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Identify an unknown substance through a series of tests that should systematically eliminate all other possible substances.

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Introduction

A2 - Assessed Practical (Skill P) Introduction. Alcohols. 'Alcohols are a family of organic compounds in which the -OH functional group is attached to a hydrocarbon chain', pg. 296 Chemistry for Advanced level, Third edition. A substance can be tested for the presence of this functional group by selective oxidation. 'A solution of acidified potassium dichromate ions is often used as the oxidising agent. In the reaction the orange dichromate (VI) ions are reduced to green chromium (III) ions' pg 308, Chemistry for Advanced Level, Third Edition, Cr2O72 - + 14H+ + 6e- 2Cr3+ + 7H2O The overall equation is: 2Cr2O72 -(aq) + 3C2H5OH(l) + 16H+(aq) 4Cr3+(aq) + 3CH3CO2H(aq) + 11H2O(l) Ketones. Oxidation of secondary alcohols produces ketones, e.g. propanone and contains a C=O bond, where the carbon atom is also bonded to a CH3 group. 'The basic method of identification of an unknown carbonyl compound is to make a solid derivative and then measuring the melting point of the derivative. The original carbonyl compound is then identified by reference to tables of melting temperatures of 2,4- dinitrophenylhydrazone derivatives.' Pg.536 Chemistry for Advanced Level, Third Edition. Aldehydes. Primary alcohols produce aldehydes, e.g. ...read more.

Middle

Apparatus. > 100ml water > Phenolphthalein indicator > Electric kettle > Sodium hydrogen carbonate > Silver nitrate solution > 6 test tubes > 2M sodium hydroxide solution > 2M ammonia solution > 1 semi-micro test tube > 5 drops of 2,4 dinitrophenylhydrazine > 5 drops of potassium dichromate (VI) > 3 drops of dilute sulphuric acid > 5 drops of ferric chloride > Pipette > Small amount of unknown substance > 2 large beakers > Test tube rack Procedure. Carry out these following tests on the unknown substance and write down observations. 1) To five drops of ferric chloride add two drops of the substance followed by 2 drops of water. If there is no reaction move onto step two. If a reaction is observed note down any colour change, in this case if the solution turns a black-purple colour this indicates that there is a phenol present. 2) Mix a few drops of the substance with about 2-3cm3 of water. Add a drop of phenolphthalein indicator, shake to mix, then add sodium hydroxide solution drop by drop, until the indicator just changes colour. Allow the solutions to stand in a beaker of hot water for a few minutes. ...read more.

Conclusion

(Flow chart is in the appendix). Variables. Pressure and temperature must be kept constant throughout this experiment, as any change in either of these will have an effect on the results. The experiment will have to take place at room temperature and pressure (25?c and 100kPa). When conducting the silver mirror test care must be taken to avoid leaving the substance under light as this will have an effect on the results. Risk assessment. * Phenolphthalein can cause skin contamination, always wear rubber gloves when handling this. * Sodium hydroxide is corrosive and is an irritant. Handle with care by wearing rubber gloves and protective eyewear. * Potassium dichromate (VI) is very toxic if inhaled, handle with care. * Ferric chloride is irritating to eyes and skin so rubber gloves and eyewear are required. Appendix. Test 1. This is the ferric chloride test and if reacted with phenol a violet complex is formed. Test 2. Using phenolphthalein indicator and reflux with sodium hydroxide. Test 3. Sodium hydrogen carbonate with water, and testing for any gas released. Test 4. Silver mirror test. Test 5. 2,4 dinitrophenylhydrazine with one drop of water. Test 6. Potassium dichromate (VI) added to dilute sulphuric acid. ...read more.

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