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3pracs2013

MELBOURNE HIGH SCHOOL

YEAR 12

UNIT 3 CHEMISTRY

2013

PRACTICAL WORK

Name ___________________________________ Teacher ___________________________________ Chem. group: _____

All entries in this document must be made in INK. Apart from results and observations, SAC prac instructions are NOT TO

BE ANNOTATED in any way whatsoever.

You must have this booklet with you while carrying out the included pracs.

A Scientific Calculator only may be used in class. Copies of Chemistry booklets are available on Chemistry domain and

MHS Connect.

This booklet contains the following practical exercises:

• Standardisation of 0.1 M hydrochloric acid • Percentage of iron in steel wool (trial SAC) - (Report pages will are at the end of this booklet) • Chromatography of inks and food colouring • Preparation of artificial fragrances and flavours • Synthesis and analysis of aspirin (SAC) – (Includes a pre-prac test)

NOTE: This booklet has: - instructions, spaces for results/observations and blank reports for all pracs.

Before any practical exercise ensure that you:

• have read the exercise thoroughly and completely, • understand the theory behind the exercise, • are aware of the safety instructions relevant to the exercise, • have completed any necessary pre-prac calculations or predictions (if there

are any for that particular exercise), • if the exercise is a SAC prac, ensured that you have planned your answers

to the questions in the report.

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General comments applicable to SAC exercises

Penalties The usual penalties apply:

• 1/2 mark is deducted each time a unit is omitted from the final numerical answer to any question or where one is required to be added to a results table. (A unit is not essential for individual burette readings but one is needed for titres.)

• A maximum of 1 mark per report is deducted for omission of symbols of state from equations and half equations.

• A maximum of 1 mark per report is deducted for incorrect numbers of significant figures in the final numerical answer to any question but not from intermediate steps within a question.

• The penalty for incorrect units or symbols of state varies depending on the severity of the crime.

Errors When discussing errors associated with practical exercises, the following types of error should be covered as appropriate:

• systematic • gross • random • procedural (eg. evaporation of volatile reactant/product) • reasoning (eg. Concepts such as yield)

The error discussion must be relevant to the particular exercise and draw on the experimental details of that exercise. General comments will not receive the same credit. Three distinct terms

• observations are what you see, hear, smell, etc. (but definitely not taste!), and deal with changes,

• interpretation is deducing something from observations, • explanation is discussing what you have observed or inferred in terms of a theory or hypothesis

about that phenomenon.

Questions

• Where a question is written in the plural, at least two points must be made in your answer, • a 'Why?' question requires an explanation,

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STANDARDISATION OF HYDROCHLORIC ACID (Time required: double period)

Calculation must be done prior to the exercise.

This exercise can readily be performed over two single periods – the preparation of the standard carbonate solution in the first period and the titrations in the second.

Background Hydrochloric acid is frequently used in the laboratory in a diluted form and for many purposes an approximate concentration is sufficient. If the acid is to be used for volumetric purposes, then its concentration must be accurately known. This is determined by comparison with a primary standard, a chemical chosen for its purity and ease of use. The primary standard must be a pure substance of known and constant composition. It must not react with any of the components of the atmosphere and it must not deteriorate on standing either in solution or as the solid. In this exercise, the reagent of choice is analytical quality (A.R.) sodium carbonate commercially available in 99.5% purity. It satisfies the above criteria (and more) although it may absorb a little water from the atmosphere if care is not taken during its use. This water, along with water of crystallisation, can readily be eliminated by heating the solid in an oven just prior to use followed by storage of the solid in a desiccator. The reaction of an acid and a carbonate produces carbon dioxide as a product:

CO32-(aq) + 2H3O+(aq) → H2CO3(aq) + 2H2O(l) and H2CO3(aq) → CO2(aq) + H2O(l)

The dissolved carbon dioxide will, at the equivalence point, produce a solution that is slightly acidic. The indicator chosen must change colour at the equivalence point and, for this reason, methyl orange is chosen (yellow if pH > 5, pink if pH < 3 and orange about pH = 4). The carbon dioxide escapes slowly from the solution as a gas but is difficult to detect. Why? For reasons of convenience and accuracy, it is desirable that the acid titre and sodium carbonate aliquot be of similar volume. Given that 20 mL aliquots of sodium carbonate will be used, its concentration will need to be about 0.05 M.

