(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. \(_9\)F and \(_{17}\)Cl; \(_{12}\)Mg and \(_{20}\)Ca. 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 could be used to determine the end-point of the following titrations:
(i) dilute hydrochloric acid against sodium hydroxide solution;
(ii) dilute hydrochloric acid against 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 red litmus paper blue. identify E, F and G.
(a)(i) Rutherford's atomic structure: the atom has a very small, dense, positively charged nucleus at the centre where nearly all the mass is concentrated, with the negatively charged electrons moving in the mostly empty space around the nucleus.
(a)(ii) Atomic number vs mass number: the atomic number is the number of protons in the nucleus of an atom; the mass number is the total number of protons and neutrons in the nucleus.
(a)(iii) Chlorine exists as two isotopes, 35Cl and 37Cl, in the fixed ratio 3:1. The relative atomic mass is the weighted average of these isotopic masses, \(\frac{(35\times3)+(37\times1)}{4}=35.5\), which is not a whole number.
(b)(i) First ionization energy: the minimum energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous singly charged positive ions.
(b)(ii) Three properties of electrovalent compounds:
- They have high melting and boiling points.
- They are usually soluble in water but insoluble in organic solvents.
- They conduct electricity when molten or in aqueous solution.
(b)(iii) Each pair has the same number of electrons in the outermost shell (F and Cl each have 7; Mg and Ca each have 2). Since chemical properties depend on the number of valence electrons, members of the same group have similar chemical properties.
(c)(i) Homologous series: a family of organic compounds with the same general formula in which successive members differ by a CH2 unit and show a gradual change in physical properties, e.g. the alkanes (CH4, C2H6, C3H8...).
(c)(ii) Heterolytic fission: the breaking of a covalent bond in which both shared electrons go to one of the atoms, producing oppositely charged ions, e.g. \(\text{HCl} \to \text{H}^+ + \text{Cl}^-\).
(d) Indicators:
- (i) HCl vs NaOH (strong acid/strong base): methyl orange or phenolphthalein.
- (ii) HCl vs NH4OH (strong acid/weak base): methyl orange.
- (iii) Ethanoic acid vs NaOH (weak acid/strong base): phenolphthalein.
(e) E = ammonium chloride (NH4Cl, a solid chloride that sublimes); F = an alkali such as sodium hydroxide (NaOH); G = ammonia (NH3), the choking gas that turns moist red litmus blue.
\[ \text{NH}_4\text{Cl} + \text{NaOH} \to \text{NaCl} + \text{H}_2\text{O} + \text{NH}_3 \]
(a)(i) Rutherford's atomic structure: the atom has a very small, dense, positively charged nucleus at the centre where nearly all the mass is concentrated, with the negatively charged electrons moving in the mostly empty space around the nucleus.
(a)(ii) Atomic number vs mass number: the atomic number is the number of protons in the nucleus of an atom; the mass number is the total number of protons and neutrons in the nucleus.
(a)(iii) Chlorine exists as two isotopes, 35Cl and 37Cl, in the fixed ratio 3:1. The relative atomic mass is the weighted average of these isotopic masses, \(\frac{(35\times3)+(37\times1)}{4}=35.5\), which is not a whole number.
(b)(i) First ionization energy: the minimum energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous singly charged positive ions.
(b)(ii) Three properties of electrovalent compounds:
- They have high melting and boiling points.
- They are usually soluble in water but insoluble in organic solvents.
- They conduct electricity when molten or in aqueous solution.
(b)(iii) Each pair has the same number of electrons in the outermost shell (F and Cl each have 7; Mg and Ca each have 2). Since chemical properties depend on the number of valence electrons, members of the same group have similar chemical properties.
(c)(i) Homologous series: a family of organic compounds with the same general formula in which successive members differ by a CH2 unit and show a gradual change in physical properties, e.g. the alkanes (CH4, C2H6, C3H8...).
(c)(ii) Heterolytic fission: the breaking of a covalent bond in which both shared electrons go to one of the atoms, producing oppositely charged ions, e.g. \(\text{HCl} \to \text{H}^+ + \text{Cl}^-\).
(d) Indicators:
- (i) HCl vs NaOH (strong acid/strong base): methyl orange or phenolphthalein.
- (ii) HCl vs NH4OH (strong acid/weak base): methyl orange.
- (iii) Ethanoic acid vs NaOH (weak acid/strong base): phenolphthalein.
(e) E = ammonium chloride (NH4Cl, a solid chloride that sublimes); F = an alkali such as sodium hydroxide (NaOH); G = ammonia (NH3), the choking gas that turns moist red litmus blue.
\[ \text{NH}_4\text{Cl} + \text{NaOH} \to \text{NaCl} + \text{H}_2\text{O} + \text{NH}_3 \]
