Acid-base titrations, pH curves and indicators Flashcards
the ‘end point’ of a titration is when the indicator
changes colour
the equivalence point is when
the acid and base have reacted together in the exact proportions as dictated by the stoichiometric equation
when titrating an aqueous solution of of a monobasic acid with an aqueous solution of a monoacidic base of the same concentration, the volumes of reactants will be
the same, so 25 cm3 of acid would react with 25 cm3 of base
the pH at the equivalence point depends on
the combination of acid and base used
titrating a strong acid with a strong base will result in a pH of……….being produced at 298K at the equivalence point because
7.00
both reactants are of equal strength
a weak acid reacting with a strong base will lead to the pH at the equivalence point being……..than 7.00
more
a weak base reacting with a strong acid will lead to the pH at the equivalence point being………than 7.00
less
you should NEVER use the term: ………………………..point
neutralisation point as the pH of the solution is not always 7.00 at 298K when the equivalence point is reached
the shape of a pH titration curve depends on
the nature of the acid and base used
when titrating a strong acid with a strong base, the curve will have the shape:
the pH falls only by very small amount from pH 14.00 until quite near the equivalence point where there is a very steep plunge. Once very low, the pH decreases very slowly once more in the acidic direction to pH 1 and less
when titrating a weak acid with a strong base, the titration curve has a shape:
a very small decline in pH from 14 up to just before the equivalence point where there is a steep, nearly vertical plunge down, but not too far down, with the equivalence point being higher than pH 7.00, between 8 and 9. then there is a small decrease in pH towards the acidic direction to pH 4
when titrating a strong acid with a weak base, the titration curve has a shape:
the pH falls slightly more from pH 11 and then vertically on the equivalence point at pH 5.50 and then a steep decline in the acidic direction to pH 1 or less
when titrating a weak acid with a weak base, the titration curve will have the shape:
a gentle incline from pH 11 to the equivalence point at pH 7 and then another gentle incline to pH 4.00
there is no steep point, instead a ‘point of inflexion’
examples of titrating a strong acid with a strong base is adding ….mol dm-3 of……………….. to ….cm3 of …..mol dm-3 of…………………
1.00 mol dm-3 HCl (aq) acid
25 cm3 of 1.00 mol dm-3 NaOH (aq) base
examples of titrating a weak acid with a strong base is adding ….mol dm-3 of……………….. to ….cm3 of …..mol dm-3 of…………………
1.00 mol dm-3 ethanoic acid
25 cm3 of 1.00 mol dm-3 NaOH (aq) base
examples of titrating a strong acid with a weak base is adding ….mol dm-3 of……………….. to ….cm3 of …..mol dm-3 of…………………
1.00 mol dm-3 HCl (aq) acid
25 cm3 of 1.00 mol dm-3 NH3 (aq) base
examples of titrating a weak acid with a weak base is adding ….mol dm-3 of……………….. to ….cm3 of …..mol dm-3 of…………………
1.00 mol dm-3 ethanoic acid
25 cm3 of 1.00 mol dm-3 NH3 (aq) base
an acid-base indicator is either………………….or…………………… but most are
a weak acid or a weak base
but most are weak acids (HIn)
for an indicator that is a weak acid, its dissociation in aqueous solution can be shown in the equation:
HIn(aq) ⇌ H+(aq) + In-(aq)
the conjugate base for the HIn molecule is
In-
for methyl orange, the colour of the HIn molecule is……………….and ……………..for the conjugate base In-
HIn is red
In- is yellow
when [H+(aq)] is of sufficiently high concentration, then the equilibrium shifts to the………and the………..colour predominates (for methyl orange)
shifts left and the red colour predominates
when the [H+(aq)] concentration is very low, then the equilibrium will shift…….and the ……colour will predominate (for methyl orange)
shifts right and the yellow colour predominates
when [HIn(aq)] = [In-(aq)], the indicator will appear……..in colour
orange
write the equation for determining the equilibrium constant, KIn, for methyl orange
KIn = [H+(aq)] [In-(aq)] / [HIn (aq)] = 0.0002 mol dm-3
when [HIn(aq)] = [In-(aq)], then the equation for determining KIn for methyl orange becomes
[H+(aq)] = KIn = 0.0002 mol dm-3
calculate the pH at which methyl orange changes colour when [H+(aq)] = 0.0002 mol dm-3
pH = -lg[H+] pH= -lg10 (0.0002) = 3.70
complete the table: indicator: | pKIn | pH range | Colour for HIn and In- methyl orange: bromophenol blue: bromothymol blue: phenol red: phenolphthalein:
complete the table:
indicator: | pKIn | pH range | Colour for HIn and In-
methyl orange: 3.70 3.10-4.40 red to yellow
bromophenol blue: 4.00 2.80-4.60 yellow to blue
bromothymol blue: 7.00 6.00-7.60 yellow to blue
phenol red: 7.90 6.80-8.40 yellow to red
phenolphthalein: 9.30 8.20-10.00 colourless to red
the rule of thumb for methyl orange colour changes is
red will predominate when [HIn(aq)] is 10 times the concentration of [In-(aq)] and vice versa as yellow dominates when [In-(aq)] is 10 times [HIn(aq)]
for an indicator to be suitable, it’s range has to fall in the………….part of the titration curve
steep part. if the indicator has a range within that steep section, then it is suitable (except for weak acid-weak base titrations)
for weak acid-weak base titrations, the suitable indicator is
none, the endpoint has to be determined by measuring the temperature changes (thermometric) or electrical conductivity changes (conductometric)
the best indicator to chose for a particular titration is the one
whose pKIn value is the closest to the pH at the equivalence point
for strong acid-strong base titrations, a good indicator(s) could be
both methyl orange and phenolphthalein as both their ranges fall within the steep section of the graph
for strong acid-weak base titrations, a suitable indicator is
methyl orange
for weak acid-strong base titrations, a suitable indicator is
phenolphthalein
