practical skills in chem Flashcards
(20 cards)
PAG ONE: heating in a crucible
- what can this method be used for
- explain the two types of methods
- why is a lid used
- why should large or small amounts not be used
- why does the crucible need to be dry
- Measuring mass loss in thermal decomposition reactions or mass gain when reacting magnesium with oxygen
- a) Oxidation reaction: mass gain when reacting magnesium with oxygen
- weigh an empty, clean dry crucible and lid
- add 2g of coiled magnesium ribbon to the crucible and weigh again
- heat strongly with a bunsen for a couple minutes
- occasionally lift lid to allow in oxygen but take care to not allow any solid to escape
- allow to cool
- weigh the crucible and contents again
- heat crucible again and reweigh until a constant mass is reached
b) decomposition reaction: water of crystallisation can be removed as water vapour by heating - weigh an empty, clean dry crucible and lid
- add 2g of hydrated calcium sulfate to the crucible and weigh again
- heat strongly with a bunsen for a couple minutes
- allow to cool
- weigh the crucible and contents again
- heat crucible again and reweigh until constant mass
- improves accuracy as it prevents loss of solid from the crucible but should be loose fitting to allow gas to escape/enter
- large amounts = reaction is likely to be incomplete
small amounts = percentage uncertainties in weighing is too high - wet crucible would cause mass loss to be too large as the water would be lost when heating
how to measure gas volumes
- using a gas syringe
- or over water with an upturned measuring cylinder in a trough of water
potential errors in using a gas syringe
- gas escapes before bung inserted
- syringe sticks
- some gases eg co2 or so2 are soluble in water so true amount of gas is not measured
how to make a solution
- weigh the sample bottle containing the mass of solid on a 2dp balance
- transfer to beaker
- reweigh empty sample bottle
- record difference in mass
- add 100cm3 of distilled water to beaker. use a glass rod to dissolve the solid. heat gently if needed
- pour solution into a 250cm3 graduated flask via a funnel
- rinse beaker and funnel and add washings from the beaker and glass rod to the flask
- make up to the mark with distilled water using a dropping pipette at the end
- invert to ensure uniform solution
diluting a solution
- pipette 25cm3 of original solution into a 250cm3 volumetric flask
- make up to the mark with distilled water using a dropping pipette at the end
- invert to ensure a uniform solution
PAG TWO: titrations
- method
- why are titrations done
- rinse equipment (burette with acid, pipette with alkali, conical flask with distilled water)
- pipette 25 cm3 of alkali into conical flask
- touch surface of alkali with pipette ( to ensure correct amount is added)
- add acid solution from burette
- make sure the jet space in the burette is filled with acid
- add a few drops of indicator and refer to colour change at end point
- use a white tile underneath the flask to help observe the colour change
- add acid to alkali whilst swirling the mixture and add acid drop wise at end point 9. note burette reading before and after addition of acid
- repeats titration until at least 2 concordant results are obtained- two readings within 0.1 of each other
to find the concentration of one substance
method for working out amount of iron in iron tablets
- weigh accurately two ‘ferrous sulfate’ tablets
- grind up the tablets with a little sulfuric acid
- through a funnel, transfer paste to 100cm3 volumetric flask. use sulfuric acid to rinse the tablets into the flask
- add sulfuric acid to make solution up to 100cm3. stopper and shake the flask.
- titrate 10cm3 portions of the solution with Kmno4-. end point = purple colour
reducing uncertainties in a titration
- replacing measuring cylinders with pipettes or burettes which have lower apparatus uncertainty
- make the titre a larger volume by increasing the volume and concentration of the substance in the conical flask or decreasing the concentration of the substance in the burette
reducing uncertainties in measuring mass
- using a balance that measures to more dp
- using a larger mass
- weighing sample before and after addition and then calculating mass
calorimetric method
- washes the equipment (cup and pipettes etc) with the solutions to be used
- dry the cup after washing
- put polystyrene cup in a beaker for insulation and support
- Measure out desired volumes of solutions with volumetric pipettes and transfer to insulated cup
- clamp thermometer into place making sure the thermometer bulb is immersed in solution
- measure the initial temperatures of the solution or both solutions if 2 are used. Do this every minute for 2-3 minutes
- At minute 3 transfer second reagent to cup. If a solid reagent is used then add the
solution to the cup first and then add the solid weighed out on a balance.