Safety Hydrochloric acid is corrosive. Any spillages must be cleaned up immediately.

Eye protection and an apron should be worn.

Materials and Apparatus

• goggles • 250 mL standard flask • apron • watchglass • anhydrous Na2CO3 in desiccator • 20.00 mL pipette • approx. 0.1 M HCl • red pipette filler • methyl orange indicator • burette and stand • deionised water • white tile • electronic balance • spatula • 2 × 100 mL beakers • dropper • 2 (or 3) 250 or 300 mL conical

flasks • small funnel

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Method Note: It is recommended that the pipette and burette be prepared concurrently by different students as

this approach uses the available time more efficiently. In the space for Calculation below, calculate the mass of anhydrous sodium carbonate required to prepare 250 mL of 0.050 M Na2CO3 solution. Weigh out approximately (within 5%) your calculated mass of anhydrous Na2CO3 on a watchglass Record the mass to three decimal places. Return the jar of Na2CO3 to the desiccator immediately. Rinse the 250 mL standard flask with a small amount of deionised water and discard the washings. Using a small dry funnel, transfer the solid to the standard flask and wash all solid adhering to the funnel into the flask using deionised water. Add deionised water until the flask is about a quarter full. Stopper the flask and shake until all the solid has dissolved. Then add more deionised water until the solution level is at the base of the neck. Stopper and shake as before. Use the dropper to make the solution up to the mark ensuring that the bottom of the meniscus is on the mark. Stopper and shake thoroughly until the solution is of uniform concentration throughout. Rinse a 20.00 mL pipette with a small amount of deionised water and then with a small amount of your carbonate solution, which you have poured into a clean, dry 100 mL beaker. Discard the washings. Check that the burette tap is closed. Using the smallest funnel that you have, rinse the burette with a small amount of deionised water followed by a small amount of the hydrochloric acid which you have poured into another clean dry beaker. Discard the washings. Using the funnel once again, fill the burette with hydrochloric acid to above the zero mark. Remove the funnel. Run out and discard sufficient acid to (i) fill the region below the tap and (ii) ensure that the bottom of the meniscus is on the scale. Using a red pipette filler correctly (see your Year 12 handbook for details), deliver a 20.00 mL aliquot of the sodium carbonate solution into each rinsed conical flask. Add three drops of methyl orange indicator to each flask. Record your initial burette reading to two decimal places. Sit the conical flask on the white tile under the burette and titrate the acid into the flask with constant swirling of the flask until the first permanent change of indicator colour occurs. This should occur with the addition of half a drop of acid. Record the final burette reading, also to two decimal places. Repeat the titration until three concordant titres are obtained. Empty the unused acid and carbonate solution down the sink and rinse the apparatus with tap water. Return all equipment to its correct place and wipe down the bench. Results Mass of anhydrous sodium carbonate: ____________________

Titration 1 2 3 4 5 Initial reading (mL)

Final reading (mL)

Titre (mL)

Calculations (Show ALL working)

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H = 1.0 C = 12.0 O = 16.0 Na = 23.0 Cl = 35.5 Calculate the mass of anhydrous sodium carbonate required to make up 250 mL of 0.050 M Na2CO3 solution. [Significant figures may be ignored in this calculation only - why?] Calculate the concentration of your sodium carbonate solution. Calculate the average of three concordant titres. Use the average titre to determine the concentration of the hydrochloric acid. Questions Why are different procedures used for washing the pipette and the conical flasks? What volume of concentrated (10 M) hydrochloric acid would be required to produce 500 mL of 0.10 M hydrochloric acid? Show your working.

