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A2 Practicals
Analysis of
Bleaches
NaCl
Find LEf
1
Slides 3- 31.
Investigation: Most cost-effective bleach
Slides 32 – 50.
A2 Practical: Investigation of Bleaches
2
CARE: Bleach attacks
the skin. If acid is
added it may give off
poisonous chlorine gas
Assorted domestic bleaches
Emergency germ-free water:
1 drops of bleach in 1 litres water
Stir and leave for 30 minutes
Bleaches “whiten” as well
3
Planning : When conducting this experiment certain
variables must be considered:
 Brand of bleach- Domestos, Tesco Value etc.
 Volume of bleach- quantitative- cm³
 Size of conical flask- quantitative- cm³
 Volume of Sulphuric acid used- quantitative- cm³
 Volume of Potassium iodide used- quantitative- cm³
 Volume of Sodium thiosulphate added- quantitative- cm³
 Volume of Starch solution added- quantitative- cm³
 The OClֿ content of the bleach- quantitative- M
4
Some of the variables must remain constant:
Volume of bleachSize of conical flask
A suitable conical flask must be chosen which can hold the required volumes
Volume of Sulphuric acid used
There must be enough sulphuric acid present in the solution of the iodide.
Volume of Potassium Iodide used
The potassium iodide must be added in excess to the bleach.
Volume of fresh Starch added
The starch is used as an indicator
5
One variable will be changed:
The brand of bleach used will be
the variable, which shall be varied.
6
One variable variable is measured:
The volume of Sodium thiosulphate
required for each brand of bleach.
7
Safety:
Always wear eye
protection
Bleach is an irritant and
will evolve toxic chlorine
on acidification
Avoid skin contact with
iodide
8
Apparatus for experiment:
 10cm³ measuring cylinder
 100cm³ conical flask
 10cm³ pipette
 funnel
 burette and stand
 dilute sulphuric acid
 10% potassium iodide solution
 0.1M sodium thiosulphate solution
fresh starch solution
 selection of commercial bleaches.
9
Method
1. Set up the apparatus as shown above and mix in a
beaker, 15cm³ of Potassium Iodide and 10cm³ of
dilute sulphuric acid. Leave to stand.
10
2. Using a measuring cylinder transfer
10cm³ of one of the bleaches to a
100cm³ beaker. Then dilute it with
90cm³ of distilled water.
If 100cm³ volumetric flasks are
available, use them instead.
11
Method
(50cm³)
3. Wash the burette out with distilled
water, followed by sodium thiosulphate.
Fill it with sodium thiosulphate.
Record the initial reading.
12
4. Mix the potassium iodide and dilute sulphuric
acid prepared in step 1 with 25cm³ of the
diluted bleach in the conical flask.
Swirl the contents.
This liberates iodine from the solution
13
Method
Iodine
solution is
red-brown
5. Titrate the liberated iodine against the sodium
thiosulphate until the solution in the conical flash
turns straw yellow in colour.
14
Method
Iodine solution
fades in colour
at the iodine is
reacted with
thiosulpahte
If the starch is added too early iodine is strongly
adsorbed onto it and accuracy is reduced.
The titre will be lower than it should be!
15
Method
Don’t wait too
long and
overshoot the
end point!
6. Add about five drops of fresh starch to
the solution when it has turned a straw
yellow colour.
16
Method
Starch turns blue-black in the
presence of iodine
17
Method
7. The titration should then be performed
fairly quickly but carefully as Iodide can
be oxidised by the air.
18
Method
Keep going........
19
Method
Repeat the titration to obtain
concordant results.
Results within 0.1cm3
20
Interpreting the Results.
 This investigation is to find the cost
effectiveness of different brands of bleache.
 The active ingredient in bleach, that kills
bacteria, is the chlorate ions (OCl- (aq)).
 The concentration of chlorate is obtained
indirectly. It is used to displace iodine from
a solution of potassium iodide in acid
By working out how much iodine had been
produced, we can determine how much chlorate
was present in the sample.
 The iodine is determined using thiosulphate.
21
The chlorate present in bleach is usually
Sodium Hypochlorate (NaOCl).
This, in the presence of acid, can oxidise
iodide ions: (I-(aq)) to iodine (I2).
OCl-aq) + 2H+(aq)+ 2I-
(q)
→I2 (aq) + Cl-a(aq) + H2O(l)
1 mole of I2 formed per 1 mole of NaOCl
The equation shows the role of the acid,
helping the oxidising agent.
22
This iodine, freed up by the chlorate, can be
measured by titration against
0.1M Sodium Thiosulphate (Na2S2O4).
