Chemistry practical questions Flashcards
Why is the funnel removed after it has been used for filling the burette (titration)
a droplet from the funnel could enter the burette, affecting the volume and readings = affects titre
Name the sources of error when using the burette to carry out a titration
- leaving the funnel in
- air bubble in jet space
- not reading at eye level and recording where the bottom of the miniscus touches
During the titration, the inside of the conical flask is rinsed with distilled water.
Suggest why rinsing improves the accuracy of the titre
Ensures all reagents are able to react
Explain why adding extra water when rinsing the conical flask during titration, does not change the volume of acid solution that is required in the titration.
Because it doesn’t change the number of moles for the reagents
State why it is imporant to fill the space below the tap in the burette with solution before beginning an accurate titration
prevent air bubbles = to fill the space during titration, which prevents the volumes being added from being too high
Suggest why 250cm3 conical flask is preferred to a 250cm3 beaker for titration
Less chance of losing liquid when swirling
the liquid doesn’t splash
Why does repeating a titration make the value of the average titre more reliable
a single titration could be anomalous
in order to obtain concordant results
How do you reduce percentage uncertainty when measuring volume?
Increase the amount of the solution used. Remember, % error = (uncertainty in equipment / reading) x 100
Uncertainty in the equipment remains constant, but if you have a larger reading (more solution) then you will have a smaller % error.
Decrease the concentration of the substance in the burette.
As a larger volume has to be delivered to add the same amount of moles to the solution.
A student heated a solid sample of Na2CO3. xH2O for 1 minute to remove water and determine a value for x
The diagram shows the apparatus used.
a) the experimental value for x was much lower than the correct value of x. Suggest a reason for this
b) suggest how the procedure could be improved, using the same apparatus to give a more accurate value of x and justify your answer
a) Failure to drive off all the water/ Failure to heat for long enough/ Not heated to constant mass
b) Heat to constant mass / heat for longer / use a smaller mass
You can be sure all / more of the water has been driven off
How do you make up a standard solution?
Using a weighbing boat, weigh out accurately the mass of salt
Transfer the solid to a 250cm^3
3)Re-weigh the weighing boat and record the mass. Calculate the difference in mass
Add about 100cm3 of deionised water. Stir the mixture to dissole the solid, adding more water if necessary to aid the dissolving process
Transfer the solution to a 250cm^3 volumetric flask through a filter funnel. Rinse the beaker, glass rod and runnel and transfer all washings into the volumetric flask
add distilled water to the volmetric flask until the bottom of the meniscus sits on the graduation mark on the neck of the flask
stopper the flask and invert it 20 times so the solution is homogeneous solution
What kind of chemical should be used in a standard solution?
- Does not absorb or lose moisture to/from the environment.
- Has accurately known relative formula mass so number of moles can be calculated
- Is very pure
- Has high relative formula mass to minimise weighing errors.
Why is a volumetric pipette more accurate than a measuring cylinder?
because it has a smaller uncertainty
Why only use small drops of indicator?
indicators tend to be weak acids - so adding too much will affect the result of the titration.
How do you investigate the rate of a reaction changes with temperature?
Measuring cylinders - measure 50cm3 of 0.05 mol of sodium thiosulphate. In another- measure 5cm3 of HCl - place them I seperate boiling tubes.
measure the initial temperature of the 2 solutions before you add one to another in a conical flask.
Add the sodium thiosulphate to conical flask first and then the HCl. The conical flask should be placed over a white tile with a black cross, start the stopwatch and measure the time it takes for the cross to disappear due to the cloudy suffer being produced.
If equipment has uncertainty, it can still be used in an experiment and it is still adequate. What would make it adequate?
If the reading is bigger than the uncertainty. (For example, for a thermometer, if the temperature change is bigger than the uncertainty it is adequate)
How do you convert m3 to dm3 and dm3 to cm3
x 1000
Why is it important to note down the temperature and pressure of the room when using a gas syringe?
because the volume of a gas depends on the pressure and temperature
What are the potential errors when using a gas syringe (3)
- gas escapes before bung inserted
- syringe sticks
- some gases like co2 or so2 are soluble in water so the true amount of gas is not measured
Why is a burette more suitable than a pipette for titrations
because it can measure variable volume
Why should we never seal the top of a condenser?
the build up of gas pressure could cause the apparatus to explode
Why do we extrapolate the temperature curve back to the time the reactants were added together?
to obtain an accurate temperature rise
when the reaction is slow it is hard to calculate without extrapolating it
How do you increase accuracy when heating in a crucible?
- make sure the crucible is dry or a wet crucible wold give inaccurate result (the mass loss would be too large as the water would be lost when heating)
- small amounts of the solid being heated should not be used as the percentage uncertainties in weighing would be too high
- lid would prevent loss of solid from the crucible (should still be a bit loose to allow gas to escape)
What are the errors usually made when measuring enthalpy change?