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If the sodium carbonate were not heated prior to the experiment and had absorbed 2% of its mass in water, would this be a systematic or a random error? Explain your reasoning. What would be the effect (ie increase, decrease or no effect) on the calculated concentration of the hydrochloric acid if the sodium carbonate had not been completely dried and still contained some water of crystallisation? Explain your reasoning. Why is it essential that the anhydrous sodium carbonate be returned to the desiccator immediately after use?

End of experimental work and report.

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PERCENTAGE OF IRON IN STEEL WOOL -Trial SAC -

(Time required: Double period for prac + one period (40 min) write-up) The report for this trial-SAC is at the end of this booklet.

Introduction Sometimes it is necessary to carry out measurements on metals or other insoluble solids, which will not conveniently react directly with the usual standard solutions. One way to overcome this is to find a reaction, which quantitatively converts the insoluble substance to a convenient soluble one. This exercise involves a reaction of that type. Background If a sample of steel wool is placed in excess dilute sulfuric acid, the iron present is converted into iron (II) ions as the metal dissolves. The reaction is

Fe(s) + 2H+(aq) → Fe2+(aq) + H2(g) The iron remains as iron (II) ions as long as oxygen is prevented from entering the solution. The iron (II) ions in the solution can now be determined by titration against standard permanganate solution, the iron (II) ions being oxidised to iron (III) ions and the permanganate ions being reduced to manganese (II) ions.

Safety - Wear protective goggles and an apron at all times, especially whilst boiling the steel wool and sulfuric acid - It is essential that the volume of the solution be maintained during the boiling by the regular addition of small amounts of deionised water. Should the volume of solution drop significantly, some of the boiling acidic mixture may be splashed from the flask. Any splashed mixture should be immediately flooded with water and then wiped up. - Potassium permanganate stains clothing, paper, the bench, etc. - Sulfuric acid is corrosive.

Materials and Apparatus

• safety goggles • 2 × 250 (or 300) mL conical flasks • apron • burette, burette stand and white

tile • 100 mL measuring cylinder • steel wool • watch glass • small glass funnel • wash bottle • dilute (1M) sulfuric acid • standardised 0.02 M potassium

permanganate solution

• bunsen burner, cement bench mat, tripod and gauze

Method NOTE: The 2 preliminary questions on the Trial SAC report must be completed at

home PRIOR to the experimental work being performed and the calculated mass of steel wool transcribed into the space below.

Assuming that the steel wool is 100% iron, calculate the mass of steel wool required to give a titre of 20.0 mL of 0.0200 M KMnO4 solution. (All working is to be done on the SAC report form.) Calculated mass of steel wool (transcribe your calculated mass from your SAC report form): ___________ g Weigh out three (3) pieces of steel wool of approximately this mass - record the exact masses of each in your table on the next page. Place one (1) piece of steel wool in a 250 mL conical flask and add about 25 mL of dilute (1 M) sulfuric acid. Place a watch glass over the top of the flask.

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Heat the flask and contents until all the steel wool has dissolved, leaving a suspension of unreacted impurities. Add some deionised water as the solution volume decreases. When the reaction has finished, wash the inside of the flask and the watch glass to return any splashings to the bulk of the solution. Fill the burette (using the smallest glass funnel available) with standard potassium permanganate solution in the usual manner. Reading the top of the meniscus, titrate the iron (II) solution until a very pale pink colour persists for at least 30 seconds. Repeat for the two other samples of steel wool. Rinse all apparatus and return it to its rightful place - the correct shelf in correct cupboard, trolley, side bench or leave set up on your bench as appropriate. Observations and Results Observations (Record all observations which include before, during and on completion of the reaction).

• steel wool and sulfuric acid

• titration Concentration of standard KMnO4 : _____________ Results table - prepare your own. (VCAA has directed that you be able to do this!)

