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4.1AS Practical Skills

Physical Chemistry Practicals — AQA A Level Chemistry Revision Notes & Topical Questions

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📚 AQA A Level Chemistry Hub / Topical Revision
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Physical Chemistry Practicals: Titration, Volumetric Solutions and Enthalpy Measurement — AQA A LEVEL CHEMISTRY — COMPLETE NOTES notes: Prepare a standard solution, carry out an accurate titration and plan an enthalpy-change measurement.Volumetric analysis is a process that uses …

  • Enthalpy change of reaction
  • Factors affecting rate
  • Worked example 1
  • Worked example 2
  • Worked example 3
  • Concordant titres

Part 1 of 4Making a volumetric solution

Volumetric analysis is a process that uses the volume and concentration of one chemical reactant to determine the concentration of another unknown solution.

Making a volumetric solution
To make a standard solution: weigh the solid accurately by difference (weigh the container, transfer, reweigh), dissolve in a beaker with distilled water, transfer to a volumetric flask using a funnel, rinse the beaker and funnel into the flask, make up to the mark with distilled water (bottom of meniscus on the line, dropper for the last drops), stopper and invert several times to mix.
Calculate concentration c = n ÷ V.

Part 2 of 4Performing a titration

1
Rinse the pipette with the solution it will deliver and the burette with the titrant; rinse the conical flask with distilled water (this does not change the moles).
2
Pipette a known volume (e.g. 25.0 cm3) into the flask and add a few drops of indicator.
3
Fill the burette (below the tap too) and record the initial volume to 0.05 cm3.
4
Do a rough titration, then repeat, adding dropwise near the end point, swirling.
5
Repeat until two concordant titres are obtained (within 0.10 cm3) and average them.

Indicators

  • methyl orange (red in acid, yellow in alkali;
    • for strong acid-weak base) and phenolphthalein (colourless in acid, pink in alkali;
    • for weak acid-strong base).
  • Calculate: moles of known → moles of unknown using the equation → concentration.
  • Uncertainty = (reading error × number of readings) ÷ value × 100.

Part 3 of 4Measuring an enthalpy change and rate

Enthalpy change of reaction

  • use a polystyrene cup with lid;
    • record temperature for a few minutes before mixing, then at regular intervals after;
    • plot temperature against time and extrapolate back to the time of mixing to find ΔT, correcting for heat loss.
  • Use q = mcΔT and ΔH = -q ÷ n.

Factors affecting rate for the reaction of sodium thiosulfate with hydrochloric acid, the time for a cross under the flask to disappear (sulfur precipitate) is measured.

Measuring an enthalpy change and rate
Vary one factor (concentration or temperature) at a time and control the others; rate ∝ 1 ÷ time.
For magnesium with acid, the volume of hydrogen collected in a gas syringe over time is recorded and the gradient of the curve gives the rate.

Part 4 of 4Worked examples

  1. Describe how to make 250 cm3 of a standard solution from a solid. [4]4 marks

  2. Titres of 24.30, 24.15, 24.10 and 24.20 cm3 were recorded. Give the mean titre. [2]2 marks

  3. Why is a graph extrapolated back to the time of mixing in calorimetry? [2]2 marks

Key terms

TermMeaning
Volumetric analysisAnalysis based on measuring volumes of solutions of known concentration.
Standard solutionA solution of accurately known concentration.
Concordant titresTitres that agree within 0.10 cm3.
End pointThe point at which the indicator changes colour.
MeniscusThe curved surface of a liquid in a tube.

Avoid these mistakes

1
Rinsing the pipette or burette with water instead of the solution used.
2
Including the rough titre in the mean.
3
Reading the burette from the top of the meniscus.
4
Rinsing the conical flask with the solution (changes the moles).
5
Not varying one factor at a time in rate experiments.

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