- energy transfer from surroundings
- assuming all solutions have the same heat capacity of water
- ignoring energy absorbed by the apparatus
- reaction may be incomplete
- density of the solution is taken to be the same as water.
How do you stop reactions and why does this method work?
stop reaction by dilution
the reactant will become less concentrated = less successful collisions = slow down the reaction
Describe the iodine clock reaction
- Add dilute sulfuric acid and starch solution to beaker
- Add sodium thiosulfate to reaction mixture
- Add potassium iodide solution
- Add hydrogen peroxide solution
- Sodium thiosulfate reacts with iodine being formed
- Once all sodium thiosulfate is used up = any more iodine formed remains in solution
- Turns starch indicator blue-black
- Varying conc. of iodide or hydrogen peroxide while keeping everything constant = different times for colour change ◦ Used to work out reaction order
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How do you measure the EMF of the cell?
Clean a piece of copper and zinc using fine grade sandpaper.
Degrease the metal using cotton wool and propanone.
Place the copper into a 100cm3 beaker with 50cm3 of 1moldm-3 of copper sulphate
Place a strip of Zinc into a 100cm3 beaker with 50cm3 of 1moldm-3 of Zinc Sulphate.
use a strip of filter paper soaked in potassium nitrate for the salt bridge
connect the 2 half cells by connecting the metals to crocodile clips attached to leads joined to the voltmeter.
How do you investigate pH changes?
Transfer 25cm3 of acid to a conical flask using a volumetric pipette.
measure the initial pH of the acid with a pH meter
Add the alkali from the burette in small amounts e.g. 2cm3 at a time. Noting the volume added.
Stir mixture to equalise the pH
measure and record the pH to one d.p.
when approaching end point add alkali in smaller volumes, drop by drop until the alkali is in excess.
How and why do you calibrate the pH meter?
Calibrate pH meter by measuring the pH of a known pH buffer solution// distilled water.
Because pH meter can lose accurate over time when in storage.
Apart from calibrating the pH meter, how else can you improve the accuracy of measuring pH
maintain a constant temperature
How do you carry our recrystallization?
dissolve the crude product in the minimum amount of hot solvent
filter the solution whilst hot to remove insoluble impurities
cool in ice bath to recrystallise/ let it cool to room tempertaure
filter with a Buchner flask under pressure
wash with a cold solvent and dry
Why is the minimum quantity of hot water used in recrystallization
- to dissolve the desired product rather than the impurities as well
- to allow for a saturated solution so the solution isn’t too dilute for cyrstals to form
Why is the flask cooled to room temperature before the crystals were filtered off
so large crystals can form = allows for a slower cooling process which decreases the chances of trapping impurities in the developing lattice
Why are solutions filtered whilst hot after they are dissolved in a hot solvent
to remove insoluble impurities
prevent crystallisation before separation of impurities
Why are the crystals compressed in a Buchner funnel
Air passes through the sample not just around it
Why is a little cold water poured through the crystals?
to wash away impurities off the crystals surface without dissolving the crystals
How is yield loss during recrystallization?
- crystals lost when filtering or washing
- some product stays in solution after recrystallisation
- other side reactions ocurring
Why do crystals sometimes have a percentage yield over 100%
because they are not dried properly
Suggest why an electric heater is used rather than a Bunsen burner in the hydrolysis of ethyl benzoate with sodium hydroxide
the ester and the ethanol are both flammable
How do you make aspirin?
add to a pear-shaped flask salicylic acid (2-hydroxybenzoic acid) and ethanoic anhydride. To this mixture add drops of phosphoric(v) acid (which acts as an acid catalyst) and swirl to mix
Fit the flask with a reflux condenser and heat the mixture on a boiling water bath for about 5 minutes. Without cooling the mixture, carefully add 2 cm3of water in one portion down the condenser.
When the vigorous reaction has ended, pour the mixture into cold water in a 100 cm3 beaker, stir and rub the sides of the beaker with a stirring rod necessary to induce crystallisation and, finally, allow the mixture to stand in ice bath to complete crystallisation. Collect the product by suction filtration and wash it with a little water.
recrystallise
How do you carry out nitration of methyl Benzoate
add methyl benzoate into a small conical flask then dissolve with concentrated sulfuric acid. When the liquid has dissolved,cool the mixture in ice
prepare the nitrating mixture by adding drop by drop of sulfuric acid to concentrated nitric acid. Cool this mixture in ice as well. [This acids react together to make NO2+]
add the nitrating mixture drop y drop to the methyl benzoate, stir the mixture with a thermometer
keep the temperature below 10 °C. [This is to prevent multiple substitution of nitrogroups on the benzene ring]
When the addition is complete, allow the mixture to stand at room temperature for another 15 minutes. After this time, pour the reaction mixture on to crushed ice and stir until all the ice has melted and crystalline methyl 3-nitrobenzoate has formed.
recrystallise
How do you purify an organic liquid?