End of experimental work. The report for this trial-SAC is at the end of this booklet.

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CHROMATOGRAPHY OF INKS AND FOOD COLOURING (Time required: Single period)

Background Many dyes are composed of a mixture of different coloured components, which may be separated by means of paper chromatography. The extent of separation of the components is determined by each component's solubility in a mobile phase (solution or liquid) known as the eluent and its adsorption on to a stationary (paper) phase. [Note that in paper chromatography, the stationary phase is also the stationery phase (joke!)]

SAFETY Wear goggles, as glass capillary tubes are fragile - do not knock them or damage them in any way, as they will easily break into many little pieces. Do NOT eat the Smarties used in this exercise - remember that substances used in the laboratory must not be tasted.

Materials and Apparatus

• 1% NaCl solution (the eluent) • water soluble fine tipped felt pens • black, brown and orange

Smarties • food colourings

• 250, 300 or 400 mL beaker • large paper clip • graphite pencil (2B) • chromatography paper • watch glasses (for preparation of • glass capillary tubes

Smarties' colour solutions) • ruler Using a capillary tube Draw the solution being tested into the capillary tube by placing one end in the solution. Touch the capillary gently and quickly on to the origin on a sheet of chromatography paper so that a very small spot forms. Allow the spot to dry - this occurs quickly - and repeat the procedure in order to make a more concentrated spot of colour. Method A. Chromatography of inks and food colourings Using a lead pencil, rule a line about 2 cm from one narrow end of a piece of chromatography paper. Using 3 different coloured felt tipped pens, apply 3 small spots of ink along this line. Pour 1% sodium chloride solution into a 250 mL beaker to a depth of about 1 cm. Straighten out a paper clip and thread it through the other narrow end so that the paper hangs with the bottom of it just touching the eluent. Ensure that the ink spots are not immersed. Allow the eluent to rise up the paper for a distance of about 5-7 cm above the origin. Do not let it rise about the support hole. Remove the paper and immediately mark the eluent front (solvent level) using the lead pencil. Also mark the positions of each of the components. Repeat the procedure using the food colouring solutions supplied - use a fresh capillary tube for each food colour. B. Food colours in Smarties Select a Smartie (black, brown or orange) and place it on a watch glass. Add a drop or two of water to dissolve the food colour (use as little water as possible so that the solution is reasonably concentrated). [This may already be done for you or your teacher may direct different groups to prepare a solution for class use.] Make a spot of colour on a piece of chromatography paper. Repeat using a Smartie of another colour. If standard food colour components are available, add 1 drop of them to your chromatography paper in the usual manner in order to assist with identification of the unknown food colours. Develop the chromatogram.

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Results and Calculations Complete the table:

Sample tested

Solvent used

Colours of components

Rf values

Why do the components of the dyes separate as they move up the chromatography paper? Why is the origin marked with a lead pencil rather than a pen? Why must the spots be above the level of the eluent at the start of the development of the chromatogram? Why do different solvents give different chromatograms when the same colour components are used?

End of experimental work and report

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PREPARING ARTIFICIAL FRAGRANCES AND FLAVOURS (Time required: Single period)

Theory Esters are commonly used as artificial flavourings in foods such as ice-cream and sweets. They are partially responsible for many familiar odours, including those of coffee, perfumes and fruit. An ester is formed when an alcohol undergoes a condensation reaction with a carboxylic acid. For example, an ester is formed during the condensation reaction between ethanol and ethanoic acid.

CH3CH2OH(l) + CH3COOH(l) → CH3COOHCH2CH3(l) + H2O(l) The ester produced during this reaction is known as ethyl acetate.

Safety

• Wear safety glasses and apron • The acids used in this experiment are corrosive, particularly sulfuric acid. Handle

them with extreme care. Dilute any spills with large quantities of cold water. • The alcohols used are highly flammable. Keep the bottles firmly stoppered and

keep well away from flames.

