NaCl Find LEf

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NaClFind LEf
A2 Practicals Analysis of Bleaches
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Slides 3- 31. Investigation Most
cost-effective bleach Slides 32 50. A2
Practical Investigation of Bleaches
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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
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• 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 OCl? content of the bleach- quantitative- M

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Some of the variables must remain constant
Volume of bleach- Size 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
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One variable will be changed The brand of
bleach used will be the variable, which shall be
varied.
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One variable variable is measured The volume
of Sodium thiosulphate required for each brand of
bleach.
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Safety Always wear eye protection Bleach is
an irritant and will evolve toxic chlorine on
acidification Avoid skin contact with iodide
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• 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.

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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.
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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.
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Method
(50cm³)
3. Wash the burette out with distilled
water, followed by sodium thiosulphate. Fill
it with sodium thiosulphate. Record the
initial reading.
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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
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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.
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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!
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Method
Dont 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.
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Method
Starch turns blue-black in the presence of iodine
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Method
7. The titration should then be performed
fairly quickly but carefully as Iodide can be
oxidised by the air.
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Method
Keep going........
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Method
Repeat the titration to obtain concordant
results. Results within 0.1cm3
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• 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.

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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.
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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
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Results of the Investigation Most Cost Effective
Bleach
Bleach Volume Added (cm3) Average Cost per Litre Cost per Mol
      (pence) (/mol)
Sainsbury's Economy   5.75   6.02   18   ?
Sainsbury's Economy   5.68   6.02   18   ?
Sainsbury's Economy   6.00   6.02   18   ?
Sainsbury's Economy   6.03   6.02   18   ?
Asda Farmhouse   11.69    11.67   15  ?
Asda Farmhouse   11.85    11.67   15  ?
Asda Farmhouse   11.45    11.67   15  ?
Asda Farmhouse   11.50    11.67   15  ?
Sainburys Bleach 20.45  20.43 65 ?
Sainburys Bleach 20.40  20.43 65 ?
Somerfield Thick Bl 34.68    34.21   52  ?
Somerfield Thick Bl 34.20    34.21   52  ?
Somerfield Thick Bl 34.30    34.21   52  ?
Somerfield Thick Bl 34.22    34.21   52  ?
Parazone Original 29.52  29.51 127 ?
Parazone Original 29.50  29.51 127 ?
Neon Original 25.45  25.45 132 ?
Neon Original 25.45  25.45 132 ?
Asda Thick Bleach 24.19  24.20 132 ?
Asda Thick Bleach 24.20  24.20 132 ?
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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. Sainsburys Economy
Thin Bleach-6.02cm³Asda Farm Stores
Bleach-11.67cm³Sainsburys Bleach-20.43cm³Somerf
ield Thick Bleach-34.21cm³Parozone-29.51cm³Neon-
25.45cm³Asda Thick Bleach-24.20cm³
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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
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Moles of Thiosulphate Used. Moles
concentration x dm3 0.1 x dm3 Taking
the Sainburys 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 mol NaClO
2
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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
Sainburys Thin Bleach
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Its 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 pence p/mol Mole/litre
Moles per litre mole The Sainsburys
Thin Bleach worked out to be 18p per litre and
contained 0.1204 moles/litre18/0.1204 1.50
per litre The same is done to the other
bleaches.
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Interpreted Results Most Cost Effective Bleach
Bleach Volume Added (cm3) Average Cost per Litre Cost per Mol
      (pence) (/mol)
Sainsbury's Economy   5.75   6.02   18   1.50  
Sainsbury's Economy   5.68   6.02   18   1.50  
Sainsbury's Economy   6.00   6.02   18   1.50  
Sainsbury's Economy   6.03   6.02   18   1.50  
Asda Farmhouse   11.69    11.67   15  0.64 
Asda Farmhouse   11.85    11.67   15  0.64 
Asda Farmhouse   11.45    11.67   15  0.64 
Asda Farmhouse   11.50    11.67   15  0.64 
Sainburys Bleach 20.45  20.43 65 1.59
Sainburys Bleach 20.40  20.43 65 1.59
Somerfield Thick Bl 34.68    34.21   52  0.76 
Somerfield Thick Bl 34.20    34.21   52  0.76 
Somerfield Thick Bl 34.30    34.21   52  0.76 
Somerfield Thick Bl 34.22    34.21   52  0.76 
Parazone Original 29.52  29.51 127 2.15
Parazone Original 29.50  29.51 127 2.15
Neon Original 25.45  25.45 132 2.59
Neon Original 25.45  25.45 132 2.59
Asda Thick Bleach 24.19  24.20 132 2.73
Asda Thick Bleach 24.20  24.20 132 2.73
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• Bleaches placed in order of decreasing value for
  money.
• Asda Farmhouse
• Somerfield Thick Bleach
• Sainsbury's Economy
• Sainburys Bleach
• Parazone Original
• Neon Original
• Asda Thick Bleach
• So cheapest is always the cheapest!
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•  

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A2 Practical Assessment. Analysis of Bleach.
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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)
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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.
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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 all your
results in a suitable table, with units.
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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.
Sample Name Cost (pence) Volume (cm3) Average titre (cm3)
A Parozone 84 750 12.00
B Domestos 75 750 12.20
C Asda Thin 12 750
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Marking Criteria Suitable Table for Results
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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
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1. (a) Calculate the number of moles of sodium
thiosulphate required for 25.0cm3
diluted bleach.
MOLES OF THIOSULPHATE 1 MARK
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(b) Calculate the number of moles of iodine
reacting with the sodium thiosulphate solution.
MOLES OF IODINE 1 MARK
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(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
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(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
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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
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(b) Suggest one possible source of chemical
error in your determination.
ONE SOURCE OF CHEMICAL ERROR 1 MARK
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3. Outline at least TWO aspects of
experimental technique necessary to minimise
error in your determination.
TWO ASPECTS TO MINIMISE ERROR 1 MARK
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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
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Skill Area 1 Manipulation, Measuring
Recording. (Analysis of Bleaches)
Code Descriptor Mark
B1A1 Apparatus set up competently and used with dexterity 2
B1A2 Some assistance given in setting up and using apparatus. 1
B1A3 Unable to set up or use apparatus without major assistance. 0
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Code Descriptor Mark
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
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Code Descriptor Mark
B1C1 Two or more titres within 0.1cm3 2
B1C2 Two or more titres within 0.2cm3 1
B1C3 Difference in titre values gt 0.2cm3 0
B1D1 Accuracy within 0.1cm3 of identified value i.e. teacher's identified value. 2
B1D2 Accuracy within 0.2cm3 of identified value 1
B1D3 Result differs by more than 0.2cm3 from identified value. 0
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Code Descriptor Mark
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 10
Total marks awarded
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Prepare well for carrying out and interpreting
the results of this experiment F Scullion.
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