Acid-base titrations, pH curves and indicators Flashcards Preview

Topic 12.2 (Acid-base titrations) > Acid-base titrations, pH curves and indicators > Flashcards

Flashcards in Acid-base titrations, pH curves and indicators Deck (35)
Loading flashcards...
1

the 'end point' of a titration is when the indicator

changes colour

2

the equivalence point is when

the acid and base have reacted together in the exact proportions as dictated by the stoichiometric equation

3

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

4

the pH at the equivalence point depends on

the combination of acid and base used

5

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

6

a weak acid reacting with a strong base will lead to the pH at the equivalence point being........than 7.00

more

7

a weak base reacting with a strong acid will lead to the pH at the equivalence point being.........than 7.00

less

8

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

9

the shape of a pH titration curve depends on

the nature of the acid and base used

10

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

11

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

12

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

13

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'

14

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

15

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

16

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

17

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

18

an acid-base indicator is either......................or........................ but most are

a weak acid or a weak base
but most are weak acids (HIn)

19

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)

20

the conjugate base for the HIn molecule is

In-

21

for methyl orange, the colour of the HIn molecule is...................and .................for the conjugate base In-

HIn is red
In- is yellow

22

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

23

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

24

when [HIn(aq)] = [In-(aq)], the indicator will appear........in colour

orange

25

write the equation for determining the equilibrium constant, KIn, for methyl orange

KIn = [H+(aq)] [In-(aq)] / [HIn (aq)] = 0.0002 mol dm-3

26

when [HIn(aq)] = [In-(aq)], then the equation for determining KIn for methyl orange becomes

[H+(aq)] = KIn = 0.0002 mol dm-3

27

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

28

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

29

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)]

30

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)