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3.1Unit 3. Practical Skills in Chemistry I

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

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3.1 Physical Chemistry Core Practicals — molar volume, enthalpy change, titration and standard solutions — IAL EDEXCEL LINEAR (CHEMISTRY) notes: Describe and analyse Core Practicals 1 to 4: molar volume of a gas, enthalpy change of reaction, acid concentration by titration and pr…

  • Molar gas volume
  • Energy transferred
  • What the symbols mean
  • Gas syringe
  • Volumetric flask
  • Concordant titres

Section 1 of 4More from the specification

More from the specification
Carbonyl organic functional group
  • Carbonyl organic functional group nucleophilic addition of cyanide across the polar carbonyl group forms cyanohydrins in synthetic pathway reactions.
Determining Enthalpy of Reaction by Calorimetry
  • Heat energy transferred during neutralisation or dissolution is determined using q = mc delta_T,
  • where m is solution mass, c is the specific heat capacity (4.18 J g−1 K−1),
  • and delta_T is the corrected temperature change.
Cooling Curve and Temperature Correction
  • Graphing temperature against time before and after reagents are added allows extrapolation back to the moment of mixing, compensating for thermal losses to the surroundings.
Gas Collection of Carbon Dioxide
  • Reacting carbonates with excess acid generates carbon dioxide gas, whose volume is captured in a gas syringe to calculate molar volume and formula mass.
Experimental Uncertainty in Titrimetric Results
  • Concordant titration results within 0.10 cm3 are averaged, factoring in graduated glassware tolerances to minimise percentage uncertainties in determined concentrations.
Carbon dioxide
  • A gas that turns limewater cloudy.
Temperature correction
  • Extrapolating a temperature-time graph back to the mixing time to estimate the true temperature change.
Concordant results
  • Titres within 0.1 cm³.

Section 2 of 4Key terms

TermMeaning
Molar gas volumeThe volume occupied by one mole of gas under stated conditions, about 24.0 dm3 at room temperature and pressure.
Gas syringeA calibrated syringe used to measure the volume of a gas produced.
Standard solutionA solution whose concentration is known accurately.
Volumetric flaskA flask calibrated to hold one exact volume.
Concordant titresTitres that agree within 0.10 cm3.
Percentage uncertaintyThe absolute uncertainty divided by the measured value, multiplied by 100.
Temperature correctionExtrapolating a temperature-time graph back to the mixing time to estimate the true temperature change.