- Put the distillate of impure product into a separating funnel
- wash product by adding
- sodium hydrogen-carbonate solution to remove unreacted acid/acid catalyst : make sure to shake and release the pressure from CO2 produce
- Saturated sodium chloride solution: this allows the aqueous and organic layers to separate and draws water from the organic layer into the aqueous layer
- Allow the layers to separate in the funnel, and then run and discard the aqueous layer.
- Run the organic layer into a clean, dry conical flask and add a few spatulas of drying agent (e.g. anhydrous sodium sulphate, calcium chloride) to dry the organic liquid aka remove any traces of water. When dry the organic liquid should appear clear.
- Carefully decant the liquid into the distillation flask
- Distil to collect pure product
How would you determine an accurate value for the melting point of aspirin
Heat melting point tube in an oil bath slowly near the melting point
Why do you add sodium hydrogen carbonate or sodium chloride to the impure product when purifying an organic liquid?
Sodium hydrogencarbonate will neutralise any remaining reactant acid. Sodium chloride will help separate the organic layer from the aqueous layer
What features should drying agents have when purifying an organic liquid
- be insoluble in the organic liquid
- not react with the organic liquid
How do you prove you have made a pure compound
measure the melting point by heating the compound in an oil bath (the compound is in a melting point tube) slowly.
compare to data book
Give a detailed method for the partial oxidation of propan-1-ol
- Place about 10 cm3 of dilute sulphuric acid in a flask and add about 3g of potassium dichromate(VI) and 2 or 3 anti-bumping granules.
- Shake the contents of the flask until solution is complete (do not warm).
- Add 1.5 cm3 of propan-1-ol in drops from a dropping pipette, shaking the flask so as to mix the contents,and then assemble distillation apparatus
- Gently heat and slowly distil
Why does water go in the bottom of the condenser?
to go against gravity which prevents back flow of water and allows more efficient cooling
Because water enters from the bottom, it means the coldest water will be at the bottom of the condenser. Therefore the distil will be at the coldest point as it leaves the apparatus
Give a detailed method for the FULL oxidation of propan-1-ol
- Measure 5 cm3 of water into a boiling tube. Add 6 g of sodium dichromate(VI), shake and set aside to dissolve.
- Put about 1.5 cm3 propan-1-ol into a 50 cm3 round bottomed flask and add about 5 cm3 of water and two or three anti-bumping granules. Put a condenser on the flask for reflux, as shown in figure below.
- Add 2 cm3 of concentrated sulphuric acid down the condenser in drops from a dropping pipette. While the mixture is still warm, start to add your sodium dichromate(VI) solution down the condenser in drops from a dropping pipette. The energy released from the reaction should make the mixture boil. Add the solution a drop at a time so that the mixture continues to boil without any external heating.
- When all the sodium dichromate(VI) solution has been added, use a low Bunsen burner flame to keep the mixture boiling for 10 minutes, not allowing any vapour to escape.
- At the end of that time remove the Bunsen burner and arrange the apparatus for distillation. Gently distil 2-3 cm3 of liquid into a test tube.
How do you carry out fractional distillation in the lab
- Heat the flask, with a Bunsen burner or electric mantle . This causes vapours of all the components in the mixture to be produced.
- Vapours pass up the fractionating column.
- The vapour of the substance with the lower boiling point reaches the top of the fractionating column first.
- The thermometer should be at or below the boiling point of the most volatile substance.
- The vapours with higher boiling points condense back into the flask.
- Only the most volatile vapour passes into the condenser.
- The condenser cools the vapours and condenses to a liquid and is collected.
In detail how do you test for halogenalkenes?
- Arrange three test tubes in a row and add three drops of halogenoalkane in the sequence 1-chlorobutane, 1-bromobutane, 1-iodobutane.
- Add 4 cm° of 0.02 M silver nitrate to each halogenoalkane.
- Without delay, put all three test tubes simultaneously in a hot water bath.
- Note the order in which precipitates appear
What does the rate of precipitate formation tell u about the haloalkenes in the test for haloakenanes with silver nitrate
- quicker precipitate is formed = formed the substitution reaction = more reactive haloalkane
- the weaker the C- halogen bond, the easier it is to break and the faster the reaction
- AgI precipitate forms the fastest as it has the weakest bond
Why is the flask in recyrstallisation sometimes cooled in ice instead of room temperature?
because ice increases the yield of the cyrstals