Materials and Apparatus

• safety goggles • semi-micro test tube rack • apron • small tongs • small volumes of the alcohols 1-

pentanol, methanol, ethanol • 250 mL beaker of boiling water

• Small amounts of various carboxylic acids: salicylic acid, glacial ethanoic acid and decanoic acid

• 2 x 250 mL beakers

• dropping bottle of concentrated sulfuric acid

• marking pen

• 6 x semi-micro test-tubes Method Label two semi-micro test-tubes ‘A’ and ‘B’. Place ten drops of 1-pentanol in each tube. Add ten drops of glacial ethanoic (acetic) acid to test tube A and a similar volume of salicylic acid to test-tube B. The add two drops of concentrated sulfuric acid to each tube. Heat the mixtures for 10 minutes in a beaker of boiling water and then pour each one into a 250 mL beaker containing 200 mL of cold water. Try to identify the odour of the esters that are produced. Wash out the beakers thoroughly and repeat steps 1 – 4 using clean semi-micro test tubes and other combinations of alcohols and carboxylic acid.

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Results and Questions Complete the following table. (Smells may be apricot, pineapple, banana, wintergreen – aka ‘Dencorub’, grape, apple, ‘fruit flavor’ or other.) Alcohol Acid Ester Smell 1-pentanol

ethanoic acid

pentyl ethanoate

1-pentanol

salicylic acid

Salicylic acid has the formula C6H4(OH)COOH. Write the equation for the reaction between methanol and salicylic acid. Draw the structure of, and name, the ester produced above. Write the equations for each of the other reactions you performed in this experiment.

•

•

•

•

•

What is the role of the concentrated sulfuric acid?

End of experimental work and report.

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SYNTHESIS AND ANALYSIS OF ASPIRIN

- (SAC PRAC) –

(Time required: three or four single periods or parts of a period plus 1 period for write up. Assessment and time allocation includes pre-prac test, synthesis, analysis and write up)

(Students may need to come in at recess, lunch time or after school to conduct the filtration.) Aim: To synthesise a sample of aspirin, purify it by a process of recrystallisation, and determine the percentage yield. To understand that the purity of aspirin can be determined by comparing the melting points of prepared samples at each stage against the actual melting point of aspirin. To investigate the purity of samples by reaction with iron (III) chloride solution and comparing to those of pure aspirin. Background Aspirin is a compound that is both an analgesic (reduces pain) and antipyretic (reduces fever). It is also used regularly - in small doses - to prevent heart attack and stroke, and as a first response in the event of heart attack, as it will act as an anti-coagulant. It is found in the inner bark of the willow tree, and was originally used as a natural product. It has been synthesised for commercial use since it was patented in 1899. The Merck Index, which is an encyclopedia of chemicals, drugs and biological agents, lists the following information under aspirin:

• acetylsalicylic acid; monoclinic tablets or needle-like crystals; melting point of 135°C (rapid heating); is odourless, but in moist air it is gradually hydrolysed into salicylic and acetic acids; one gram dissolves in 300 mL of water at 25oC and in 100 mL of water at 37 oC.

Synthesis of aspirin (acetylsalicylic acid – C6H4(COOH)OCOCH3) commercially involves the reaction between salicylic acid ( C6H4(OH)COOH) and glacial acetic acid (CH3COOCOCH3). The structural equation is shown below:

The synthesis of the salicylic acid from phenol is by the Kolbe-Schmitt reaction as described below.

(http://en.wikipedia.org/wiki/User:Beetstra/aspirin)

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Safety

• Eye protection and an apron should be worn. • Concentrated sulfuric acid and acetic anhydride are highly corrosive. • Handle them with extreme care and avoid contact with the skin and eyes. Use only in the fume

cupboard. • Ethanol and ethanoic acid are flammable. Do not use near an open flame. • Do not ingest the aspirin prepared in this experiment.

