AQA A LEVEL CHEM PRACTICALS -
PAPER 3
Acid-base titration - correct answer- pipette 25cm³ acid and add indicatir, place beaker on a magnetic
stirrer and place magnet in the beaker, fill burette with base
- rinse electrode with distilled water as readings may have changed over time, dip it into the acid in the
beaker, recording pH
-switch on stirrer and ensure magnet doesnt strike the electrode.
- add 5cm³ base and take readings of pH and indicator
- continue adding base taking 5cm³ readings each time then 1cm³ around end point, then 5cm³ util most
base has been added
- plot a graph of pH (Y axis) against volume of base added
Determining purity of the organic product (aspirin) - correct answermelting point of product formed can
be established by placing a small amount of the product in a capillary tube and heating the tube slowly
near the melting point. compare against databook value for the compound.
- pure sample has sharp melting point at the same point of the data book, if impurities are present, the
melting point will be lower and will melt over a range of several °C
* To increase purity, re-crystallise more than once
* to increase percentage yield, take more care in transfer process to reduce mechanical losses and use a
more pure starting product
distillation - correct answer
Examples of continuous monitoring methods - correct answergas volume, change in pH, loss of mass,
colour via colorimeter to see concentration changes
Examples of initial rate methods - correct answertime taken for pH to change, time taken for set vol of
gas to be evolved, time taken for change in colour to occur
, How can number of molecules of water of crystallisation be determined in a practical? - correct
answertitrate the aq solution, with known volume in a conical flask and acid in burette, repeat titration
until concordant results found.
Use added volume to work out moles using molar ratio.
calculate mr using known mass and number of moles found - then calculate 'x' using found mr - mr of
each molecule in substance, divided by 18 to find number of H2O molecules
How does a buffer maintain an approximately constant pH when a small amount of HCl is added? -
correct answerThe buffer neutralises the addition of acid (equilibrium shifts left/right dependent on
what is added)
identifying organic functional groups - correct answer1 - alkene test, add few drops of 0.01moldm3
bromine water, record any colour change (orange to colourless)
2 - primary/secondary alcohol. add 1cm³ acidified potassium dichromate and warm. positive test =
orange to green (alcohol is oxidised to an aldehyde)
3 - test for aldehydes, mix equal amounts of fehlings 1 & 2. add few drops to sample and warm gently,
positive test = blue to brick red (aldehyde oxidised to carboxylic acid)
4 - test for carboxylic acids, add Na₂CO₃ and warm, positive test = effervesence (CO₂ produced)
in terms of electrons, state what is happening at the negative electrode - correct answernegative
electrode is supplying the electrons to the solution which are transferred to the positive electrode. This
is where oxidation occurs, this metal is the more reactive metal.
Investigating how rate changes with temperature - correct answer- mark black cross on piece of white
paper
- fill 250cm³ beaker one third full with hot water, place over black cross and measuer temp using
thermometer (should be around 60°C)
- Using a measuring cylinder put 10cm³ of 0.05moldm⁻³ sodium thiosulfate into boiling tube and heat
- add 1cm³ HCl to test tube and placein water bath, allow reactants to heat
- record temp of boiling tube and add acid to the sodium thiosulfate, look down through test tube and
record the time for the cross to dissapear from view
(ppt of sulfur forms) - record temp when reaction ends.
- repeat experiment at a lower temperature until 5 different temperatures have been looked at
PAPER 3
Acid-base titration - correct answer- pipette 25cm³ acid and add indicatir, place beaker on a magnetic
stirrer and place magnet in the beaker, fill burette with base
- rinse electrode with distilled water as readings may have changed over time, dip it into the acid in the
beaker, recording pH
-switch on stirrer and ensure magnet doesnt strike the electrode.
- add 5cm³ base and take readings of pH and indicator
- continue adding base taking 5cm³ readings each time then 1cm³ around end point, then 5cm³ util most
base has been added
- plot a graph of pH (Y axis) against volume of base added
Determining purity of the organic product (aspirin) - correct answermelting point of product formed can
be established by placing a small amount of the product in a capillary tube and heating the tube slowly
near the melting point. compare against databook value for the compound.
- pure sample has sharp melting point at the same point of the data book, if impurities are present, the
melting point will be lower and will melt over a range of several °C
* To increase purity, re-crystallise more than once
* to increase percentage yield, take more care in transfer process to reduce mechanical losses and use a
more pure starting product
distillation - correct answer
Examples of continuous monitoring methods - correct answergas volume, change in pH, loss of mass,
colour via colorimeter to see concentration changes
Examples of initial rate methods - correct answertime taken for pH to change, time taken for set vol of
gas to be evolved, time taken for change in colour to occur
, How can number of molecules of water of crystallisation be determined in a practical? - correct
answertitrate the aq solution, with known volume in a conical flask and acid in burette, repeat titration
until concordant results found.
Use added volume to work out moles using molar ratio.
calculate mr using known mass and number of moles found - then calculate 'x' using found mr - mr of
each molecule in substance, divided by 18 to find number of H2O molecules
How does a buffer maintain an approximately constant pH when a small amount of HCl is added? -
correct answerThe buffer neutralises the addition of acid (equilibrium shifts left/right dependent on
what is added)
identifying organic functional groups - correct answer1 - alkene test, add few drops of 0.01moldm3
bromine water, record any colour change (orange to colourless)
2 - primary/secondary alcohol. add 1cm³ acidified potassium dichromate and warm. positive test =
orange to green (alcohol is oxidised to an aldehyde)
3 - test for aldehydes, mix equal amounts of fehlings 1 & 2. add few drops to sample and warm gently,
positive test = blue to brick red (aldehyde oxidised to carboxylic acid)
4 - test for carboxylic acids, add Na₂CO₃ and warm, positive test = effervesence (CO₂ produced)
in terms of electrons, state what is happening at the negative electrode - correct answernegative
electrode is supplying the electrons to the solution which are transferred to the positive electrode. This
is where oxidation occurs, this metal is the more reactive metal.
Investigating how rate changes with temperature - correct answer- mark black cross on piece of white
paper
- fill 250cm³ beaker one third full with hot water, place over black cross and measuer temp using
thermometer (should be around 60°C)
- Using a measuring cylinder put 10cm³ of 0.05moldm⁻³ sodium thiosulfate into boiling tube and heat
- add 1cm³ HCl to test tube and placein water bath, allow reactants to heat
- record temp of boiling tube and add acid to the sodium thiosulfate, look down through test tube and
record the time for the cross to dissapear from view
(ppt of sulfur forms) - record temp when reaction ends.
- repeat experiment at a lower temperature until 5 different temperatures have been looked at