2S2O3 (aq) + I2(aq) → 2I (aq) + S4O62-(aq)
2 mol of thiosulphate for every 1 mol Iodine
Recall: 1 mole of I2 formed per 1 mole of NaOCl
Putting these two equations together:
1 mole Na2S2O3 = 0.5 mol I2 = 0.5 mol NaOCl
23
Results of the Investigation: Most Cost Effective Bleach
Bleach
Sainsbury's Economy
Asda Farmhouse
Sainbury‘s Bleach
Somerfield Thick Bl
Parazone Original
Neon Original
Asda Thick Bleach
Volume Added (cm3) Average Cost per Litre Cost per Mol
(pence)
(£/mol)
5.75
5.68
6.00
6.03
11.69
11.85
11.45
11.50
20.45
20.40
34.68
34.20
34.30
34.22
29.52
29.50
25.45
25.45
24.19
24.20
6.02
18
?
11.67
15
?
20.43
65
?
34.21
52
?
29.51
127
?
25.45
132
?
24.20
132
?
24
Results
All of the results calculated to 2 decimal places.
Below are the averages of each bleach, taken by
averaging the two concordant results for each
bleach.
Sainsbury’s Economy Thin Bleach-6.02cm³
Asda Farm Stores Bleach-11.67cm³
Sainsbury’s Bleach-20.43cm³
Somerfield Thick Bleach-34.21cm³
Parozone-29.51cm³
Neon-25.45cm³
Asda Thick Bleach-24.20cm³
25
Recall the key chemical reactions.
Their partial ionic equations are shown below:
OCl
(aq)
+
+ 2H (aq)+ 2I (aq)→ I2(aq) + Cl (aq)+ H2O(l)
2S2O32- (aq)
+ I2(aq) → 2I (aq) + S4O6² (aq)
1 mole of I2 formed per 1 mole of NaOCl
2 mol of thiosulphate for every 1 mol Iodine
Putting these two equations together:
1 mole Na2S2O3 = 0.5 mol I2 = 0.5 mol NaOCl
26
Moles of Thiosulphate Used.
Moles = concentration x dm3
= 0.1 x dm3
Taking the Sainbury’s Thin Bleach as the example.
Moles Thiosulphate = 0.00602 x 0.1 = 0.000602 mol
Recall again.
1 mole Na2S2O3 = 0.5 mol I2 = 0.5 mol NaOCl
0.000602 = 0.000301
2
mol NaClO
27
Recall that only 25 cm³of the 100 cm³of
diluted bleach was used in each of the titrations
Amount of Chlorate in 100cm³.
= 4 x 0.000301 mol = 0.001204 mol NaClO
This was contained in 10cm³ of the original bleach.
Volume in a litre (dm3):
100 x 0.001204 mol NaClO = 0.1204 moldm-3 NaClO
in Sainbury’s Thin Bleach
28
It’s the moles of active ingredient that counts.
Working out the cost per mole.
Simply divide the cost per litre by the molar
concentration of the chlorate.
Pence/litre = Pence per litre
Mole/litre = Moles per litre
= pence = p/mol
mole
The Sainsbury’s Thin Bleach worked out to be
18p per litre and contained 0.1204 moles/litre
18/0.1204 = £1.50 per litre
The same is done to the other bleaches.
29
Interpreted Results : Most Cost Effective Bleach
Bleach
Sainsbury's Economy
Asda Farmhouse
Sainbury‘s Bleach
Somerfield Thick Bl
Parazone Original
Neon Original
Asda Thick Bleach
Volume Added (cm3) Average Cost per Litre Cost per Mol
(pence)
(£/mol)
5.75
5.68
6.00
6.03
11.69
11.85
11.45
11.50
20.45
20.40
34.68
34.20
34.30
34.22
29.52
29.50
25.45
25.45
24.19
24.20
6.02
18
11.67
15
20.43
65
34.21
52
29.51
127
25.45
132
24.20
132
1.50
0.64
1.59
0.76
2.15
2.59
30
2.73
Bleaches placed in order of decreasing
value for money.
1. Asda Farmhouse
2. Somerfield Thick Bleach
3. Sainsbury's Economy
4. Sainbury‘s Bleach
5. Parazone Original
6. Neon Original
7. Asda Thick Bleach
So cheapest is always the cheapest!
31
A2 Practical Assessment.
Analysis of Bleach.
32
ANALYSIS OF BLEACHES
A solution of domestic bleach contains sodium chlorate (I),
NaClO. If this solution is acidified, it produces chlorine.
ClO
(aq)
+ 2H+(aq) + Cl
(aq)
 Cl2
(g)
+ H2O(l)
The amount of chlorine may be produced from a fixed
amount of bleach is referred to as “available chlorine”.
The amount of “available chlorine” in bleach may be
determined by using the bleach to liberate iodine from
acidified aqueous sodium iodide.
2I
(aq)
+ 2H+(aq) + ClO
(aq)
 I2(aq) + H2O(l) + Cl
(aq)
33
The liberated iodine is then titrated with aqueous
sodium thiosulphate.
