Question

1. (a)(i) What is the structure of the atom as proposed by Rutherford? (ii) Distinguish between the atomic number and the mass number of an element. (iii) Explain briefly why the relative atomic mass of chlorine is not a whole number.
(b)(i) What is meant by first ionization energy? (ii) List three properties of electrovalent compounds. (iii) Consider the following pairs of elements: I. ₉F and ₁₇Cl; II. ₁₂Mg and ₂₀Ca. Explain briefly why the elements in each pair have similar chemical properties.
(c) Explain briefly the following terms using an appropriate example in each case: (i) homologous series; (ii) heterolytic fission.
(d) State the indicator(s) which would be used to determine the end-point of the following titrations: (i) dilute hydrochloric acid against sodium hydroxide solution; (ii) dilute hydrochloric acid and ammonium hydroxide solution; (iii) ethanoic acid against sodium hydroxide solution.
(e) A solid chloride E which sublimed on heating reacted with an alkali F to give a choking gas G, G turned moist litmus paper blue. Identify E, F and G.
2(a) What is diffusion?

Accepted Answer

1(a)(i) Rutherford proposed that an atom comprises a positively charged nucleus (situated at the centre) with electrons revolving around it.
(ii) The atomic number of an element is the number of protons present in the nucleus of its atom, while the mass number is the sum of the proton number and the neutron number.
(iii) The relative atomic mass of chlorine is not a whole number because chlorine is an isotopic mixture; the conventional relative atomic mass for an isotopic element is the average weight of the various isotopes.

(b)(i) The first ionization energy is the energy required to remove the most loosely bound electron from a gaseous atom to form a unipositive gaseous ion.
(ii) Electrovalent compounds are composed of ions, rather than molecules; they conduct electricity in molten or aqueous form; they have high melting and boiling points.
(iii) The elements in each pair have similar chemical properties because they have equal number of valence electrons.

(c)(i) A homologous series is a family of organic compounds which are closely related. Features: members have a general molecular formula; they have the same functional group; they have a general method of preparation; they have similar chemical properties; they show gradation in physical properties; consecutive members differ in their molecular formula by a CH2 group and in their relative molecular mass by 14.
(ii) Heterolytic fission refers to a process by which bond breaking is done in such a way that the electron pair is completely transferred to one of the atoms, leaving the other atom electron-deficient.

(d)(i) For dilute HCl and NaOH, any indicator (methyl orange, litmus, phenolphthalein etc.) can be used. (ii) For HCl and NH4OH, methyl orange is used. (iii) For CH3COOH and NaOH, phenolphthalein is used.

(e) E is ammonium chloride, F can be NaOH, KOH, Ca(OH)2, CaO etc., G is ammonia.

2(a) Diffusion is defined as the movement of particles from the region of higher concentration to the region of lower concentration, until even distribution is attained.

WAEC Chemistry 2012 Theory — Question 1

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