6.2Unit 6. Practical Skills in Chemistry II

Advanced Inorganic & Organic — IAL Edexcel Modular (Chemistry) Revision Notes & Topical Questions

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6.2 Advanced Inorganic & Organic Chemistry Core Practicals — IAL EDEXCEL MODULAR (CHEMISTRY) notes: Carry out and analyse redox titrations, a complex preparation, qualitative analysis tests and the aspirin preparation.Reaction — MnO4- + 8H+ + 5Fe2+ → Mn2+ + 5Fe3+ + 4H2O, so the r…

Acidification
Starch timing
Dark blue colour
Role of ethanol
Percentage yield
Group 2 ions

Section 1 of 10Method and calculation for the iron titration

Weigh an iron(II) sample accurately and dissolve it in dilute sulfuric acid.

Make up to 250.0 cm³ in a volumetric flask and invert to mix.

Pipette 25.0 cm³ into a conical flask and add excess dilute H₂SO₄.

Titrate with standard KMnO₄ to the first permanent pale pink.

Repeat for concordant titres.

Calculate moles of MnO₄^- and multiply by 5 for moles of Fe²⁺.

Scale up to the whole sample and find the percentage of iron.

Uses
- finding the percentage purity of iron supplements and the formula of hydrated ethanedioic acid.
- Percentage purity = mass of pure substance / mass of impure sample x 100.

Section 2 of 10General sequence for redox titration calculations

Write the half-equations for the oxidant and the reductant.

Deduce the overall equation.

Calculate moles of manganate(VII) or dichromate(VI) used.

Use the equation to find the ratio of moles of oxidant to reductant.

Calculate moles of reductant in the sample solution.

Scale up to moles in the original solution.

Find the concentration of the original solution or the percentage of reductant in the sample.

Section 3 of 10Preparing a transition metal complex

Weigh 1.4-1.6 g of copper(II) sulfate by difference using a test tube.

Add 4 cm³ of water with a graduated pipette.

Dissolve the solid by standing the test tube in a hot water bath and stirring gently.

In a fume cupboard wearing gloves, add 2 cm³ of concentrated ammonia while stirring.

Pour the mixture into 6 cm³ of ethanol and cool in an ice bath.

Filter the crystals with a Buchner funnel, then rinse with cold ethanol.

Pat the crystals dry between filter papers.

Weigh the dry crystals.

Equation

CuSO₄.5H₂O + 4NH₃ → Cu(NH₃)₄SO₄.H₂O + 4H₂O.

Dark blue colour

Dark blue colour comes from the [Cu(NH₃)₄(H₂O)₂]²⁺ complex ion.

Role of ethanol

Role of ethanol it is less polar than water, so the ionic complex is less soluble and crystallises.
Cold ethanol is used for washing because the crystals dissolve in hot ethanol.

Section 4 of 10Yield, hazards and errors in the complex preparation

Percentage yield

Percentage yield actual mass / theoretical mass x 100, with a 1 : 1 mole ratio between CuSO₄.5H₂O and the product.
Values above 100% usually mean the crystals are not fully dry.

Losses

Losses the reaction may not go to completion, or product may stay in solution.

Gains

Gains impure or wet crystals.

Safety

Safety concentrated ammonia is corrosive and toxic, so it is handled in a fume cupboard with gloves.
Copper salts are harmful and environmentally dangerous.
Ethanol is flammable and kept away from flames.

Section 5 of 10Qualitative analysis of inorganic ions

Qualitative analysis of inorganic ions

Preparation

Preparation dissolve solids in deionised water.

Group 2 ions

Group 2 ions add NaOH dropwise to excess to compare Ca²⁺, Mg²⁺, Sr²⁺ and Ba²⁺, and repeat with ammonia and sulfuric acid.

Ammonium ions

Ammonium ions warm with NaOH(aq) in a water bath; ammonia turns damp red litmus paper blue.

Halide ions

Halide ions add dilute nitric acid and then silver nitrate.
White precipitate AgCl for chloride.
Cream precipitate AgBr for bromide.
Yellow precipitate AgI for iodide.
Ammonia solution then distinguishes the silver halides.

Hydroxide ions

Hydroxide ions turn red litmus paper blue and give a clearly alkaline pH on universal indicator paper.

Carbonate ions

Carbonate ions add dilute HCl; the CO₂ released turns limewater milky.

Sulfate ions

Sulfate ions acidify with dilute HCl and add aqueous BaCl₂; a white BaSO₄ precipitate forms.
HCl is added first to remove carbonates that would also give a precipitate.

Section 6 of 10Qualitative analysis of organic compounds

Test	Reagent	Positive result
Alkene (unsaturation)	bromine water	orange solution decolourises
Alcohol -OH	PCl₅ at room temperature	steamy fumes of HCl
Primary or secondary alcohol	acidified K₂Cr₂O₇	orange to green
Primary or secondary alcohol	acidified KMnO₄	purple to colourless
Aldehyde or ketone	2,4-DNPH	deep orange precipitate
Aldehyde only	Tollens' reagent, warmed	silver mirror
Aldehyde only	Fehling's or Benedict's, warmed	brick-red precipitate
Carboxylic acid	Na₂CO₃(s) or NaHCO₃(aq)	effervescence of CO₂

2,4-DNPH precipitate

2,4-DNPH precipitate can be purified by recrystallisation and its melting point compared with literature values to identify the carbonyl compound.

