Practical Chemistry Flashcards
What is a standard solution?
a solution whose conc is accurately known
What is a titre?
the required volume delivered from the burette
What is the nearest measurement a burette reads to?
0.05cm3
How do you ensure all water is removed from a thermal decomp expt?
heat to a constant mass
Why is a reful reaction used?
allows prolonged heating without loosing material through evapouration
What is an advantage of using a water bath rather than a bunsen burner?
better control - heating at a constant temp can be acheived
What are distillations used for?
used to purify organic liquids based on different BP
How do you test for Br2?
alkenes and phenols will decolourise Br2
What colour are phenols when universal indicator is added?
red- orange
What happens when you add sodium carbonate to a phenol and a carboxylic acid?
carboxyl = fizzes
phenol= nothing
What happens when you add warm tollen’s reagent to aldehydes?
aldehyde gives a silver mirror
What happens when you add warm acidic potassium dichromate to alcohols and aldehydes?
solution turns from orange to green
What are the 2 methods you can use to determine the amount of moles?
measuring gas collected
measuring mass
How do you produce a standard solution?
- weigh specific amount of solute into a weighing boat then transfer to 250ml beaker
- add small vol of distilled water to the beaker and stir until the solid completely dissolves
- transfer solution to a 250ml volumetric flask using a funnel
- rinse beaker and glass rod with distilled water into the volumetric flask
- fill the volumetric flask up to the graduted line using distilled water
- insert a bung and invert several times
What indicator is used for titrations?
methyl orange
What is the colour change of methyl orange in titrations?
yellow to orange = neutralised
What are the errors of titrations?
- some of the weighed solid may not be transferred from the weighing boat
- do not spill soild/ solution
- not swirling = higher titre than expected
- colour change is not clearly visible
How do you determine the enthalpy change of neutralisation styrofoam cup prac?
- measure equal volumes of acid and base
- record the initial temp of acid in styrofoam cup
- add base and record temp at regular intervals for 10 mins
- stir the solution each interval
Describe the test for halide ions
add dilute nitric acid with carbonate ions so no Ag2CO3 (white solid) forms
add silver nitrate and precipitate forms
How can you test for the presence of Magnesium?
add sulfate solution = mg is the only g2 element that is soluble
How can you test for barium?
add chromate solution
barium is the only g2 metal that is insoluble
What type of reaction is a haloalkane formed from an alcohol?
nucleophilic sub
What are the 3 main stages of sythesising a haloalkane from an alcohol?
- preparation
- separation
- distillation
Why do 2 layers form in the separating funnel when synthesising a haloalkane from an alcohol>
the aq layer and organic layer have different denisties and they are immiscible (do not mix)
What is the purpose of anti-bumping granules?
prevents the formation of large gas bubbles that cause violent boiling
Why is a base added to an organic layer after synthesing it from an alcohol?
to react with any hydrogen-halide remaining
Why is a base added to an organic layer after synthesing it from an alcohol?
to remove any hydrogen-halide remaining
Why do you need to open the stopper on the separating funnel regulary?
release build up of pressure and reduce risk of appartus breaking
What can you use to heat flammable reactants instead of a bunsen burner?
water bath or electric heater
What happens during preparation of synthesis of a haloalkane from an alcohol?
add alcohol and hydrogen-halide into a separating funnel
place stopper on separating funnel and shake vigorously for 20 mins - release pressure when required
How do you separate organic layer from aq layer in a separating funnel?
open and close the tap to collect the bottom aq layer
add a base like sodium hydrogen carbonate to separating funnel
shake and release pressure
allow mixture to separate
remove bottom aq layer
repeat until no pressure build up
collect organic layer
add anhydrous MgSO4 and swirl until stops clumping
filter into a round bottom flask
How is cyclohexene formed from cyclohexanol?
dehydration
1. reflux
2. distillation
What is the catalyst for dehydration of alcohols?
phosphoric acid
What apparatus is used to purify cyclohexene?
distillation
Why is anhydrous calcium chloride added to any mixture?
remove any traces of water
What catalyst is used to add a substituent to a benzene ring?
sulfuric acid
Describe filteration of an organic solid
- place a piece of filter paper in a buckner funnel
- place the funnel in a buchner flask. connect the side arm of the flask to a water pump
- connect the water pump to a tap and turn the tap on to create a vacuum. Pour the solution into the funnel
- once the solution has drained through, rinse the soild on the filter paper with a small volume of solvent
Describe the process of recrystallisation
- dissolve the solid in the minimum volume of hot solvent
- filter to remove insoluble impurities
- cool the filtrate
- filter to remove soluble impurities
- product remains on filter paper
Why is product of recrystallisation dissolved in a minimum amount of solvent?
if there is a large volume of solvent when the mixture is cooled, some of the product may remain in solution
How does recrytallisation improve the purity of a product?
desired product is dissolved in an appropriate solvent that impurities are insoluble in
filtration leaves insoluble impurities on the filter paper
Why the solution cooled in ice immediately after recrystallisation?
to decrease solubility of product so that it crystallises
Why must crystals be completely dry before measuring their mass?
excess solvent on crystals could falsely increase the yield
Why might % yield not be 100% after recrystallisation?
product may be lost when filtering or transferring between apparatus
some product may stay in solution after recrystallisation
other side reactions may occur
How can you investigate the purity of a product?
thin layer chromatography
During chromatography, why is a clear watch glass placed on the beaker and TLC plate?
prevent evapouration of solvent
should be clear to allow the plate to be monitored
In chromatography, what is the solvent front?
the furthest distance travelled by the solvent up the TLC plate
How do you calc a Rf value?
Rf = distance travelled by substance/ distance travelled by solvent
How can the purity of a sample be assessed using melting points?
pure = has specific MP
impure = range of MP
How can the purity of a sample be assessed using melting points?
pure = has specific MP
impure = range of MP
smaller range - more pure
What is observed from a reaction of an alkane with bromine water?
stays orange
What is observed from a reaction of a cycloalkane with bromine water?
two separate layers form
top layer = orange
bottom layer = water
Do alkenes react with bromine water?
yes
What is observed from a reaction of an alkene with bromine water?
orange —> colourless
How do you test for haloalkanes?
- heat the sample with a solution of sodium hydroxide under reflux to release halide ions
- add dilture nitric acid
- add silver nitrate and observe precipitate formed
What is the solubility of AgCl in ammonia?
soluble in dilute and conc ammonia
What is the solubility of AgBr in ammonia?
soluble in conc ammonia
What is the solubility of AgI in ammonia?
insoluble in ammonia
How do you test for carboxylic acids?
add sodium carbonate
effervesence - CO2 produced = carboxylic produced
test for CO2 with limewater
How do you test for phenol?
add bromine water = decolourises
How would you determine the empirical formula of magnesium oxide by a prac?
measure the mass of Mg and heat in air to form MgO and measure the mass of product
What colour is methyl orange in acid?
red
What colour is methyl orange in alkali?
yellow
What colour is methyl orange to indicate neutralisation?
orange
What would you add to a sample to test for the presence of a haloalkane?
ethanol
HNO3
AgNO3
What is used instead of benzene for aromatic reactions as benzene is too hazardous?
methyl benzoate
benzene ring with COOCH3
How would you continuously measure the rate of reaction by a prac?
add HCl to a conical flask and a strip of Mg
insert a bung immediately with a gas syringe attached via tubing
record the volume of gas every 15 sec time intervals for 5 mins
repeat for diff concs of HCl
What are some possible errors in the iodine clock prac?
inaccurate timing of appearance of the blue colour - use an average value recorded by 2 students
adding starch changes the vol slightly which affects the conc of reactants
