Chemical Tests and Practical Skills Flashcards
Test for primary or secondary alcohol
Add acidified potassium dichromate
Colour change for orange to green
Test for aldehydes (Tollen’s Reagent)
- Tollens’ reagent [Ag(NH3)2]+.
- add silver(I) nitrate solution
- then a drop of sodium hydroxide solution to give a brown precipitate of silver(I) oxide
- add just enough dilute ammonia solution to redissolve precipitate
- add suspected aldehyde solution
- warm gently in a hot water bath for a few minutes
- aldehyde present if silver mirror precipitate forms
Test for aldehyde (Fehling’s Test)
- Add Fehling’s solution to a test tube
- Add aldehyde and add in hot water bath for a few minutes
- Colour change from blue to brick red
Test for carboxylic acid
- Add sodium hydrogen carbonate
- Produces effervescence/bubbles
OR - Universal Indicator/ blue litmus paper
- Turns red
Suggest how you could distinguish between a primary an secondary alcohol
Acidified potassium dichromate turns from orange to green Apparatus set up for distillation Tollens test (if silver mirror forms then primary)
Suggest how the conditions for producing an aldehyde differ from producing a carboxylic acid
Aldehyde - (dilute H2SO4) - dilute potassium dichromate - distillation IMMEDIATELY Carboxylic acid - (concentrated H2SO4) - EXCESS potassium dichromate - reflux
Investigating rate of reactions (gas produced)
- Add reactants together
- Measure change in mass over regular time intervals
Or - Measure volume of gas produced using a gas syringe over regular time intervals
Suggest why deionised water is used over tap water
Does not contain any ions which can react with reaction mixture and give false results
Suggest why an electric mantle is used over a Bunsen burner
- even heating
- more control over temperature
- less chance of overheating
- safer
Accuracy
How close values are to true value - improved through correctly calibrated equipment
Precision
How close values are to each other - improved through using high resolution equipment
Test for NH4+
- add sodium hydroxide solution and heat gently
- ammonia gas given off
- pungent smell/turns damp red litmus paper blue
Test for CO32-
- add dilute hydrochloric acid
- carbon dioxide gas given off
- turns limewater (calcium hydroxide) cloudy
Test for OH-
Turns red litmus paper blue
Test for NO3-
- add to NaOH solution
- then add powdered aluminium
- damp red litmus paper to test for ammonia gas (turns blue)
Test for SO42- ions
- add hydrochloric/nitric acid to remove carbonate ions (forms carbon dioxide)
- add barium chloride to form barium sulphate (white precipitate)
Suggest why barium chloride used to test for sulphate ions is acidified
Acid reacts with carbonate impurities often found in salts which would form a white barium carbonate precipitate and give a false positive result
Suggest why barium chloride is used to test for sulphate ions
- forms BaSO4 precipitate
- insoluble in water
Suggest why aluminium is added to solution when testing for nitrate ions
- aluminium reduces nitrate NO3- to NH4+
- NH4+ + OH- -> NH3 + H2O
- NH3 detected by red litmus paper
Describe how to prepare a pure sample of silver bromide from a colourless solution containing a mixture of sodium chloride and sodium bromide
- add silver nitrate to form AgCl and AgBr (give equations)
- add DILUTE ammonia in excess to dissolve AgCl
- filter AgBr
- wash and dry to removed soluble impurities
Test for halogenoalkane
- add NaOH then ACIDIFIED AgNO3
- forms white, cream or yellow precipitate based on which halogen is present
