(a)(i) Give the two reasons why soda lime is used instead of caustic soda in the preparation of methane.
(ii) List two physical properties of methane.
(iii) A hydrocarbon with a vapour density of 29 contains 82.76% carbon and 17.24% hydrogen. Determine the: I. empirical formula; II. molecular formula of the hydrocarbon. [ H = 1.00 C = 12.00 ]
(b)(i) What is meant by the term isomerism?
(ii) Draw the structures of the two isomers of the compound with the molecular formula C\(_2\)H\(_6\)O.
(iii) Give the name of each of the isomers in (b)(ii).
(iv) State the major difference between the isomers.
(c) Give three deductions that could be made from the qualitative and quantitative analysis of a given organic compound.
(d) Give one chemical test to distinguish between propene and propane.
(a) Methane
- (i) Why soda lime rather than caustic soda: caustic soda (NaOH) is deliquescent and hard to handle as a dry solid, while soda lime is a dry, easily handled solid; caustic soda also attacks (corrodes) glass on heating, whereas soda lime does not. So soda lime is safer and more convenient for heating with sodium ethanoate.
- (ii) Two physical properties of methane: it is a colourless, odourless gas; it is almost insoluble in water and lighter (less dense) than air.
- (iii) Vapour density 29, so molar mass \(= 2\times29 = 58\). From composition: C \(= \dfrac{82.76}{12} = 6.90\); H \(= \dfrac{17.24}{1.00} = 17.24\). Ratio \(C:H = 6.90:17.24 = 1:2.5 = 2:5\). I. Empirical formula = C\(_2\)H\(_5\) (mass \(= 24+5 = 29\)). II. \(n = \dfrac{58}{29} = 2\), so molecular formula = C\(_4\)H\(_{10}\) (butane).
(b) Isomerism
- (i) Isomerism is the existence of two or more compounds having the same molecular formula but different structural arrangements of their atoms (different structural formulae).
- (ii) Isomers of C\(_2\)H\(_6\)O: ethanol, CH\(_3\)CH\(_2\)OH (an O-H group joined to the carbon chain), and methoxymethane, CH\(_3\)-O-CH\(_3\) (an oxygen atom between two methyl groups).
- (iii) Names: ethanol and methoxymethane (dimethyl ether).
- (iv) Major difference: they have different functional groups, so different properties, ethanol is an alkanol (hydroxyl group, reacts with sodium to give hydrogen, higher boiling point) while methoxymethane is an ether (does not react with sodium, much lower boiling point).
(c) Deductions from analysis of an organic compound: the identity of the elements present (qualitative analysis); the empirical formula, from the masses/percentages of each element; the molecular formula and hence molar mass (from empirical formula and relative molecular mass); and, from these, the possible functional group(s)/class of the compound.
(d) Test to distinguish propene from propane: add bromine water (or acidified KMnO\(_4\)) to each. Propene (an alkene, unsaturated) rapidly decolourises the reddish-brown bromine water, while propane (a saturated alkane) does not decolourise it.
(a) Methane
- (i) Why soda lime rather than caustic soda: caustic soda (NaOH) is deliquescent and hard to handle as a dry solid, while soda lime is a dry, easily handled solid; caustic soda also attacks (corrodes) glass on heating, whereas soda lime does not. So soda lime is safer and more convenient for heating with sodium ethanoate.
- (ii) Two physical properties of methane: it is a colourless, odourless gas; it is almost insoluble in water and lighter (less dense) than air.
- (iii) Vapour density 29, so molar mass \(= 2\times29 = 58\). From composition: C \(= \dfrac{82.76}{12} = 6.90\); H \(= \dfrac{17.24}{1.00} = 17.24\). Ratio \(C:H = 6.90:17.24 = 1:2.5 = 2:5\). I. Empirical formula = C\(_2\)H\(_5\) (mass \(= 24+5 = 29\)). II. \(n = \dfrac{58}{29} = 2\), so molecular formula = C\(_4\)H\(_{10}\) (butane).
(b) Isomerism
- (i) Isomerism is the existence of two or more compounds having the same molecular formula but different structural arrangements of their atoms (different structural formulae).
- (ii) Isomers of C\(_2\)H\(_6\)O: ethanol, CH\(_3\)CH\(_2\)OH (an O-H group joined to the carbon chain), and methoxymethane, CH\(_3\)-O-CH\(_3\) (an oxygen atom between two methyl groups).
- (iii) Names: ethanol and methoxymethane (dimethyl ether).
- (iv) Major difference: they have different functional groups, so different properties, ethanol is an alkanol (hydroxyl group, reacts with sodium to give hydrogen, higher boiling point) while methoxymethane is an ether (does not react with sodium, much lower boiling point).
(c) Deductions from analysis of an organic compound: the identity of the elements present (qualitative analysis); the empirical formula, from the masses/percentages of each element; the molecular formula and hence molar mass (from empirical formula and relative molecular mass); and, from these, the possible functional group(s)/class of the compound.
(d) Test to distinguish propene from propane: add bromine water (or acidified KMnO\(_4\)) to each. Propene (an alkene, unsaturated) rapidly decolourises the reddish-brown bromine water, while propane (a saturated alkane) does not decolourise it.
