(a) List three characteristics of a homologous series (b) Give one example of; (i) alkanes; (ii) alkynes. (c) A hydrocarbon contains 7.7% by mass of hydroge...
Question 1 Report
(a) List three characteristics of a homologous series
(b) Give one example of;
(i) alkanes; (ii) alkynes.
(c) A hydrocarbon contains 7.7% by mass of hydrogen and 92.3% by mass of carbon. The relative molar mass of the compound is 78.
(i) Derive the empirical formula of the compound and hence its molecular formula.
(ii) Name the hydrocarbon and write its structural formula. (H=1, C=12)
(d) Two hydrocarbons, X and Y were treated separated with acidified potassium tetraoxomanganate (VII) solution. X decolorized the solution and Y did not. Which of X and Y will undergo
(i) substitution reaction only,
(ii) both addition and substitution reactions.
(iii) polymerization?
(e) If ethanol is to be converted into ethanoic acid
(i) What are the conditions required?
(ii) name the type of reaction that will be involved and write the equation
Answer Details
(a) Three characteristics of a homologous series
- All members can be represented by the same general formula.
- Consecutive members differ by a -CH2- group (a difference of 14 in relative molar mass).
- They have similar chemical properties because they contain the same functional group, and show a gradual change in physical properties as molar mass increases.
(b) Examples
- (i) Alkane: methane, CH4.
- (ii) Alkyne: ethyne, C2H2.
(c) Formula from composition
| Element | C | H |
|---|---|---|
| % mass | 92.3 | 7.7 |
| ÷ atomic mass | 92.3/12 = 7.69 | 7.7/1 = 7.7 |
| Ratio | 1 | 1 |
Empirical formula = CH (empirical mass = 12 + 1 = 13).
\[n = \frac{78}{13} = 6\]
Molecular formula = (CH)6 = C6H6.
(ii) The hydrocarbon is benzene. Its structural formula is a hexagonal ring of six carbon atoms with alternating single and double bonds, each carbon carrying one hydrogen atom (often drawn as a hexagon with an inscribed circle).
(d) X decolorized acidified KMnO4, so X is unsaturated (an alkene/alkyne); Y did not, so Y is saturated (an alkane).
- (i) Substitution only: Y.
- (ii) Both addition and substitution: X.
- (iii) Polymerization: X.
(e) Ethanol to ethanoic acid
- (i) Conditions: heat (reflux) the ethanol with an acidified oxidizing agent such as acidified potassium tetraoxomanganate(VII) or acidified potassium dichromate(VI).
- (ii) Type of reaction: oxidation. \[CH_3CH_2OH + 2[O] \rightarrow CH_3COOH + H_2O\]
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