Task 1 - Qualitative analysis PDF

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Task 1 - Qualitative chemical analysis Chemical tests are used by analysts to give an indication of the nature of substances that may be present in a sample. For example, a chemical test may be used to determine the presence of a particular metal ion in a sample of water taken from a polluted river. A forensic scientist may use a simple chemical test to identify components in a soil sample taken from a crime scene and compare this with a soil sample taken from a suspect’s shoes. Simple chemical tests can be used to identify a range of molecules. In your portfolio you will have to show that you are able to: • explain the uses of qualitative inorganic chemical analysis; • prepare a sample for analysis. You will have to produce a report on one qualitative analysis: • giving details of the standard procedure you used to - carry out a chemical test to identify cations - carry out a chemical test to identify anions; • analyse and interpret your results and report accurately the outcomes of your analysis; • describe the limitations of your qualitative techniques. Scenario An environmental field officer working for the environment agency has discovered 3 unknown substances on the sand near Newborough. They believe that the substance has either been dumped on the beach illegally or it has come from a passing ship. Your task is to separate your substance from the sand and analyse it to identify the unknown compound. You must carry out the analysis carefully and repeat your results for confirmation. Standard procedure Part A - Separating out the substance from sand  Wear safety glasses.  Test the sand and soil mixture with 1M sulphuric acid. If the mixture fizzes and the gas given off turns limewater milky a carbonate is present and this will be insoluble in water but soluble in acid in order to determine the cation present in the sample.  If no fizzing is observed place the sample in a 100cm3 beaker containing approximately 20cm3 of hot distilled water.  Stir using a glass rod to dissolve the unknown compound.  Filter to remove the sand.  Test the solution of the sample in order to identify the unknown compound by following the standard procedures. Part B – Identify cations i)

Flame tests If using a nichrome wire solid samples are needed, therefore the solution of the sample will have to undergo crystallisation first. If using a splint, the sample will be soaked in the solution before testing. Place the nichrome wire (must be cleaned first by dipping the wire into concentrated hydrochloric acid) or splint in a blue flame (air-hole half open). Record any colour observed or inconclusive if no colour is detected. Find out the colours of different metal ions. http://www.chemguide.co.uk/inorganic/group1/flametests.html http://21008isabella-csi.blogspot.co.uk/2011/08/flame-test.html http://www.docbrown.info/page13/ChemicalTests/ChemicalTestsc.htm

http://www.nuffieldfoundation.org/practical-chemistry/reactions-positive-ions-sodium-hyroxidemicroscale-version ii)     

Using 1.0M Sodium hydroxide solution Wear safety glasses. Add 2cm3 of the unknown solution to a test-tube. Then add 3cm3 of 1M NaOH to the solution and shake. Record any observation. Add a further 10cm3 of NaOH to the cation solution and record any changes observed.

Inference for some cations precipitates will occur. Give appropriate equations. Use the results of the above two tests to find out the identity of the cation in the unknown sample.

Part C – Identifying anions i)

Testing for carbonates

All carbonates are insoluble in water apart from those of group 1 and the ammonium ion.  

To a sample of either 1 spatula of solid or 3cm3 of solution add 1M sulphuric acid. Bubble any gas produced through limewater.

Inference The solution will effervesce if a carbonate is present. The carbon dioxide gas given off can be tested using limewater. Give the equation ii)   

Testing for sulphates Place 3cm3 of solution to be tested into a test-tube. Acidify with 1M hydrochloric acid. (Test on a piece of litmus paper). Add 1cm3 of 0.2M barium chloride to the solution.

Inference – if a thick white precipitate is seen then a sulphate ion was present.

iii)

Testing for nitrates  Transfer 2cm3 of the solution to be tested into a test-tube along with 2cm3 of 1M NaOH and half a spatula of aluminium powder.  Place a piece of damp red litmus over the mouth of the test tube and heat gently.

Inference – If a nitrate is present ammonia gas will be given off which will change the red litmus blue. Give the equation. iv)   

Testing for halides – (chlorides, bromides and iodides) Transfer 3cm3 of solution to be tested into a test tube. Acidify with dilute nitric acid (1M) – test with blue litmus paper. Then add 2cm3 of silver nitrate (0.02M).

Inference: Chloride – A white precipitate will be observed which dissolves in dilute (1M) ammonia solution. Bromide – A cream precipitate will be observed which dissolves in concentrated ammonia solution. Iodide – A yellow precipitate will be observed which does not dissolve even in concentrated ammonia solution. Give relevant equations.

Results and analysis Carry out the standard procedures on the unknown sample. Use your results to identify the compound. Collect results in a table with the following format. Test e.g. testing for cations with NaOH

Observation Blue precipitate formed

Inference Copper ions present Cu2+ + 2OH-  Cu(OH) 2

Conclusion State the name of the unknown compound giving the reasons as to why you have reached this conclusion.

Evaluation Discuss the limitations of this technique in detail. Explain some of the new modern methods that are used for sample analysis and that these techniques have replaced traditional inorganic analysis. Bibliography Reference any material used: If candidates use the same wording as a published source, they must place quotation marks around the passage and state where it came from. Candidates must give detailed references even where they paraphrase the original material. A reference from a printed book or journal should show the name of the author, the year of publication and the page number. For example: (Morrison, 2000 p 29). For material taken from the internet, the reference should show the date when the material was downloaded and must show the precise web page, not the search engine used to locate it. This can be copied from the address line. For example: http://www.bbc.co.uk/schools/16/sosteacher/history/40766.shtml. Candidates should be encouraged as a means of good practice to state the actual date when the material was downloaded. 5.3 Candidates must also include a bibliography which lists the full details of publications used to research and support their coursework, even where these are not directly referred to, for example: Morrison, A (2000) ‘Mary Queen of Scots’, London: Weston Press.

Qualitative Analysis – Checklist and guide. To be completed by ___________________________ Requirement Introduction to the assignment. Taking samples and how to prepare them for analysis.

Details of each standard procedure.

Detail Give the scenario to set the scene. Make sure sample is well mixed. Use random sampling from different parts of the sample. How to prepare the sample for testing (removing the sand). Give details of how each cation and anion test.  Give the method  The observation expected  The inference – a chemical symbol equation.

Risk assessment

Must be completed on the generic sheet. On the pupil drive. Must be detailed!! Mention which concentration is used for each chemical and give the correct hazard relating to that concentration. Also included risks for glassware and gas if being used.

Determination of the unknown sample.

Give the method of each practical carried out in turn. Record your observations. Give an inference. This works well as a table with the three headings. Results and equations in the inference must be shown clearly. Remember repeating results is good practice to confirm your findings. Report back on the scenario. Give details of your findings using your evidence to back it up. Discuss the limitations of this technique in detail. Explain some of the new modern methods that are used for sample analysis and that these techniques have replaced traditional inorganic analysis.

Analysis and interpretation

Evaluation

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