Section 3 of 4Syllabus checklist

  • The gas produced can be collected in a gas syringe or by displacement of water.
MethodAdvantageLimitation
Gas syringeDirect, accurate reading (±0.5 cm3)Limited to about 100 cm3; plunger can stick
Inverted measuring cylinder over waterCheap, larger volumesSoluble gases such as CO2 dissolve, giving a low reading
Inverted burette over waterPrecise for small volumesSmall capacity
Mass loss on a balanceEasy continuous readingsPoor for light gases such as H2
  • Read the syringe when the plunger stops moving and the apparatus has returned to room temperature.
1
Measure a fixed volume of hydrochloric acid, e.g. 25.0 cm3, into a conical flask.
2
Add a known mass of sodium carbonate (or CaCO3, or Mg).
3
Immediately connect the gas syringe delivery tube.
4
Allow the reaction to go to completion and record the volume of CO2.
5
Repeat with different masses.
Na2CO3 + 2HCl → 2NaCl + H2O + CO2: one mole of carbonate gives one mole of CO2.
Assumptions
  • Negligible gas is lost before the tube is connected.
  • The apparatus is airtight.
  • The reaction goes to completion.
Plot mass of carbonate (x) against gas volume (y), ignore anomalies and draw one line of best fit.
Molar gas volume
  • Molar gas volume volume of gas (dm3) ÷ moles of gas, with cm3 converted to dm3;
    • about 24.0 dm3 mol−1 at room temperature and pressure.
  • A low value usually means gas escaped before the bung was inserted or dissolved in the liquid.
The method identifies an unknown metal M in MCO3: moles of CO2 = moles of MCO3, so Mr = mass ÷ moles, then Ar(M) = Mr − 60.
1
Hazards depend on the type and concentration of the acid.
2
Dilute acids are irritants or have no symbol, so wear eye protection.
3
Moderately concentrated acids are corrosive, so also wear gloves.
4
Some concentrated acids, e.g. nitric acid, are oxidising and can intensify fires; use eye protection, gloves and often a fume cupboard.
Calorimetry measures enthalpy changes using a polystyrene cup, a vacuum flask or a metal can.
Energy transferred
  • Energy transferred q = mcΔT, with c = 4.18 J g−1 K−1.
    What the symbols mean: E, Q or q = energy transferred (J) · m = mass · c = specific heat capacity (J kg−1 °C−1; water 4.18 J g−1 °C−1) · ΔT or Δθ = temperature change (°C or K)
    • (kg; g in chemistry calorimetry)
  • Convert q from J to kJ.
  • ΔH = −q ÷ n, with n the moles of the limiting reagent (rise in temperature gives a negative ΔH).
    What the symbols mean: ΔH = enthalpy change (kJ mol−1) · q = heat energy measured (J, divide by 1000 for kJ) · n = amount of the limiting reactant (mol); negative sign = exothermic
Sample method for zinc and copper(II) sulfate
  • Put 25 cm3 of 1.0 mol dm−3 CuSO4 into the polystyrene cup.
  • Weigh about 6 g of zinc powder, an excess.
  • Record the temperature every half minute for 2.5 minutes while stirring.
  • At exactly 3 minutes add the zinc, without recording that reading.
  • Keep stirring and record for a further 6 minutes.
Assumptions
  • The solution has the heat capacity (4.18 J g−1 K−1) and density (1 g cm−3) of water.
  • The heat capacity of the container is ignored.
  • The reaction is complete and heat loss is negligible.
  • A slow reaction loses heat before the maximum temperature, so a graph is used.
1
Record the temperature for a few minutes before mixing.
2
Add the second reagent and keep recording.
3
Extrapolate the cooling line back to the time of mixing.
4
Read ΔT from the two extrapolated lines.
ErrorEffectImprovement
Heat loss to surroundingsΔT too small, ΔH less negativeInsulated polystyrene cup with lid; extrapolate
Heat capacity of cup ignoredUnderestimates heat releasedUse polystyrene, low heat capacity
Incomplete combustionLess energy releasedGood air supply, shield the flame
Evaporation of fuelMass burnt too highReplace the cap on the spirit burner quickly
  • Hess's law experiments combine two measured reactions;
    • combustion enthalpies use a spirit burner and copper calorimeter.
  • Burettes are marked to 0.10 cm3;
    • the uncertainty of one reading is ±0.05 cm3, and a titre is ±0.10 cm3 because two readings are made.
1
Rinse the burette with the solution it will contain and the pipette with the solution it delivers; rinse the conical flask with distilled water only.
2
Pipette 20 or 25 cm3 of one solution into a conical flask and add a few drops of indicator.
3
Fill the burette, usually to 0.00 cm3, and place a white tile under the flask.
4
Add the solution portion by portion, swirling, then dropwise near the end point so one drop causes the colour change.
5
Repeat until titres are concordant (within 0.1 cm3).
Core Practical 3: titration to find an acid concentration
Record the initial and final readings to ±0.05 cm3, average only concordant titres and discard others.
Calculate the moles of the known solution, use the stoichiometry, then divide by the volume (dm3); e.g. NaHCO3 + HCl is 1:1.
The pipette (25.0 cm3) has an uncertainty of about ±0.06 cm3 and a 250 cm3 volumetric flask about ±0.2 cm3.
Core Practical 4: preparing a standard solution
A standard (volumetric) solution has an accurately known concentration.
Concentration is expressed as moles per unit volume, mass per unit volume or parts per million.
Calculate the mass needed: moles = concentration × volume (dm3); mass = moles × Mr.
1
Weigh the solid accurately by difference on a three-decimal-place balance.
2
Dissolve it in a beaker with distilled water.
3
Transfer the solution and all washings into a volumetric flask.
4
Add distilled water until the bottom of the meniscus is on the mark.
5
Invert the flask several times to mix.
  • Apparatusbeaker, burette, volumetric pipette, conical flask and volumetric flask.

Section 4 of 4Exam technique

1
Using a measuring cylinder instead of a volumetric pipette — a pipette is more precise.
2
Rinsing the pipette with distilled water only — rinse with the solution it will deliver.
3
Averaging non-concordant titres — use only concordant results.
4
Forgetting to double the burette reading uncertainty for a titre — two readings are made.
5
Making up a standard solution above the mark — the bottom of the meniscus must sit on the line.

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