5.6.6Unit 5. Transition Metals and Organic Nitrogen Chemistry
Organic Techniques – Purity — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 6 parts Lesson 43 of 43 Unit quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 6Melting Point Testing
Melting range is a quick check of purity for solids.
Pure solid
- Pure solid A pure crystalline solid melts sharply over a narrow range of about 1 °C at a fixed temperature.
Impure solid
- Impure solid Impurities lower the melting point and make the solid melt over a wide range of temperatures.
Method
- Method A small sample in a capillary tube is heated slowly in an oil bath or melting point apparatus while the range is recorded.
Identification
- Identification The measured range can be compared with a data book value to help identify the compound.
| Sample | Melting range | Interpretation |
|---|---|---|
| Pure | Narrow, close to data book value | High purity |
| Slightly impure | Lower and wider | Contaminated |
| Mixture | Very wide | Little product |
Part 2 of 6Boiling Point and Chromatography
Boiling Point and Chromatography
Boiling point
- Boiling point A pure liquid boils at a fixed temperature at a given pressure, while a mixture boils over a range.
Thin-layer chromatography
- Thin-layer chromatography A spot of the sample is placed on a coated plate and a solvent carries different components different distances.
One spot
- One spot A single spot suggests the sample is pure, though a second compound could have an identical Rf value.
Rf comparison
- Rf comparison Comparing Rf values with a pure standard run on the same plate helps confirm the identity.
Part 3 of 6Running a TLC Plate
1
Draw a pencil line near the bottom of the plate and mark the positions where each sample will be spotted.
2
Spot small amounts of the product and any reference sample on the line using a capillary tube.
3
Place the plate upright in a tank with the solvent level below the pencil line and cover with a lid.
4
Remove the plate when the solvent front is near the top and mark the front at once with a pencil.
5
Visualise the spots with ultraviolet light or a developing agent and calculate Rf = distance moved by spot ÷ distance moved by solvent front.
Part 4 of 6Evaluating Purity
Spectroscopy
- Spectroscopy An IR or NMR spectrum with no unexpected peaks supports a pure product.
Yield above 100%
- Yield above 100% A percentage yield above 100% suggests that the product is wet or still contains solvent.
Repeat purification
- Repeat purification If the melting range is still wide, recrystallise again and measure the melting point again.
Limitations
- Limitations No single test proves purity, so more than one method is combined.
Part 5 of 6Worked Example
Question
- On a TLC plate the solvent front moves 6.0 cm and the product spot moves 2.4 cm.
- Calculate Rf.
- Rf is distance moved by the spot divided by the distance moved by the solvent front.
- Rf = 2.4 ÷ 6.0 = 0.40.
Answer Rf = 0.40
Part 6 of 6Exam Checklist & Tips
5.6.6 – Key Points for the Exam
- Melting range The span of temperatures from the first liquid appearing to the last solid melting.
- Rf value Distance travelled by a spot divided by the distance travelled by the solvent front.
- Purity Absence of other substances in a sample.
- Stationary phase The solid coating on a TLC plate that holds components back.
Common Mistakes
- Stating that impurities raise the melting point; they lower it.
- Drawing the pencil baseline below the level of the solvent so the sample dissolves into the tank.
- Using ink for the baseline, as the dye runs with the solvent.
- Concluding purity from one TLC spot without checking another solvent or method.
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