YEAR 12 HOMEWORK

SHEET 2 - QUALITATIVE ANALYSIS

Read pp 8 - 16 of Chemistry 2 and run the program "The Bohr atom" on the Scott Visual Chemistry Program for background on absorption and emission spectra.

Also check out the online chemistry course at: http://tqd.advanced.org/3310/lographics/index.html (especially the experiments "Nichrome wire flame test", "Watch glass flame test" and the videos of these pracs).

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1. Compare and contrast the techniques of paper chromatography and thin layer chromatography by listing the advantages and disadvantages of each.

2.The dyes in a red jelly bean were extracted and separated using paper chromatography. Two spots were measured. One of these spots moved 0.9cm while the other moved 3.7cm. The solvent front moved 7.2cm.

The R_f values of some known dyes are:
carmoisine 0.08
green S 0.11
erythrosine 0.48
croceine orange

(a) Draw the chromatogram for the jelly bean

(b) What are the likely dyes present in the bean?

(c) What possible reasons may there be for small differences in the R_f values?

(d) Why is the R_f value never more than 1?

(e) What is meant by the terms mobile phase, stationary phase, absorption and solvent?

3. The amino acids in a sample of fruit juice can be determined using paper chromatography.
Three samples (A, B and C) were separated and the chromatogram obtained.

Amino Acid	R_f value
Alanine	0.56
Arganine	0.61
Isoleucine	0.82
Proline	0.89
Serine	0.27
Taurine	0.33
Valine	0.75

(a) Calculate the R_f value of each spot on the chromatogram
R_f spot at A (red) =
R_f spot at B (purple) =
R_f spot at C (small - yellow) =
R_f spot at C (large- green) =

(b) Attempt to identify the amino acids in each juice

(d) Which amino acid was most strongly adsorbed to the paper?

(e) Which amino acid showed the strongest attraction to the mobile phase?

(f) Do any of the spots show signs of incomplete separation of 2 components? Explain.

(g) Why is the origin marked with pencil not biro?

(h) Why is the solvent level kept below the origin?

4. How can samples of potassium, sodium, barium and calcium be quickly distinguished? How accurate is this?

5. In a flame test, both calcium and lithium produce red light:
(a) What is the origin of this light?

(b) How could we use this light to distinguish between the two elements?

(c) How is this better than flame tests?

6. Below is a represntation of some of the possible energy transitions in a given atom. Each return to a lower energy level is accompanied by the emission of a photon of light.

(a) Are these the only possible transitions for this atom? Explain?

(b) Why are there so many lines in an emission spectrum?

(d) Which transition is most likely to be red? Explain

(e) Is it possible for light to be absorbed by atoms? Explain.