(a)i. State two characteristics of a homologous... | WAEC SSCE - Chemistry - 2021 (Essay)

Question 1 Report

(a)i. State two characteristics of a homologous series.

(ii) Explain briefly why there are differences in the reaction of ethane and ethene.

(b) When crystals of sodium chloride were warmed with concentrated tetraoxosulphate( VI) acid, a gas was evolved.
(i) Name the gas.
(ii) State two physical properties of the gas.
(iii) Write a balanced chemical equation for the reaction.

(c)i. What are hydrocarbons?
(ii) State two natural sources of hydrocarbons.
(iii) A hydrocarbon contains 83% of carbon by mass. Calculate its empirical formula. [H=1.0, C=12.0]

(d) Draw and label a diagram of a set-up that could be used to electroplate a copper ornament with silver

Answer

(a)(i) Characteristics of a homologous series

All members have the same functional group and can be represented by the same general formula.
Successive members differ by a \(-CH_2-\) unit, that is, by 14 in relative molecular mass.
They have similar chemical properties and show a gradual change in physical properties such as boiling point.

(a)(ii) Ethane is a saturated hydrocarbon containing only carbon-carbon single bonds. It undergoes mainly substitution reactions. Ethene is unsaturated and contains a carbon-carbon double bond, \(C=C\), which is reactive; it therefore readily undergoes addition reactions.

(b)(i) The gas evolved is hydrogen chloride, \(HCl\).

(b)(ii) Hydrogen chloride is a colourless gas with a pungent, choking smell. It is denser than air and is very soluble in water.

(b)(iii)

\[\mathrm{NaCl_{(s)} + H_2SO_{4(l)} \rightarrow NaHSO_{4(s)} + HCl_{(g)}}\]

(c)(i) Hydrocarbons are organic compounds that contain carbon and hydrogen only.

(c)(ii) Natural sources of hydrocarbons include crude oil (petroleum) and natural gas.

(c)(iii) Empirical formula

Assume 100 g of the hydrocarbon.

Element	Mass (g)	Moles	Mole ratio
C	83	\(\frac{83}{12}=6.92\)	\(\frac{6.92}{6.92}=1\)
H	\(100-83=17\)	\(\frac{17}{1}=17\)	\(\frac{17}{6.92}=2.46\approx2.5\)

The simplest ratio is \(\mathrm{C:H}=1:2.5\). Multiplying throughout by 2 gives \(2:5\).

\[\boxed{\text{Empirical formula }=\mathrm{C_2H_5}}\]

(d) Electroplating a copper ornament with silver

The silver electrode is connected to the positive terminal and is the anode. The copper ornament is connected to the negative terminal and is the cathode. Aqueous silver trioxonitrate(V), \(\mathrm{AgNO_3(aq)}\), is used as the electrolyte. Silver dissolves from the anode and is deposited on the copper ornament.

Read lesson note on Chemistry Of Carbon Compounds (Nigeria Only) (WAEC)

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