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NaClFind LEf

A2 Practicals

Analysis ofBleaches

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Slides 3- 31. Investigation: Most cost-effective bleach

Slides 32 – 50.A2 Practical: Investigation of Bleaches

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Assorted domestic bleaches

Emergency germ-free water:1 drops of bleach in 1 litres waterStir and leave for 30 minutes Bleaches “whiten” as

well

CARE: Bleach attacks the skin. If acid is added it may give off poisonous chlorine gas

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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 usedThere must be enough sulphuric acid present in the solution of the iodide.

Volume of Potassium Iodide usedThe potassium iodide must be added in excess to the bleach.

Volume of fresh Starch addedThe 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

3. Wash the burette out with distilled water, followed by sodium thiosulphate. Fill it with sodium thiosulphate. Record the initial reading.

(50cm³)

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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

5. Titrate the liberated iodine against the sodium thiosulphate until the solution in the conical flash turns straw yellow in colour.

Iodine solution is red-brown

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Method

Iodine solution fades in colour at the iodine is reacted with thiosulpahteIf 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

6. Add about five drops of fresh starch to the solution when it has turned a straw yellow colour.

Don’t wait too long and overshoot the end point!

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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) + S4O6

2-(aq)

2 mol of thiosulphate for every 1 mol IodineRecall: 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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BleachVolume Added

(cm3) AverageCost per

Litre Cost per Mol      (pence) (£/mol)

Sainsbury's Economy

 

5.75 

6.02 

18 

?5.686.006.03

Asda Farmhouse 

11.69 

 11.67 

15  ?11.8511.4511.50

Sainbury‘s Bleach20.45

 20.43 65 ?20.40

Somerfield Thick Bl

34.68 

 34.21 

52  ?34.2034.3034.22

Parazone Original29.52

 29.51 127 ?29.50

Neon Original25.45

 25.45 132 ?25.45

Asda Thick Bleach

24.19 24.20 132 ?

24.20

Results of the Investigation: Most Cost Effective Bleach

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ResultsAll 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-6.02cm³Asda Farm Stores Bleach-11.67cm³Sainsbury’s Bleach-20.43cm³Somerfield 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 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 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 Sainbury’s Thin Bleach

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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 = pence = p/molMole/litre = Moles per litre 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.

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BleachVolume Added

(cm3) AverageCost per

Litre Cost per Mol      (pence) (£/mol)

Sainsbury's Economy

 

5.75 

6.02 

18 

1.50 

5.686.006.03

Asda Farmhouse 

11.69 

 11.67 

15  0.64 11.8511.4511.50

Sainbury‘s Bleach20.45

 20.43 65 1.5920.40

Somerfield Thick Bl

34.68 

 34.21 

52  0.76 34.2034.3034.22

Parazone Original29.52

 29.51 127 2.1529.50

Neon Original25.45

 25.45 132 2.5925.45Asda Thick

Bleach24.19

 24.20 132 2.7324.20

Interpreted Results : Most Cost Effective Bleach

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Bleaches placed in order of decreasing value for money.

1.Asda Farmhouse2.Somerfield Thick Bleach

3.Sainsbury's Economy4.Sainbury‘s Bleach5.Parazone Original

6.Neon Original7.Asda Thick Bleach

So cheapest is always the cheapest!

 

 

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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

NameCost

(pence)Volume (cm3)

Average titre (cm3)

A Parozone 84 750 12.00

BDomesto

s75 750 12.20

C Asda Thin 12 750

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Marking Criteria

Suitable Table for Results

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TEACHER VALUEcm3

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

TOTAL10

MARKS

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1. (a) Calculate the number of moles of sodium thiosulphate required for 25.0cm3 “diluted” bleach.

MOLES OF THIOSULPHATE1

MARK

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(b) Calculate the number of moles of iodine reacting with the sodium thiosulphate solution.

MOLES OF IODINE1

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 ERROR1

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 ERROR1

MARK

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4. Write a concise report justifying the conclusions reached.

About 1 page, so less than 400 words

NOMENCLATURE AND TERMINOLOGY1

MARK

APPROPRIATE NUMBER OF SIGNIFICANT FIGURES

1 MARK

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Skill Area 1 Manipulation, Measuring & Recording. (Analysis of Bleaches) Code Descriptor Mark

B1A1Apparatus set up competently and used with dexterity

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B1A2Some assistance given in setting up and using apparatus.

1

B1A3Unable to set up or use apparatus without major assistance.

0

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Code Descriptor Mark

B1B1Uses apparatus safely with due care and attention

2

B1B2Intervention needed to ensure safety and care in the use of equipment.

1

B1B3Major intervention needed to prevent harm to personnel or equipment.

0

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Code Descriptor Mark

B1C1Two or more titres within 0.1cm3 2

B1C2Two or more titres within 0.2cm3 1

B1C3Difference in titre values > 0.2cm3 0

B1D1Accuracy within 0.1cm3 of identified value i.e. teacher's identified value.

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B1D2Accuracy within 0.2cm3 of identified value

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B1D3Result 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.

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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

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