Materials and Apparatus

• safety glasses • glass rod • apron • Buchner funnel • 3g salicylic acid • filter paper • 10.0 mL acetic anhydride • 130 mL iced water • 1 mL concentrated sulfuric acid • electronic balance • phenol • disposable gloves • 1% iron III chloride solution • methylated spirits • 250 mL conical flask • 10 mL measuring cylinder • filter flask

Method (Record all results as you take measurements). Synthesis (Leave overnight to allow for maximum crystalisation) Day 1 (about 15 - 20 minutes) Place an accurately weighed mass of about 3 g of salicylic acid in a conical flask. In the fume cupboard, add 10.0 mL of acetic anhydride and 1 mL of concentrated sulfuric acid to the flask. Swirl the mixture, without external heating, until all the solid has ‘dissolved’. At this stage the reaction should be complete and the flask can be taken out of the fume cupboard. Allow the mixture to cool and then add 100 mL of iced water. The resulting oil should be ‘scratched’ with a glass rod to induce crystallisation. (Alternatively a seed crystal of aspirin could be added.) Day 2 (about 15 - 20 minutes. This can be completed during a lunch break) When crystallisation is complete, collect the solid by vacuum filtration in a Buchner funnel and wash with iced water. Allow the crystals to dry overnight and weigh them. Day 3 a) Melting Point Determination – Students will view a video of the procedure. Information regarding the procedure – note you will not be conducting this determination. In schools, melting temperatures are usually determined by placing a small amount of sample into a capillary tube. This is done by tapping the open end of the capillary tube into a sample of dry, finely powdered solid. Vibrating the capillary tube aids settling the powder which should fill the tube to no more than 5mm. Gentle tapping on the side of the upright capillary tube also settles and compacts the powder. The capillary tube with sample is then attached, using a rubber band, to a thermometer so sample and thermometer bulb are in line. The thermometer is then clamped into a beaker of paraffin oil. The oil is heated slowly but steadily with the Bunsen burner, stirring all the time. A sharp melting temperature provides excellent evidence of purity. Since the impurity particles are only in the solid state, they interfere with the melting process. Unless the impure particles are of a similar size and structure to the material under consideration they will disrupt the crystal lattice and thereby decrease the effect of the intermolecular forces. Consequently the melting process will take place at a lower temperature and over a wider range, than it would for the pure material. b) Percentage Yield Determination When the product is completely dry, weigh the product, accurately & calculate the percentage yield. c) Iron III Chloride Test for Salicylic Acid Formation of an iron-phenol complex with Fe3+ gives a definite color ranging from red to violet, depending upon the particular phenol present. The iron III chloride should react with the salicylic acid to produce a purple colour but not with the aspirin.

