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6.1Unit 6. Practical Skills in Chemistry II

Advanced Physical Chemistry Core Practicals — IAL Edexcel Modular (Chemistry) Revision Notes & Topical Questions

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📚 IAL Edexcel Modular (Chemistry) Hub / Topical Revision
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6.1 Advanced Physical Chemistry Core Practicals — IAL EDEXCEL MODULAR (CHEMISTRY) notes: Describe the titrimetric, clock, activation energy, Ka and electrochemical cell core practicals and analyse their results.Aim — find how [I2] affects the rate of the acid-catalysed reaction C…

  • Sampling and quenching
  • Practical tips
  • Rate equation
  • Clock reaction
  • Hydrogen peroxide concentration
  • Calculation

Section 1 of 6Analysing the iodine titration results

Graph
  • Graph plot titre (proportional to [I2]) on the y-axis against time.
  • A straight line with a negative gradient means a constant rate.
  • The rate is independent of [I2], so the reaction is zero order with respect to iodine.
Rate equation
  • Rate equation the reaction is first order in propanone and in H+, so rate = k[CH3COCH3][H+].
  • [I2] does not appear because its order is zero.

Section 2 of 6EMF calculation and analysis

1Theoretical EMF
  • E°cell = E° - E°(negative electrode), that is the most positive E° minus the most negative E°.
    • (positive electrode)
  • The more positive (less negative) half-cell is the positive electrode.
2Comparing with measurements
  • Comparing with measurements values are unlikely to match exactly because standard conditions (1.0 mol dm-3, 298 K) are hard to achieve.
  • The relative EMFs of the cells should still match the theoretical order.
  • A higher EMF means a larger difference in reactivity between the two metals.
3Standard conditions
  • 298 K, 1.00 mol dm-3 solutions and 100 kPa for gases.
4Half-cells
  • Half-cells metal / metal ion half-cells use the metal as electrode and ion / ion or gas half-cells use platinum.

Section 3 of 6Titration technique and errors

Equipment
  • Equipment use pipettes and burettes for precision and record burette readings to ±0.05 cm3.
Concordant titres
  • Concordant titres repeat until titres agree within 0.10 cm3.
Percentage uncertainty
  • Percentage uncertainty x 100.
    • (uncertainty / measured value)
Burette readings
  • Burette readings two readings are taken, so the absolute uncertainty is doubled.
Reducing uncertainty
  • Reducing uncertainty a larger titre gives a smaller percentage uncertainty.
Starch
  • Starch add it only near the end point, when the iodine is pale straw, because too much iodine forms an insoluble complex.

Section 4 of 6Key terms

TermMeaning
QuenchingStopping the reaction in a withdrawn sample, for example by adding sodium hydrogencarbonate, so it can be titrated.
Clock reactionA reaction with a sudden visible change after a fixed amount of product forms, where 1/t is proportional to rate.
Half-neutralisationThe point where half of a weak acid has been neutralised, so [HA] = [A-] and pH = pKa.
Salt bridgeA strip soaked in saturated KNO3 that completes the circuit by letting ions move.
Concordant titresTitres that agree within 0.10 cm3.
Zero orderA reactant whose concentration has no effect on the rate.