2S2O3 2 –(aq) + I2
(aq)
 S4O62 –(aq) + 2I -(aq)
The colour of the iodine disappears at the end point.
This can be made sharper by the addition of starch
solution near the end point.
You will investigate one of three bleaches supplied.
Carry out the titration and collect your results for the
other two bleaches.
Use these results in your calculation.
34
Procedure
Using a pipette filler, pipette exactly 10.0cm3 of the
bleach supplied to you into a 250cm3 volumetric flask and
make it up to the mark with deionised water.
Transfer 25.0cm3 of this solution to a conical flask; add
1.5g (approximately) of potassium iodide and 20.0 cm3 of
dilute sulphuric acid (2M).
Swirl the mixture.
Titrate the contents of the flask with standard 0.1M
sodium thiosulphate solution adding 1ml starch solution
when the contents of the flask are a straw colour.
Continue adding sodium thiosulphate until the colour
changes from blue/black to colourless.
Repeat until you obtain two concordant results. Record 35
all
your results in a suitable table, with units.
Record the names of the bleaches, the total volume of
each bottle and the cost of the bleach.
Record the volume of 0.1M sodium thiosulphate
required to react with the released iodine from the
other two samples.
Volume
(cm3)
Average
titre
(cm3)
Sample
Name
Cost
(pence)
A
Parozone
84
750
12.00
B
Domestos
75
750
12.20
C
Asda Thin
12
750
36
Marking Criteria
Suitable Table for Results
37
TEACHER VALUE
cm3
APPARATUS SET UP
2 MARKS
SAFE APPARATUS USE
2 MARKS
TWO OR MORE TITRES
WITHIN 0.1cm3
2 MARKS
WITHIN 0.1cm3 OF TEACHER
IDENTIFIED VALUE
2 MARKS
CLEAR NEAT PRESENTATION
2 MARKS
TOTAL
10 MARKS
38
1. (a) Calculate the number of moles of sodium
thiosulphate required for 25.0cm3 “diluted” bleach.
MOLES OF THIOSULPHATE
1 MARK
39
(b) Calculate the number of moles of iodine
reacting with the sodium thiosulphate solution.
MOLES OF IODINE
1 MARK
40
(c) Determine the number of moles of “available
chlorine” in 25.0cm3 of diluted bleach.
MOLES OF “AVAILABLE
CHLORINE” 25.0 cm3 DILUTED
1 MARK
41
(d) Calculate the number of moles of “available
chlorine” in the 10.0cm3 of undiluted bleach.
MOLES OF “AVAILABLE CHLORINE”
10.0 cm3 DILUTED
1 MARK
42
2 (a) The volume of each bottle of bleach and their
cost are given on the front sheet. Evaluate which
bleach gives the best value for money in terms of
“available chlorine”.
Value for money
1 MARK
43
(b) Suggest one possible source of chemical
error in your determination.
ONE SOURCE OF CHEMICAL ERROR
1 MARK
44
3. Outline at least TWO aspects of
experimental technique necessary to
minimise error in your determination.
TWO ASPECTS TO MINIMISE ERROR
1 MARK
45
4. Write a concise report justifying the
conclusions reached.
About 1 page, so less than 400 words
NOMENCLATURE AND TERMINOLOGY
1 MARK
APPROPRIATE NUMBER OF
SIGNIFICANT FIGURES
1 MARK
46
Skill Area 1 Manipulation, Measuring &
Recording. (Analysis of Bleaches)
Code
Descriptor
Mark
Apparatus set up competently
B1A1
and used with dexterity
2
Some assistance given in
B1A2
setting up and using apparatus.
1
Unable to set up or use
B1A3 apparatus without major
assistance.
0
47
Code
Descriptor
B1B1
Uses apparatus safely with due
care and attention
2
B1B2
Intervention needed to ensure
safety and care in the use of
equipment.
1
B1B3
Major intervention needed to
prevent harm to personnel or
equipment.
0
Mark
48
Code
B1C1
B1C2
B1C3
B1D1
B1D2
B1D3
Descriptor
Two or more titres
within 0.1cm3
Two or more titres
within 0.2cm3
Difference in titre
values > 0.2cm3
Accuracy within 0.1cm3 of
identified value i.e. teacher's
identified value.
Accuracy within 0.2cm3 of
identified value
Result differs by more than
0.2cm3 from identified value.
Mark
2
1
0
2
1
0
49
Code
Descriptor
B1E1
Clear neat presentation, with suitable
table (showing initial, final
values and volume delivered), correct
units given and titre figures to at 2
least one decimal place.
2
B1E2
one major omission (-1)
1
B1E3
two major omissions (-2)
0
Total marks available
Total marks awarded
Mark
10
50
Prepare well for
carrying out and
interpreting the
results of this
experiment
F Scullion. JustChemy.Com
51