Chem 3/4 Flashcards
(50 cards)
Define fractional distillation
technique used to separate liquids with narrow range of b.p
Define simple distillation
Technique used to separate liquids with significantly different b.p
How can purity be determined by M.P
PURE substances have a sharply defined M.P (range of 1-2deg) while impure substances have both lower and broader melting temp.
Why does impure have lower M.P
Bc the impurities disrupt and weaken the overall molecular or lattice structure, meaning less energy is required to overcome the forces of attraction in the solid.
What may the results of MP techniques suggest
-If the range is lower and broader than that of the known pure sample, then compound is not pure or is a mixture of impure compounds
-If lower yet well-defined, the unknown substance is likely a mixture of pure compounds
Why do impurities increase B.P
-Impurities can introduce additional intermolecular forces
-Impurities can disrupt the regular lattice structure which can increase bp
(-decrease vapour pressure)
Describe difference between simple and fractional distillation
The fractioning column allows continuous changes from l to g and back, this results in a much purer fraction than with simple distillation where mixtures need to be distilled multiple times to obtain pure distillate.
Why choose distillation over MP
Measuring bp is preferred method for many volatile compounds as the mp is too low.
What are 4 sources for error (titration)
Error standardising _____ solution
Another reductant other than ______ reacting with _____
Oxidised by atmoshpheric O2 -> unknown M of solution
Endpoint may have been inaccurate reflection of equivalence point
Question
Answer
K-Perman. test
react KMnO4 + unsat…. reduction of purple coloured KMnO4 to colourless solution (and brown precipitate-Mn02- if alkaline)
Describe the Lucas Test
Used to determine presence of an alcohol (p/s/t). The suspected alcohol is mixed with the reagent (mix of conc. HCl and ZnCl), if an alcohol is present it will produce a cloudy liquid.
Describe the acidified K dichrom. test
Added to solution containing suspected alcohol. If p or s alcohol is present the Cr6+ ions reduced to Cr3+ and turns from orange to green. ALSO shows positive result for aldehydes (not ketones) - link to oxidation
Describe the silver mirror test
Mix suspected oxidised alcohol (from acid. K-dichrom. test) with TOLLENS reagent (ammoniacal silver nitrate solution). If aldehyde present silver substance is formed (therefore was a p alcohol)
List 3 tests for carboxylic acids
Acid-base indicator test
React w/ metal carbonate
Esterification test
List 3 tests for -OH groups
Acidified K dichromate test
Lucas test
Silver mirror test
List 2 tests for double bonds
Br water test
K permanganate test
Question
Answer
Errors in a titration
Misjudging the colour of the indicator near the end point
Misreading the volume
Using contaminated solutions
Using solutions of wrong concentration
Using wrong amount of indicator
Rinsing burette with wrong solution
Not filling burette properly
Not transferring all solid/liquid when preparing samples
Not transferring all the volume
Titrating at wrong temperature (other than glassware was calibrated for).
Losing solution - too vigorous swirling can end in splashing
Leaking burette
Do measurements need to be taken at even increments?
No. As long as readings taken carefully then any trends will still evident.
What are some potential changes to improve cell experiments?
GENTLY stirring the electrolyte.