8.If using a solid reagent then use ‘before and after’ weighing method - stirs mixture (ensures that all of the solution is at the same temperature)
- Record temperature every minute after addition for several minutes
draw a filtration apparatus
PAG 5: purifying an organic liquid
- method
- what does sodium hydrogencarbonate do
- what does sodium chloride do
- which layer is the organic layer
- what should the drying agent be
method:
1.Put the distillate of impure product into a separating funnel
2. wash product by adding either
a) sodium hydrogencarbonate solution , shaking and
releasing the pressure from CO2 produced.
b) Saturated sodium chloride solution
3. Allow the layers to separate in the funnel, and then run and
discard the aqueous layer.
4. Run the organic layer into a clean, dry conical flask and add three spatula loads of drying agent (e.g. anhydrous sodium sulfate, calcium chloride) to dry the organic liquid. When dry the organic liquid should appear clear.
5. Carefully decant the liquid into the distillation flask
6. Distill to collect pure product
neutralise any remaining reactant acid
help separate the organic layer from the aqueous layer
upper layer
- be insoluble in the organic liquid
- not react with the organic liquid
draw a distillation and reflux apparatus
- when is distillation used
- why are electric heaters used in distillation
- to separate an organic product from its reacting mixture
- organic chemicals are usually flammable
measuring boiling point
use distillation and refer to data book
how to purify an organic solid: recrystallisation
- why might there be a loss of yield in this process
- dissolve the impure compound in a minimum amount of hot solvent
- hot filter solution through filter paper
- cool filtered solution by inserting beaker in ice
- suction filtrate with a buchner flask to separate out crystals
- wash crystals with distilled water
- dry crystals between absorbent paper
- crystals lost when filtering or washing
- some product stays in solution after recrystallisation
- other side reactions
reflux
- draw apparatus
- when is it used
- why is the end of the condenser never sealed
- why are anti-bumping granules used
- when heating organic reaction mixtures for long periods
- build up of gas could cause the apparatus to explode
- to prevent vigorous, uneven boiling by forming small bubbles instead
measuring melting point
- draw apparatus
- how to measure
Melting point can be measured in an electronic melting point machine or by using a practical set up where the capillary tube is strapped to a thermometer immersed in some heating oil.
In both cases a small amount of the sample is put into a capillary tube. The tube is heated up and is heated slowly near the melting point
TLC method
a) Wearing gloves, draw a pencil line 1 cm above the bottom of a TLC plate and mark spots for each sample, equally spaced along
line.
b) Use a capillary tube to add a tiny drop of each solution to a
different spot and allow the plate to air dry.
c) Add solvent to a chamber or large beaker with a lid so that is no more than 1cm in depth
d) Place the TLC plate into the chamber, making sure that the level of the solvent is below the pencil line. Replace the lid to get a tight seal.
e) When the level of the solvent reaches about 1 cm from the top of the plate, remove the plate and mark the solvent level with a pencil. Allow the plate to dry in the fume cupboard.
f) Place the plate under a UV lamp in order to see the spots. Draw around them lightly in pencil.
g) Calculate the Rf values of the observed spots.
functional group test for an aldehyde
- reagent
- conditions
- reaction
- observation
- equation
- tollens reagent: formed by mixing aqueous ammonia and silver nitrate. active substance is the complex ion of [Ag(NH3)2]+
- heat gently
- aldehydes only are oxidised into a carboxylic acid and silver ions are reduced to silver atoms
- with aldehydes, a silver mirror forms on the inside of the test tube.
CH3CHO + 2Ag+ + H2O = CH3COOH + 2Ag + 2H+
techniques to investigate rates of reaction
- measurement of the change in volume of a gas
- measurement of change of mass - works if there is a gas produced
- titrating samples of reaction mixture with acid, alkali etc
- colorimetry
- measuring change in electrical conductivity
- measurement of optical activity
