Separation relies on differences in intermolecular forces
Lesson 1.2.7 covered filtration, evaporation, and distillation in terms of macroscopic properties such as solubility and boiling point. AP topic 3.9 asks you to explain at the particle level: chromatography and distillation separate mixtures because the components have different intermolecular forces.
Chromatography: stationary and mobile phases
Chromatography has two phases: a stationary phase固定相 that stays put (the cellulose of paper in paper chromatography, usually silica gel in thin-layer and column chromatography) and a mobile phase流动相 (a solvent) that flows past it. Each component of a mixture is constantly divided between the two phases:
- the more strongly it is attracted to the stationary phase (the stronger the intermolecular forces), the more time it spends held there, and the slower and shorter it travels;
- the more strongly it is attracted to the mobile phase, the more time it moves along with the solvent, and the faster and farther it travels.
The surface of silica gel is covered with –OH groups, making it a very polar stationary phase: less polar components travel far, and more polar ones (especially those that can hydrogen-bond) travel a short distance. So chromatography answers the question “do these components differ in polarity (intermolecular forces)?”, not whether they differ in mass, density, or speed.
Rf values
Rf is between 0 and 1. With the same stationary and mobile phases, a substance always has the same Rf, so an unknown can be run next to known substances and identified by comparing Rf values.
Changing the polarity of the mobile phase: on silica gel, a more polar mobile phase attracts the components more strongly and competes with the stationary phase for them, so every spot travels farther (larger Rf). With a mobile phase that is too nonpolar, polar components stay on the origin line. A mobile phase of moderate polarity separates the components best.
Column chromatography柱色谱 works the same way: silica gel is packed in a column, solvent carries the mixture through it, and the component least attracted to the stationary phase comes out first. A column can handle larger samples, so it is used to collect usable amounts of pure components; paper and thin-layer chromatography are mainly for analysis.
Drag the slider to change how much ethyl acetate is in the mobile phase, develop the TLC plate, compare the Rf values of three dyes, and identify the unknown:
Distillation: weaker forces come off first
Distillation heats a liquid mixture so that the lower-boiling component vaporizes first; its vapor is condensed and collected. Boiling point depends on intermolecular forces (U3.1): the weaker the forces, the lower the boiling point and the sooner the component distills. When a mixture of ethanol (78.3 °C) and water (100 °C) is distilled, the first distillate is rich in ethanol, which hydrogen-bonds less than water.
Liquids with close boiling points (such as the hydrocarbons in petroleum) are separated by fractional distillation分馏: in a fractionating column the vapor condenses and re-vaporizes again and again, each time becoming richer in the lower-boiling component, until fairly pure fractions are collected.
To recover a solid dissolved in a solution (such as salt), evaporate the solvent.
Example 1: Reading a TLC plate
Two compounds, X and Y, are run on a silica gel TLC plate. The solvent front moves 6.0 cm, the spot of X moves 4.5 cm, and the spot of Y moves 1.5 cm. Find each Rf and decide which compound is more polar. How would the results change with a more polar mobile phase?
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Calculate Rf. Measure everything from the starting line:
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Decide polarity. Silica gel is a polar stationary phase. Y travels less far, so it is attracted more strongly to the silica: Y is more polar.
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Change the solvent. A more polar mobile phase attracts both compounds more strongly and competes with the stationary phase for them, so both spots travel farther and both Rf values increase.
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Note. The conclusion rests on the strength of attractions, not on molar mass or size; Rf is always between 0 and 1.
Example 2: Distilling two alkanes
Hexane (, 68.7 °C) and octane (, 125.6 °C) are miscible. When a mixture of them is distilled, which compound is the first distillate richer in, and why?
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Compare the intermolecular forces.
- Hexane: nonpolar, 50 electrons, weaker dispersion forces, bp 68.7 °C.
- Octane: nonpolar, 66 electrons, a longer chain with stronger dispersion forces, bp 125.6 °C.
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Link to volatility. Hexane’s weaker attractions give it the higher vapor pressure at any temperature, so the vapor is richer in hexane than the liquid is.
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Conclusion. The first distillate is richer in hexane; as distillation continues, the temperature rises and the distillate contains more and more octane.
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A better separation. With a boiling-point difference of about 57 °C, a fractionating column gives purer fractions.
- Separate the dyes in black ink
The dyes differ in their attraction to the paper and the solvent.
- Get fairly pure ethanol from an ethanol–water mixture
The two have different boiling points.
- Recover salt from salt water
Evaporate the water, leaving the dissolved solid.
- Remove sand from water
Sand doesn't dissolve in water.
- Find out whether two dyes differ in polarity
Compare their Rf values under the same conditions.
- Split petroleum into fractions with different boiling points
Fractional distillation.
- Get fresh water from seawater
The water vaporizes and is condensed; the salt stays behind.
- Recover sugar from sugar water
Evaporate the solvent.
