Required Practicals

• What is a reading?

  A value found from a single measurement
• What is a measurement?

  Values taken a the difference taken between two values
• What can increase uncertainty?

  • Human error- such as using a stopwatch
  • Resolution of equipment- such as using a measuring cylinder instead of a pipette
  • Air bubbles in a titration
  • Using smaller volumes
  • Using a lower concentration
• How can we improve the calorimetry experiment?

  • Use a lid
  • Use a burette instead of a measuring cylinder
  • Use a polystyrene cup rather than a glass beaker
  • Measure the temperature of the solution at regular intervals before the addition of another substance to ensure the solution is at a constant temperature
  • Measure the temperature of the solution after addition of another substance at regular intervals for at least 10 minutes
  • Plot a graph of time against temperature and extrapolate the lines of best fit
  • Extrapolate to the point of addition to determine temperature change at the point of addition
• Give two reasons why an experimental value may be smaller than the data book in calorimetry.

  • Heat loss
  • Incomplete combustion
• Give a reason why the calculated bond enthalpy for the N-H bond may be different than the average bond enthalpy.

  Different compounds contain N-H bonds other than NH3 as it is an average value
• Acidified potassium dichromate and primary alcohol?

  Aldehyde initially and then oxidised again to carboxylic acid
• Acidified potassium dichromate and secondary alcohol?

  Ketone, cannot be oxidised further
• Conditions for acidified potassium dichromate and secondary alcohol?

  Conc sulphuric acid and reflux
• Conditions or acidified potassium dichromate and primary alcohol to carboxylic acid?

  Concentrated sulphuric acid and reflux
• How do we represent acidified potassium dichromate in reflux?

  2[O]
• Conditions for acidified potassium dichromate and primary alcohol to aldehyde?

  Dilute sulphuric acid, less acidified potassium dichromte and distillation
• Why do we cool the distillate in distillation of primary alcohols?

  To prevent further oxidation of aldehyde to carboxylic acid
• What can we to after refluxing primary alcohols to obtain pure sample?

  Distill to obtain carboxylic acid
• An experiment measuring the concentration of cisplatin is measured at one-minute intervals. Explain how graphical methods can be used to process the measured results, to confirm that the reaction is first order.

  • Plot concentration against time and take tangents of the graph
  • Work out gradients to calculate rates
  • Plot a graph of rate against concentration
  • Straight line through the origin indicates first order
• . A(aq) + B(aq) + C(aq) → D(aq) + E(aq) In aqueous solution, A, B, C and D are all colourless but E is dark blue. A reagent (X) is available that reacts rapidly with E. Explain how you could use a series of experiments to determine the order of this reaction with respect to A. In each experiment you should obtain a measure of the initial rate of reaction.

  • Measure known volumes of some reagents
  • Measure known volumes of X
  • Into separate containers
  • Start clock at time of mixing
  • Record time for blue colour to appear using a disappearing cross
  • Use same concentration of B and C and same volume of X
  • Same temperature, controlled using a water bath
  • Repeat with different concentrations of A
  • 1/t is measure of rate
  • Plot 1/t against concentration of A
  • Interpret order from shape of graph
• Explain the continuous monitoring method for the disappearing cross experiment.

  • Measure known volumes of A, B and C
  • Use a colorimeter
  • Put A, B and C in separate containers
  • Start clock at time of mixing
  • Take a series of colorimeter readings at regular intervals
  • Use same volume and concentration of B and C and same temperature and repeat at different concentrations of A
  • Plot absorbance against time and calculate gradient at 0
  • Plot gradient against concentration of A
  • Interpret order from shape of graph
• Why does water go in at the bottom in reflux?

  Water will fill the condenser so the condenser is cool enough, it will be effective at cooling
• Why do we add sodium carbonate in reflux?

  Neutralise and remove acid
• Why is there a build up of pressure in the separating funnel?

  Carbon dioxide gas is produced
• Why do two layers form in the separating funnel?

  Esters are insoluble and immiscible
• Why is the organic solvent the upper layer?

  Lower density than water
• Why do we add anhydrous calcium chloride?

  Drying agent, remove excess water
• Why may percentage yield not be 100%?

  Losses during transfer, reaction is reversible, incomplete distillation, side reaction, impurities
• Why is effervescence seen with Fe2+ and carbonate but not Fe3+?

  • Fe3+ is more acidic than Fe2+
  • Fe3+ is smaller and more polarising than Fe2+ with a higher charge density
• how would you carry out an experiment to work out the enthalpy of solution of magnesium chloride?

  • Measure water using a measuring cylinder of 200cm3
  • Use a polystyrene cup
  • Measure a mass of MgCl2 into a weighing boat and add, weigh weighing boat after to determine mass of MgCl2 added
  • Measure initial temperature for 5 minutes before added solid at minute intervals
  • Add solid and record temperature at regular intervals every minute for 10 minutes
  • Plot a graph of temperature against time
  • Extrapolate lines to when the solid was added and work out temperature change
  • q=mc delta T
  • Work out mols added using mass/mr
  • Solution enthalpy = q/mols
• On a Gibbs free energy graph of delta g against T, what is the gradient equal to?

  negative delta S
• How do anti-bumping granules prevent bumping in reflux?

  Prevent formation of large bubbles
• Why do we use an excess?

  To ensure all reactant has fully reacted
• Why do we use reflux?

  Allows reactant vapours to re-condense back into the mixture and does not allow any to escape
• Describe how we purify organic solid.

  • Dissolve organic solid in minimum volume of hot solvent
  • Filter
  • Cool to recrystallise
  • Filter under reduced pressure with Buchner funnel
  • Wash with cold distilled water and dry
• Why do we use the minimum volume of hot solvent?

  To ensure the solution is saturated and to dissolve the compound
• Why do we filter hot?

  To remove insoluble impurities
• How to set up and use a Buchner funnel.

  • Flat-bottomed funnel containing filter paper
  • Air-tight connection between flask and funnel
  • Side-arm flask connected to vacuum pump
  • Insert filter paper and wet it with solvent
  • Add organic crystals solution and turn on
• Why do we wash crystals with distilled water?

  To remove any impurities
• A sample of the highest oxide of phosphorus was prepared in a laboratory. Describe a method for determining the melting point of the sample. State how the result obtained could be used to evaluate its purity.

  • Put sample in a melting point apparatus
  • Heat gradually
  • A lower melting point than data book value or a broad range of melting points indicates presence of impurities
• In separation, why do we use sodium carbonate to wash the distillate?

  To neutralise the acid catalyst
• Why do we have to open the tap periodically in separation?

  Avoid pressure build up
• What makes a good drying agent?

  Absorbs water and does not dissolve or react with the distillate
• Diagram of a vacuum pump

  Filter paper, buchner funnel, side arm flask to vacuum pump, bung