Tollens' reagent

Ag⁺ oxidises the aldehyde to a carboxylic acid and is reduced to silver.

Fehling's solution

Cu²⁺ oxidises the aldehyde and is reduced to Cu₂O.

Ketones

Ketones give no reaction with Tollens' or Fehling's reagents.

Section 7 of 10Aspirin preparation and purification

Add 6.0 g of salicylic acid (2-hydroxybenzoic acid), 10 cm³ of ethanoic anhydride and 5 drops of concentrated sulfuric acid to a conical flask.

Swirl and hold in a water bath at about 60 °C for about 20 minutes.

Cool, then add the contents to 75 cm³ of cold water so the aspirin crystallises.

Filter the crystals using Buchner filtration and leave to dry.

Recrystallise using ethanol as solvent: dissolve the solid, pour over 40 cm³ of cold water, and filter as before.

Reaction

Reaction salicylic acid reacts with ethanoic anhydride to give aspirin and ethanoic acid as the by-product.

Recrystallisation

Recrystallisation use the minimum hot solvent, hot-filter solid impurities, cool slowly for larger crystals, wash with cold solvent and dry.
Repeating improves purity but lowers yield.

Melting point

Melting point pure aspirin melts at about 135 °C.
Use dry, finely powdered sample, heat slowly, and repeat three times.
Impurities lower the melting point and broaden the range.
Quote a range (±) and compare with the data book value.

Section 8 of 10Key terms

Term	Meaning
Self-indicating	A titrant, such as KMnO₄, whose own colour change marks the end point.
Concordant titres	Titres that agree within 0.10 cm³.
Recrystallisation	Purification of a solid by dissolving it in hot solvent and cooling so the pure solid crystallises.
Buchner filtration	Filtration under reduced pressure, which is faster than gravity filtration.
Tollens' reagent	Ammoniacal silver nitrate, which gives a silver mirror with aldehydes.
Percentage yield	Actual mass of product divided by theoretical mass, multiplied by 100.

Section 9 of 10Syllabus checklist

Reaction

MnO₄^- + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O, so the ratio MnO₄^- : Fe²⁺ is 1 : 5.
Manganate(VII) is the oxidising agent and iron(II) the reducing agent.

Acidification

Acidification excess dilute sulfuric acid is added to the iron(II) before the titration.
Excess acid also prevents formation of brown MnO₂.
Dilute sulfuric acid does not react with manganate(VII) or get oxidised under these conditions.
Hydrochloric acid is unsuitable because Cl^- would be oxidised to Cl₂ by manganate(VII).

Indicator

Indicator potassium manganate(VII) is self-indicating.
Mn²⁺ is so dilute that the solution looks colourless.
The end point is the first permanent pale pink tinge from excess MnO₄^-.

Burette

Burette use one with white numbering because black numbers are hard to read behind purple solution.

Reaction

2S₂O₃^2- + I₂ → 2I^- + S₄O₆^2-, a 2 : 1 ratio.

Use

Use finds the concentration of an oxidising agent that oxidises iodide to iodine, such as chlorate(I) in bleach: ClO^- + 2I^- + 2H⁺ → Cl^- + I₂ + H₂O.

Add the oxidising agent to excess potassium iodide, acidified, to liberate iodine (brown solution).

Titrate with sodium thiosulfate until the solution is pale yellow (straw).

Add starch, which gives a blue-black colour.

Continue until the blue-black colour disappears.

Use the 2 : 1 ratio to calculate the moles of iodine and then of oxidising agent.

Starch timing
- add starch only near the end point, because too much iodine forms an insoluble complex with starch.

Section 10 of 10Exam technique

Using hydrochloric acid to acidify a manganate(VII) titration — Cl⁻ is oxidised to Cl₂, so dilute sulfuric acid is used.

Adding starch too early in the iodine-thiosulfate titration — an insoluble complex forms.

Washing the copper complex with hot ethanol — the crystals dissolve, so cold ethanol is used.

Adding acid after barium chloride in the sulfate test — acidify first to remove carbonates.

Using Tollens' or Fehling's reagent to test ketones — only aldehydes give a positive result.

Last updated: 13 October 2026

Section 1 of 10Method and calculation for the iron titration

Section 2 of 10General sequence for redox titration calculations

Section 3 of 10Preparing a transition metal complex

Section 4 of 10Yield, hazards and errors in the complex preparation

Section 5 of 10Qualitative analysis of inorganic ions

Section 6 of 10Qualitative analysis of organic compounds

Section 7 of 10Aspirin preparation and purification

Section 8 of 10Key terms

Section 9 of 10Syllabus checklist

Section 10 of 10Exam technique

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