- Collect a few grams of one pure component on a silica gel column
Column chromatography handles larger samples.
- Separate two miscible liquids whose boiling points differ greatly
The lower-boiling liquid distills first.
Common mistakes
Mistake 1: “Chromatography separates molecules by mass or size.” Chromatography relies on differences in the intermolecular attractions of the components to the stationary and mobile phases. Different Rf values mean different polarities.
Mistake 2: “The larger the Rf, the more polar the substance.” On a polar stationary phase such as silica gel or paper, it’s the reverse: the more polar the component, the more strongly the stationary phase holds it, the shorter it travels, and the smaller its Rf.
Mistake 3: “The first liquid to come off in a distillation is the one present in the largest amount.” The first to come off is the component with the lower boiling point and weaker intermolecular forces, regardless of how much of it there is.
Quick check
Three compounds developed on a silica gel TLC plate have Rf values of 0.15, 0.48, and 0.82. Which is most polar, and why?
Silica gel is a polar stationary phase, so the more polar a component, the more strongly it is held and the smaller its Rf. C uses mass as the reason; D is wrong because Rf also depends on the component itself.
A mixture is separated by silica gel TLC, and the mobile phase is changed from 20% ethyl acetate (the rest hexane) to 50% ethyl acetate. How does the Rf of component B change?
Ethyl acetate is more polar than hexane. A more polar mobile phase competes with the stationary phase for the components, so they travel farther with it. B: Rf depends on both the stationary and the mobile phase.
Two dyes have different Rf values under the same chromatography conditions. What is the most reasonable conclusion?
Chromatography separates by intermolecular forces. A, C, and D are not properties that chromatography reveals directly.
Which method is best for obtaining enough of a pure component from a mixture to use in further experiments?
Column chromatography can handle larger samples and lets you collect each component as it comes off. Paper and thin-layer chromatography use tiny samples and are mainly for analysis.
When a mixture of ethanol (boiling point 78.3 °C) and water (100 °C) is distilled, which is the first distillate rich in, and why?
The weaker the intermolecular forces, the lower the boiling point and the sooner the liquid vaporizes. A and B use mass as the reason; C confuses amount with boiling point.
On a TLC plate, the solvent front travels 8.0 cm and a spot travels 3.2 cm. What is its Rf?
Rf = 3.2 ÷ 8.0 = 0.40. A inverts the fraction; Rf is always between 0 and 1.
AP-style free response
Three compounds are separated by silica gel TLC: A is a hydrocarbon (nonpolar), B contains a C=O group (polar, but no hydrogen-bond donor), and C contains an –OH group (can hydrogen-bond). The mobile phase is a 4 : 1 mixture of hexane and ethyl acetate.
(a) Predict the order of the Rf values from largest to smallest, and justify your prediction using intermolecular forces.
(b) The solvent front travels 7.5 cm and spot B travels 3.0 cm. Calculate the Rf of B.
(c) If the mobile phase is changed to pure hexane, how does the Rf of C change? Explain.
(d) If the three compounds are separated on a silica gel column, which comes out of the column first?
Model answer and scoring
(a) A > B > C. The surface of silica gel is covered with –OH groups, making it a polar stationary phase. A is nonpolar, attracted to the silica only by dispersion forces, the weakest attraction, so it travels farthest. B also has dipole–dipole attractions to the silica (and its C=O oxygen can accept hydrogen bonds from the silica’s –OH groups). C’s –OH group hydrogen-bonds strongly to the silica, the strongest attraction, so it travels the shortest distance.
(b) Rf = 3.0 ÷ 7.5 = 0.40.
(c) It decreases (C barely leaves the origin). Pure hexane is nonpolar and attracts the polar C very weakly, so it can’t pull C off the silica gel.
(d) A comes out first: it is least attracted to the stationary phase, so it spends the most time in the mobile phase.
Scoring: (a) correct order, comparing each compound’s attraction to the silica gel; (b) 0.40; (c) decreases, explained by the less polar mobile phase; (d) A, explained by its weakest attraction to the stationary phase.
Summary
- Chromatography: components are divided between the stationary and mobile phases; the more strongly a component is attracted to the stationary phase, the shorter it travels. On a polar stationary phase, the more polar the component, the smaller its Rf.
- Rf = distance traveled by the spot ÷ distance traveled by the solvent front; under the same conditions it identifies a substance. A more polar mobile phase makes the spots travel farther.
- Column chromatography works the same way and can collect larger amounts of pure components.
- Distillation: the weaker the intermolecular forces, the lower the boiling point and the sooner a component distills; use fractional distillation when boiling points are close. Evaporation recovers a dissolved solid.
AP tip: To explain a chromatography result, say which component has the stronger intermolecular attraction to the stationary (or mobile) phase, and name the type of force (hydrogen bonding, dipole–dipole, dispersion). “Heavier molecules” and “faster molecules” are not reasons.