg colourless to pink change of phenolphthalein is easier to see than red to yellow in methyl orange
Why is universal indicator not suitable for titration
. It has gradual colour changes as it is a mixture of indicators that change colour at different PHs
Other indicators also aren’t suitable as they colour change over around two units
What indicator is most suitable for strong acid and strong base
Eg HCl and NaOH
. Graph of PH against volume of base added
. Phenolphthalein and methyl orange would both work here because both touch the equivalence point
(So the end point and equivalence points are the same)
. Eg phenolphthalein is at around 8-10 whilst methyl orange is from 3 to 4 ish
What indicator is most suitable for weak acid and strong base
. In this graph, phenolphthalein end point overlaps with equivalence point however methyl oranges does not.
. Methyl orange indicator will change colour at the wrong place whilst phenolphthalein will change colour sharply at 25cm^3 which is the equivalence point
What indicator is most suitable for weak acid and weak base
Neither phenolphthalein nor methyl orange are suitable indicators because the vertical portion of the graph doesn’t overlap
So equivalence point and end point won’t correspond with each other
What indicator is most suitable for strong acid and weak base
. Methyl orange can change sharply at equivalence point but phenolphthalein doesn’t overlap with vertical part of graph.
What is a half neutralisation point
. If you look at titration curves, there is always very gently sloping almost horizontal part to curve before you reach vertical line
. As you add acid or base there is a very small change to PH, almost up to volume of equivalence point
The point halfway between zero point and equivalence point is half neutralisation point
Why is half neutralisation point significant
. The knowledge that you can add acid or base up to this point with the certainty that PH will change very little is relevant to theory of buffers
How can you find pKa of a weak acid using half neutralisation point
At this point half the HA has been converted to A- and half remains so
HA + OH- —> H2O + A-
[HA] = [A-]
Ka = [H+][A-] / [HA]
Ka = [H+]
-log10ka = -log10[H+]
Pka = PH