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Add 10 drops of aqueous 1% iron III chloride solution to a test tube containing a few crystals of the compound to be tested dissolved in 5 mL water and note the color. Do this test with 1) phenol, 2) salicylic acid, 3) crude product and 4) re-crystallised product (after preparation and drying). d) Re-crystallisation The re-crystallisation technique provides a mechanism for the purification of substances by making use of differing solubilities of various components in the reaction mixture. The crude substance is dissolved in a minimum amount of the hot solvent, the solution can be filtered to remove the insoluble impurities and, upon cooling, crystals of ‘pure’ material are deposited, leaving the majority of the impurities still in solution. (We will not be conducting a hot filtration). Transfer as much as possible of the crystallised solid from the filter paper to a 100 mL beaker. Dissolve the solid with minimal amount of hot methylated spirits. 10 mL should be enough. (The methylated spirits should be in a hot water bath at no more than 60°C. One 500 mL beaker with about 250 mL of methylated spirits placed in the water bath should be enough for the class). Add the methylated spirits using a calibrated pipette dropper. After adding 8 – 10 mL place the beaker in hot water bath and heat the solution, swirling if required. Make sure that the product is completely dissolved. Add more methylated spirits if the solid does not dissolve. No more than 15 mL should be added but this may depend on evaporation. (Hot water bath - added about 100mL of hot water from the urn into a 250 mL beaker, place your 100 mL beaker with aspirin into this). After all the solid has dissolved add 20 mL of cold water. Cool the contents and leave the beaker for approx 15 minutes to one hour to allow for maximum recrystallisation, preferably in an ice bath. (You may need to do the next stage at lunchtime) Filter the crystals using the Buchner funnel, wash with small amounts of cold water. Day 4 Dry overnight, weigh and calculate the percentage yield of this recrystallised product. Calculate the % recovery of recrystallised material from crude material. Record the reaction with iron III chloride. Observations Record these on the next page. Results Synthesis and Purification Mass of salicylic acid used ________________ g Moles of salicylic acid used ________________ mol Amount of acetic anhydride used ________________ mL Limiting reagent ________________________ Mass of aspirin obtained (before recrystallisation) _______________ g Mass of aspirin obtained (after recrystallisation) _______________ g Iron III chloride test Colour of test with salicylic acid __________________________ Colour of test with phenol __________________________ Colour of test with crude aspirin product __________________________ Colour of test with re-crystallised aspirin product __________________________

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Observations Initial observations – pre-reaction During the synthesis of Aspirin During the first crystallisation process and vacuum filtration

End of experimental work.

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Synthesis of Aspirin - SAC Report Name : ____________________________ Partner : _____________________________ Chem Group: ____

All entries in this document must be made in ink. (Penalties apply for use of pencil and evidence of erasings).

(Mr(salicylic acid)=138.12, Mr(acetic anhydride)=102.09, Mr(aspirin)=180.16)

Results and observations Observations: What did you observe - a) during the synthesis of Aspirin b) during the first crystallisation process and vacuum filtration Results: Synthesis and Purification * indicates values that will be given to you during the report write-up session. (Do not use results that you obtained during the prac. Your answers should be based on the values you are given at the start of the report write-up). * mass of salicylic acid used ________________ g Amount of acetic anhydride used ________________ mL Limiting reagent ________________________ * mass of aspirin obtained before re-crystallisation _______________ g * mass of aspirin obtained after re-crystallisation _______________ g Iron III chloride test Colour of test with aspirin product before re-crystallisation __________________________ Colour of test with aspirin product after re-crystallisation __________________________ Colour of test with salicylic acid __________________________ Colour of test with phenol __________________________

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Calculations (show all working, includes formula, substituted values and answer to sig figs) 1) Moles of salicylic acid used ________________ mol 2) Moles of acetic anhydride used (density = 1.08 g mL-1) ________________ mol 3) Theoretical moles of aspirin that could be produced ________________ mol 4) Theoretical mass of aspirin if 100% yield was obtained ________________g 5) % Yield [(actual mass/theoretical mass) x 100] for the first crystallisation ________________________ % 6) % Yield after re-crystallisation ________________________ % Equations When aspirin is exposed to moist air the compound decomposes to salicylic acid and ethanoic acid. Write a balanced equation to show the hydrolysing of aspirin.

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Questions 1. Can you explain what happened to the -OH group in the salicylic acid when the acetic anhydride was added and what effect this had on the end product? 2. Why do you think the aspirin was washed with cold water during the filtration process? Explain, with this intention only in mind, the effect on product yield? 3. Another student, Gehardt Bayer, forgets to add concentrated sulfuric acid to the reaction mixture. What would you expect the iron III chloride test to show? 4. A total of 20 mL of ice water are used in processing the final reaction mixture (including the water used to rinse the precipitate). How many grams of aspirin could be lost in this process? (Assume the solubility of aspirin in ice water is the same as at 25oC.)

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5a. Identify one source of error within the experimental procedure that would result in an increase in yield. 5b Identify one source of error within the experimental procedure that would result in a decrease in yield. 6 What is the purpose of re-crystrallisation and explain how is this achieved? 7 If you obtained a melting temperature of 119-124°C for your first sample, what would this indicate about the purity of the sample?