Section 5 of 6Syllabus checklist

Aim
  • Aim find how [I2] affects the rate of the acid-catalysed reaction CH3COCH3 + I2 → CH3COCH2I + H+ + I-.
Design
  • Design propanone and the acid are in large excess, so their concentrations stay effectively constant.
Sampling and quenching
  • Sampling and quenching the iodine concentration is found at different times by withdrawing samples and titrating them.
1
Mix 25 cm3 of 1.0 mol dm-3 propanone with 25 cm3 of 1.0 mol dm-3 sulfuric acid in a beaker.
2
Add 50 cm3 of 0.02 mol dm-3 iodine solution and start the timer immediately.
3
At regular intervals pipette out a 10 cm3 sample into a conical flask.
4
Quench the sample by adding a spatula of sodium hydrogencarbonate, which removes the acid catalyst.
5
Titrate with 0.01 mol dm-3 sodium thiosulfate using starch as indicator: 2S2O32- + I2 → 2I- + S4O62-.
6
Record time and titre in a table.
  • Practical tips
    • set everything up first and keep the timer running throughout.
    • Record the time at which the hydrogencarbonate is added, because that is when the reaction stops.
Core practical 9b: iodine clock reaction
Clock reaction
  • Clock reaction a reaction that shows a sudden, sharp colour change after a period of time.
  • Starch gives a sudden blue-black colour when iodine appears.
Reaction
  • H2O2 + 2I- + 2H+ → I2 + 2H2O.
  • Adding sodium thiosulfate removes the iodine as it forms and acts as the timer: 2S2O32- + I2 → 2I- + S4O62-.
  • When the thiosulfate is used up, iodine accumulates and the blue-black colour appears.
Procedure
  • Procedure measure solutions in burettes and place them in a small beaker.
  • Sulfuric acid is in excess, so a measuring cylinder is enough.
  • Start the reaction by adding 1 cm3 of 0.25 mol dm-3 hydrogen peroxide and start the timer.
  • Stop the timer when the blue-black colour appears and vary one concentration at a time.
Hydrogen peroxide concentration
  • '10 vol' (3%) gives 10 cm3 of oxygen per cm3 and is about 0.979 mol dm-3.
Analysis
  • Analysis rate is proportional to 1/time.
  • Plot rate against concentration.
  • A straight line through the origin shows the reaction is first order with respect to iodide.
  • Reaction
    • BrO3- + 5Br- + 6H+ → 3Br2 + 3H2O, with bromine removed by phenol: C6H5OH + 3Br2 → C6H2Br3OH + 3HBr.
    • When the phenol is used up, bromine bleaches the indicator (methyl red or methyl orange), marking the end of the reaction.
1
Pipette 10.0 cm3 of phenol solution and 10.0 cm3 of bromide / bromate solution into one boiling tube.
2
Add four drops of methyl red indicator.
3
Pipette 5.0 cm3 of sulfuric acid into a separate boiling tube.
4
Prepare a water bath at 37 °C (±1 °C) and stand both tubes in it.
5
When both are at the bath temperature, mix them by pouring from one tube to the other and back, and start the clock.
6
Leave the mixture in the water bath and time until the methyl red disappears.
7
Repeat at different temperatures, using ice for the lowest.
  • Analysis
    • rate is proportional to 1/t.
    • Plot ln(1/t) against 1/T with T in kelvin.
    • The gradient is -Ea/R, so Ea = -gradient x 8.31 J mol-1 K-1.
  • Principle
    • at the half-equivalence point [HA] = [A-], so Ka = [H+] and pKa = pH.
1
Calibrate the pH probe or meter.
2
Pipette 25 cm3 of 0.1 mol dm-3 ethanoic acid into a conical flask with phenolphthalein.
3
Titrate with 0.1 mol dm-3 sodium hydroxide until the indicator just turns pink.
4
Add a further 25 cm3 portion of acid to the neutralised solution, which makes it a half-neutralised sample.
5
Measure the pH.
Calculation
  • [H+] = 10-pH, which equals Ka.
Uncertainty
  • Uncertainty comes from pipette and burette measurements and judging the end point.
  • A pipette has a single reading, while a burette has two readings (initial and final), doubling the uncertainty.
  • Equipment
    • two small beakers, strips of metals such as Cu, Zn, Fe and Ag, 1.0 mol dm-3 solutions of their ions, a high-resistance voltmeter, two wires with crocodile clips and a salt bridge.
    • The salt bridge is filter paper soaked in saturated potassium nitrate.
1
Clean the metal strips with sandpaper to remove oxide coatings.
2
Fill each beaker about two thirds full with a metal ion solution and fold a metal strip over the side, held with a crocodile clip.
3
Dip a strip of filter paper in saturated KNO3 using tongs and place it between the beakers with both ends immersed.
4
Connect the clips to the voltmeter, wait for a steady reading and record it.
  • Practical tips
    • if the reading is negative, swap the terminals, which shows the relative polarity of the metals.
    • Use a fresh salt bridge each time to avoid cross-contamination.

Section 6 of 6Exam technique

1
Stopping the clock when the quench sample is titrated — the time to record is when the sodium hydrogencarbonate is added.
2
Using temperatures in °C for the Arrhenius plot — T must be in kelvin.
3
Reading the ln(1/t) gradient as +Ea/R — it is -Ea/R.
4
Taking the pH at the end point as pKa — pKa equals the pH at half-neutralisation.
5
Using the same salt bridge for every cell — a fresh strip prevents ions crossing between half-cells.

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