A chemist is analysing an unknown organic compound using thin-layer chromatography (TLC). The chemist runs two samples side by side on the TLC plate: the unknown compound and a reference standard. After development, the chemist calculates the Rf values of the spots and compares them. Explain how the chemist can use the results of this TLC experiment to determine whether the unknown compound is the same as the reference standard, and describe ONE limitation of using Rf values alone to confirm the identity of a compound.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Thin-layer chromatography (TLC) is an analytical technique used to separate and identify the components of a mixture. Each component travels a characteristic distance relative to the solvent front, giving a value called the Rf value.
Model answer (5 marks)
1. Rf is calculated as the distance the spot travels divided by the distance the solvent front travels.
2. If the unknown and the reference give the same Rf value, this suggests they may be the same compound.
3. If the Rf values differ, the unknown cannot be the same as the reference.
4. A limitation is that two different compounds can give the same Rf value in a particular solvent system, so a match does not prove identity.
5. To confirm identity, the chemist would need to run the TLC in a different solvent or use another analytical method such as NMR or IR.
2. If the unknown and the reference give the same Rf value, this suggests they may be the same compound.
3. If the Rf values differ, the unknown cannot be the same as the reference.
4. A limitation is that two different compounds can give the same Rf value in a particular solvent system, so a match does not prove identity.
5. To confirm identity, the chemist would need to run the TLC in a different solvent or use another analytical method such as NMR or IR.
Examiner tips
- Show the Rf formula first to earn the first point. Use the word ‘suggests’ when linking equal Rf values to possible identity. Remember to mention the need for a second test to confirm identity for the final point.
Common mistakes
- Assuming equal Rf values give absolute identity without mentioning a second test. Using the wrong order of distances in the Rf calculation. Failing to state that a different solvent or another technique is required for confirmation.
Mark scheme (5 marks)
- The Rf value is calculated as the distance travelled by the spot divided by the distance travelled by the solvent front
- If the unknown compound has the same Rf value as the reference standard, this suggests they may be the same compound
- If the Rf values are different, the unknown compound is not the same as the reference standard
- A limitation is that two different compounds can have the same Rf value in a given solvent, so a match does not confirm identity with certainty
- A further technique (e.g. running the chromatogram in a different solvent, or using an additional analytical technique) would be needed to confirm identity
Key terms in this question
Rf value · thin-layer chromatography · reference standard
Related
- All OCR A-Level Chemistry B: Salters (H433) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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