End of report Marks Observations: 2marks Results: 2 marks Equation: 2 marks Calculations: ½+ ½ + ½ + ½ + ½ + ½= 3 marks Questions: 2 + 1 + 1 + 2 + 2 + 2 + 1 = 11 marks Pre-Prac test: 5 marks TOTAL: 25 marks

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PERCENTAGE OF IRON IN STEEL WOOL TRIAL SAC PRACTICAL REPORT

Name : _____________________________Partner : _____________________Date:_______ Group:____

All entries in this document must be made in ink. (Penalties apply for use of pencil and evidence of erasings).

Preliminary Questions 1 & 2 must be completed in ink AT

HOME prior to the experimental work. No alteration to these answers will be permitted.

H = 1.0, O = 16.0, S = 32.1, K = 39.1, Mn = 54.9, Fe = 55.8

Preliminary Questions Write the overall equations for the reaction between the iron (II) ions and the acidified potassium permanganate solution. oxidation: Fe2+(aq) Fe3+(aq) + e-

reduction: MnO4-(aq) + 8H+(aq) + 5e- Mn2+ (aq) + 4H2O(l)

overall: ______________________________________________________________________

Assuming that the steel wool is 100% iron, calculate the mass of steel wool required to give a titre of 20 mL of 0.02 M KMnO4 solution. Transcribe this answer to your instruction booklet. [Significant figures may be ignored in this calculation only as it is a theoretical calculation that doesn't use experimental data.] Observations

• steel wool and sulfuric acid

• titration

.../over

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Results Concentration of standard KMnO4: ___________________ Results table: Calculations For each titration separately, calculate the percentage of iron in your steel wool sample. Clearly label the steps in your calculations.

• Average percentage of iron in steel wool: _________________________ Questions The conical flasks are covered before heating. What are the likely sources of error if this is NOT done?

.../over

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Why is it necessary to read the top of the meniscus in this titration rather than the bottom of the meniscus as is done in most titrations? In most titration exercises, the titres are averaged before doing the calculations. Why is this inappropriate in this exercise? Why isn't an indicator used in this exercise? The amount of additional potassium permanganate solution required to reach the endpoint is minute. This excess will notionally increase the calculated amount of iron present but, in practice, the extra has no effect on the calculated amount. Why not? If the KMnO4 solution has partially decomposed and hence its concentration is actually lower than that stated, what effect would this have on the calculated percentage of iron in the steel wool? Explain your reasoning. The amount of iron in steel wool can also be calculated using gravimetric analysis. The steel wool can be dissolved in excess dilute sulfuric acid forming Fe2+ ions and hydrogen gas. The Fe2+ can be precipitated using an excess of potassium sulfide solution. FeS is insoluble.

.../over

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A 5.080 g sample of steel wool was added to excess dilute sulfuric acid and was fully dissolved. Excess potassium sulfide solution was then added and a precipitate formed. The solution was filtered and the solid washed and then allowed to dry to constant mass. The final mass of the precipitate was 7.636 g a Calculate the percentage Fe in steel wool in this sample. (Show all working)

b How would the following effect the calculated purity of the sample ? Experimental fault Calculated purity is increased or

decreased Explanation

Solid is not adequately washed

Solid is not fully dried

Some of the Fe2+ is oxidized to Fe3+ before the potassium sulfide is added.

End of report.

Marks:

• preliminary questions 1/2 + 1 = 11/2 • observations 1 + 1 = 2 • results table 2 • calculations 1 x (3 clear steps + answer) + 1 (average calc) = 5 • questions Q 1-1,2- ½, 3-1, 4-1, 5-1, 6-1 (=5½) + Q 7 (1 + 1 x 3 = 4 ) Total = 9½ • Total 20 marks

End of Booklet