Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 4 - 15.22 Rf values-Study Notes - New Syllabus
Edexcel International A Level (IAL) Chemistry (YCH11) -Unit 4 – 15.22 Rf values- Study Notes- New syllabus
Edexcel International A Level (IAL) Chemistry (YCH11) -Unit 4 – 15.22 Rf values- Study Notes -International A Level (IAL) Chemistry (YCH11) – per latest Syllabus.
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Edexcel International A Level (IAL) Chemistry (YCH11) -Concise Summary Notes- All Topics
15.22 Rf Values in Paper and Thin-Layer Chromatography (TLC)
In chromatography, substances move different distances depending on their attractions to the:
- Mobile phase
- Stationary phase
The movement of a substance can be measured using its: Rf value
Definition of Rf Value
Rf stands for: Retention factor
Formula
\( \mathrm{Rf = \dfrac{distance\ travelled\ by\ substance}{distance\ travelled\ by\ solvent\ front}} \)
Important Points
- Distances are measured from the baseline.
- \( \mathrm{Rf} \) has no units.
- \( \mathrm{Rf} \) values are always between:
\( \mathrm{0\ and\ 1} \)
Calculating Rf Values
Step-by-Step Method
- Measure distance travelled by substance spot.
- Measure distance travelled by solvent front.
- Divide substance distance by solvent distance.
Example 1
A dye spot moves:
\( \mathrm{4.5\ cm} \)
Solvent front moves:
\( \mathrm{6.0\ cm} \)
Calculation
\( \mathrm{Rf = \dfrac{4.5}{6.0}} \)
\( \mathrm{Rf = 0.75} \)
Interpretation of Rf Values
High \( \mathrm{Rf} \) value:
- Substance travels further.
- More soluble in mobile phase.
- Less strongly attracted to stationary phase.
Low \( \mathrm{Rf} \) value:
- Substance travels less distance.
- Stronger attraction to stationary phase.
Reasons for Differences in Rf Values
Different substances have different \( \mathrm{Rf} \) values because of differences in:
- Solubility in the mobile phase
- Attraction to the stationary phase
Factors Affecting Rf Values
Nature of the Solvent
- Different solvents change solubility of substances.
- Therefore different solvents produce different \( \mathrm{Rf} \) values.
Stationary Phase Used
- Polar stationary phases attract polar substances strongly.
- Strong attraction lowers \( \mathrm{Rf} \).
Temperature
- Temperature affects solubility and movement of substances.
Polarity of Substance
- Polar substances interact more strongly with polar stationary phases such as silica gel.
- Therefore they move more slowly.
Using Rf Values
- Compare unknown substances with known standards.
- Help identify compounds.
- Test purity of substances.
Important Limitation
Rf values are reliable only when:
- Same solvent used
- Same stationary phase used
- Same temperature used
Paper Chromatography vs TLC
| Paper Chromatography | TLC |
|---|---|
| Stationary phase = paper | Stationary phase = silica/alumina |
| Usually slower | Usually faster |
| Less sensitive | More sensitive |
Key Features
- \( \mathrm{Rf} \) compares movement of substance with solvent front.
- \( \mathrm{Rf} \) values range from 0 to 1.
- Higher \( \mathrm{Rf} \) means greater movement with solvent.
- Solvent, temperature and stationary phase affect \( \mathrm{Rf} \).
- Used to identify and compare substances.
Example 2:
In a TLC experiment, a spot travelled \( \mathrm{3.2\ cm} \) while the solvent front travelled \( \mathrm{8.0\ cm} \). Calculate the \( \mathrm{Rf} \) value.
▶️ Answer/Explanation
Calculation
\( \mathrm{Rf = \dfrac{3.2}{8.0}} \)
\( \mathrm{Rf = 0.40} \)
Example 3:
Explain why a polar substance may have a low \( \mathrm{Rf} \) value in TLC using silica gel.
▶️ Answer/Explanation
Silica gel is polar and strongly attracts polar substances.
Therefore the substance moves more slowly with the solvent and travels a shorter distance.
This gives a lower \( \mathrm{Rf} \) value.