Chemistry 3 Nov/Dec 2011

Chemistry Paper 3,Nov/Dec 2011

Questions:		1	2	3		Main

General Comments
Weakness/Remedies
Strength

Question 2

(a)Copy and complete the following table showing characteristic test for each class of organic compound indicated.

	Test	Observation	Inference
(i)	Compound + NaHCO3(aq)	---------------------- ----------------------	Alkanoic acid
(ii)	Compound + K2Cr2O7/H+((aq) + heat	---------------------- ----------------------	Alkanol
(iii)	Compound + ………………………………….	---------------------- ----------------------	Alkyne
(iv)	Compound + …………………………………..	---------------------- ----------------------	Reducing sugar

(b) Aqueous solutions of aluminum chloride, calcium chloride, iron (III) chloride and zinc trioxonitrate (V) are contained in four separate reagent bottles whose labels have been accidentally removed. Using only aqueous ammonia solution, show how to identify the content of each bottle. Tabulate your results.

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OBSERVATION

Candidates were expected to copy and complete gaps in the table as demanded by the question.
Many of the candidates gave a good account of themselves. However, some of them lost marks as a result of their inability to march organic test with the appropriate observation despite the fact that the inference was provided in each of the four cases. Majority of the candidates could not differentiate between ordinary white precipitate and white gelateneous precipitate.
A few candidates did not tabultate their responses hence they lost marks.

The expected answers are:
(a)

	Test	Observation	Inference
(i)	Compound + NaHCO3(aq)	Colourless gas/effervescence Gas turns lime water milky	Alkanoic acid
(ii)	Compound + K2Cr2O7/H+((aq) + heat	Orange solution turns green	Alkanol
(iii)	Compound + Ammoniacal soln of Cu2Cl2 (accept CuCl) OR Ammoniacal soln of AgNO3 [Accept + Br2/CCl4 OR + KMnO4/H+(aq)]	Reddish-brown ppt OR White ppt. [Reddish-brown colour of Br2, discharged or decolourized OR Purple colour decolourized].	Alkyne
(iv)	Compound + Benedict/Fehling’s solution + heat	Brick-red precipitate	Reducing sugar

(b)

	Test	Observation	Inference
(i)	Sample solution + NH3(aq) in drops; in excess	White gelatinuous precipitate Precipitate insoluble	A13+ or Zn2+ present A13+ present A1Cl3 indicated
(ii)	Sample solution + NH3(aq) in drops; then in excess	No precipitate formed/No visible reaction	Ca2+ present CaCl2 indicated
(iii)	Sample solution + NH3(aq) in drops; then in excess	Reddish-brown (gelatinuous) precipitate Precipitate insoluble	Fe3+ present FeCl3 indicated
(iv)	Sample solution + NH3(aq) in drops; then in excess	White gelatinous precipitate Precipitate dissolves/soluble	Zn2+ or A13+ present Zn2+ present Zn(NO3)2 indicated