practicals

• How to test if paracetamol is pure

  • find the melting point
  • Press the open end of the capillary tube onto the solid
  • Invert the tube and tap onto hard surface so the solid drops to the bottom of the tube
  • Use melting point apparatus
  • It should melt at a sharp, specific point
• Purifying paracetamol:

  • Add deionised water to the paracetamol sample in the conical flask and heat until the water is just below boiling point
  • Slowly add hot water until the solid has fully dissolved
  • Once all solid has dissolved allow flask to cool
  • Set up a vacuum filter
  • weigh a clean watch glass and record the weight
  • when sample is completely cool and no precipitate has formed, filter it
  • Wash with some cold water and leave under vacuum filtration to dry the product
  • Transfer to watch glass and dry
• How to make paracetamol:

  • Add 4-aminophenol to a round-bottomed flask and add deionised water using a measuring cylinder, swirl to mix
  • Place the flask onto the heating mantle and assemble the condensing apparatus
  • Use a measuring cylinder to add ethanoic anhydride to the flask and heat under reflux for 15 mins - mixture should become colourless
  • Allow the flask to cool to room temperature
  • Collect crystals using vacuum filtration
  • Wash the solid with ice-cold water and leave under suction to remove any ethanoic acid
  • Transfer the solid to a conical flask to recrystallise
• how to calibrate a pH meter:

  • Rinse the pH probe thoroughly with deionised water and shake it gently to remove the excess water
  • Place the probe in the standard pH 4 buffer solution ensuring the bulb is fully immersed
  • Repeat this process using the standard pH 7 and 9 buffer solutions
  • Rinse the pH probe thoroughly with deionised water before taking each reading
  • Record the pH readings in a table
• How to set up an electrochemical cell:

  • half fill 3 beakers with zinc sulphate, copper sulphate and magnesium sulphate solution
  • clean each metal with sandpaper and add to their respective solutions
  • make a salt bridge - submerge some filter paper in KNO3 solution
  • Rest one end of the filter paper in the zinc sulphate solution and the other end in the magnesium sulphate solution
  • connect the zinc strip to a wire then the wire to a voltmeter using crocodile clips and repeat for magnesium
  • record the voltage then repeat for the zinc/copper and magnesium/copper cells
• iodine clock experiment:

  • Use a burette to add hydrogen peroxide to a test tube
  • Pour the hydrogen peroxide into a conical flask containing iodine, start timing immediately and swirl to mix the contents
  • Record the time taken for the blue-black iodine complex to start to appear
  • Repeat for different concentrations
• test for aldehydes:

  • Prepare a sample of tollens by adding 5 drops of sodium hydroxide to 2cm3 of silver nitrate in a test tube
  • add just enough dilute ammonia to dissolve the brown ppt completely
  • Add 10 drops of aldehyde to the tollens in the test tube and warm gently
  • silver mirror should form
• Effect of temperature on rate:

  • Use a measuring cylinder to add sodium thiosulfate to a conical flask
  • Use a measuring cylinder to add HCl to a test tube
  • Place the conical flask onto a piece of paper marked with a cross, add the acid and start the stopwatch
  • Gently stir the mixture with the thermometer and record the initial temperature
  • Observe the cross from above and stop the stopwatch as soon as the cross disappears
  • record the final temperature of the mixture
• How to plot the graph for the enthalpy change of NaHSO4:

  • plot a graph of temperature (y axis) against time
  • join the points before the time of addition, then join the points after addition
  • Extrapolate both lines to the fourth minute - this shows temperature change
• enthalpy change for decomposition of sodium hydrogen carbonate:

  • Weigh 3g of anhydrous sodium carbonate and record the mass in a table
  • Use a pipette to add 25cm3 of 2M HCl into a clean polystyrene cup and place the cup inside a beaker
  • Put it under a tripod so that when a thermometer is added it is supported by the tripod
  • Stir and record the temperature every minute for 3 minutes
  • At 4 minutes add the anhydrous sodium carbonate to the cup and continue to stir and record the temperature every minute up to 10 minutes or until the temperature remains constant for 5 minutes
• titrating NaHSO4:

  • add 100 cm3 of the sodium hydrogen sulphate solution to a clean dry beaker. Use a small volume of NaHSO4 to rinse the burette before filling it with the same solution
  • Pour 100 cm3 of the sodium hydroxide solution into a second beaker
  • Rinse a 25 cm3 pipette with the sodium hydroxide solution and pipette exactly 25 cm3 of sodium hydroxide into a 250 cm3 conical flask
  • Add 2 to 3 drops of phenolphthalein indicator to the solution in the conical flask and record the colour of the indicator in alkali
• preparation of a volumetric solution e.g. sodium hydrogen sulphate:

  • measure the mass of the empty weighing boat using a mass balance
  • add 3g of NaHSO4 into the boat and record the weight
  • pour the contents of the weighing boat into a beaker and reweigh the weighing boat
  • add some deionised water to the beaker and stir until all the solid dissolves
  • using a funnel pour it into a volumetric flask
  • make the flask up to the graduated mark by adding deionised water from a wash bottle
  • add a stopper, shake and invert a minimum of 10 times
• testing for group 2 metals:

  • place 4 test tubes in a test tube rack
  • add 10 drops of barium chloride solution to the first test tube
  • add 10 drops of dilute NaOH to the same test tube and swirl
  • continue to add NaOH dropwise until in excess
  • repeat this for the other test tubes using calcium bromide, magnesium chloride and strontium chloride
  • it can also be done with ammonia solution and H2SO4
• positive results - group 2 metals:

  A) white precipitate

  B) no change

  C) no change

  D) no change

  E) white precipitate

  F) white precipitate

  G) white precipitate

  H) no change

  I) colourless solution

  J) white precipitate

  K) white precipitate

  L) white precipitate

  
• test for ammonium ions:

  • add 10 drops of an ammonium-containing solution to a clean test tube
  • add 10 drops of NaOH using a pipette and swirl
  • place the test tube in a beaker of water and put above a Bunsen Burner to heat gently
  • will produce fumes of ammonia gas
  • use a pair of tongs to hold damp red litmus paper at the mouth of the test tube
  • damp red litmus paper turns blue in presence of ammonia gas
• test for hydroxide ions:

  • use a pipette to add around 1cm3 of solution to a test tube
  • test the pH of the solution using red litmus paper - should turn blue in presence of hydroxide ions
• test for carbonate ions:

  • use a pipette to add 1cm3 of dilute HCl to a test tube
  • use a clean pipette to add 1cm3 of sodium carbonate solution to the test tube and put a bung with a delivery tube on the test tube straight away
  • the delivery tube transfers the gas produced into a different test tube containing some limewater - Ca(OH)2
  • carbonate ions react with H+ ions to produce CO2 which turns limewater cloudy
• test for sulphate ions:

  • add dilute HCl and a few drops of barium chloride solution to the sample
  • if sulphate ions are present, an insoluble white precipitate of barium sulphate is formed
• preparation of ethanal:

  A) heat

  B) lower boiling point

  C) distilled

  D) condensed

  E) Liebig condenser

  F) warm alcohol and acidified potassium dichromate

  
• oxidation of ethanol to ethanal:

  • set up the liebig condenser apparatus, putting anti-bumping granules into the round-bottom flask
  • use a measuring cylinder to add 15cm3 of acidified potassium dichromate into a round bottom flask
  • measure 5cm3 of ethanol using a measuring cylinder and add the ethanol to the flask slowly using a pipette
  • shake gently to mix
  • heat gently to distil approximately 5cm3 of aldehyde into a test tube immersed in a beaker of cold water
• test for alcohol:

  • add some acidified potassium dichromate to a test tube containing alcohol sample
  • gently mix
  • place the test tube in a hot water bath to warm
  • positive result - solution turns from orange to green
• measuring rate by continuous monitoring:

  • measure HCl and add to a conical flask
  • place an empty weighing boat onto a mass balance and set to zero, weigh 5g of calcium carbonate into the weighing boat
  • remove the weighing boat from the balance and set to zero
  • place both the weighing boat containing CaCO3 and the conical flask onto the balance and record the mass
  • add the calcium carbonate to the conical flask and start the timer
  • record the mass every 15 seconds for 3 minutes
• investigating pH changes:

  • rinse a burette with ethanoic acid then fill it with the same solution and add 20cm3 to a clean beaker
  • rinse a second burette with sodium hydroxide solution then fill it with the same solution
  • rinse the pH probe with deionised water and clamp so the bulb is fully immersed in the beaker
  • stir and record the pH reading in a table
  • use the second burette to add 2cm3 of NaOH to the beaker of ethanoic acid. stir gently then measure and record the pH
  • add the NaOH in 2cm3 portions to the beaker until 40cm3 has been added - take a pH reading after each addition
• iodide ions are oxidised to iodine by hydrogen peroxide in acidic conditions. In an experiment to determine the order with respect to [H+] a reaction mixture is made containing H+ with a concentration of 0.5 mol dm-3. samples of the reaction mixture are removed at timed intervals and titrated with alkali to determine [H+] - state and explain what must be done to each sample before titration


  • stop the reaction
  • by dilution/cooling/adding a reagent to react with H2O2 or I-
• enthalpy change:

  • place 25cm3 using a measuring cylinder of solution into a polystyrene cup
  • weigh x amount of solid - is an excess so it doesnt need to be accurate
  • draw a table to record initial temperature and then temperature and time every 30s up to 9.5 minutes
  • put a thermometer into the cup, stir and record the temperature every 30 seconds for 2.5 minutes
  • at 3 minutes add the solid to the cup - DO NOT record the temperature
  • continue stirring and record temp for an extra 6 minutes

  A) thermometer

  B) lid

  C) polystyrene cup

  
• enthalpy change pt2:

  • plot a graph of temperature against time
  • draw two lines of best fit
  • extrapolate each line to the 4th minute
• enthalpy of combustion:

  • fill copper can with 100cm3 of water and record the initial temperature with a thermometer
  • measure and record the mass of an empty spirit burner, add fuel and record mass
  • light the spirit burner, heat water whilst stirring continuously for a set time
  • record the temperature of the water
  • measure mass of burner and any remaining fuel and subtract from initial mass to find mass of fuel burnt

  A) lid

  B) thermometer

  C) water

  D) copper can

  E) spirit burner

  
• how to reduce percentage error in titrations

  • use a lower concentration of alkali (solution in the burette)
  • leads to a higher volume
  • higher number on the bottom of the % uncertainty equation leads to a smaller error
• describe what happens when a reaction mixture is refluxed and why it is necessary

  1. reaction mixture is heated to boil for a prolonged period of time
  2. vapour is formed which escapes from liquid mixture, condenses and returns to the liquid
  3. any reactant that initially evaporates is condensed and can react so the reaction is fully complete
• determining the order of reaction

  • measure known volumes of some reagents and measure known amount of X into two separate beakers, add A, B, C last
  • start timer when both substances are mixed together
  • record the time taken for the blue colour to appear/when the cross disappears
  • same concentrations and volumes of B and C, same volume/amount of X
  • same temperature - use a water bath
  • repeat with different concentrations of A
  • calculate rate by doing 1/time taken
  • plot graph of 1/T against concentration of A
  • interpret order of reaction from the shape of the graph
• describe a test to distinguish between sodium oxide and P4O10

  1. react with water
  2. add litmus paper OR measure pH with pH meter
  3. litmus paper turns blue with Na2O and red with P4O10
  4. sodium hydroxide has a pH of 12, P4O10 has a pH of 0
• anhydrous MgCl2 can absorb water to form the hydrated salt. Suggest one reason why the enthalpy change for this reaction can't be determined directly by calorimetry

  • some solid will dissolve as it is soluble/absorbs water so the mass will change
• enthalpy of solution

  • weigh an empty weighing boat using a mass balance and record the mass
  • measure x amount of solid using the mass balance and weighing boat, reweigh the weighing boat + solid and record mass
  • measure 25cm3 water in a measuring cylinder and add to a clean polystyrene cup
  • put the cup in a large beaker, add a thermometer and measure and record the temperature every minute for 3 minutes
  • at the fourth minute add the solid to the cup
  • measure the temperature from the 5th minute every minute until 12 minutes
  • plot a graph of temp against time, draw two lines of best fit and extrapolate to the 4th minute
  • equations
• state why reflux is used in hydrolysis

  • allows reactant vapours to return to the reaction mixture
  • doesn't allow any reactant vapour to escape the mixture
• describe the method the student should use to purify benzoic acid

  • wash it with a MINIMUM VOLUME of hot solvent to dissolve
  • filter it hot using filter paper and a funnel to remove insoluble impurities
  • leave to cool and recrystallise
  • filter under REDUCED PRESSURE using a Buchner funnel and pump
  • wash with cold solvent
  • leave to dry on a watch glass in a warm, dry place