(a)(i) Determine the oxidation number of sulphur in Na\(_2\)S\(_2\)O\(_3\)
(ii) Name the allotropes of sulphur.
(iii) State two ways in which the structure of graphite and diamond are similar.
(b)(i) Name two green-house gases.
(ii) State one effect of an increased level of green-house gases on the environment.
(iii) State one source from which nitrogen (I) oxide is released into the environment.
(iv) Write a chemical equation to show the effect of heat on each of the following compounds: I. KNO\(_{3(s)}\) II. AgNO\(_{3(s)}\)
(C)(i)Describe briefly how pure crystals of calcium chloride could be obtained from a solution of calcium chloride
ii) Explain briefly each of the following observations:
I. ammonia gas is highly soluble in water;
II. boiling ploint of chlorine is lower than that of iodine
(d) Consider the reaction represented by the following equation:- 2NaCl + H\(_2\)SO\(_{4(s)}\) \(\to\) Na\(_2\)SO\(_{4(s)}\) + 2HCl\(_{(g)}\)
Calculate the volume of HCl gas that can be obtained at s.t.p. from 5.85 g of sodium. chloride. [ Na = 23.0, Cl = 35.5, Molar volume of gas at s.t.p. = 22.4 dm\(^3\)]
(a)(i) Oxidation number of sulphur in \(Na_2S_2O_3\). Let it be \(x\); Na is \(+1\), O is \(-2\), and the compound is neutral:
\[2(+1) + 2x + 3(-2) = 0\]
\[2 + 2x - 6 = 0 \;\Rightarrow\; 2x = 4 \;\Rightarrow\; x = +2\]
Sulphur has an average oxidation number of +2.
(ii) The allotropes of sulphur are rhombic sulphur and monoclinic sulphur (plastic sulphur is also sometimes listed).
(iii) Two ways in which graphite and diamond are similar: both are allotropes of carbon (made up of carbon atoms only), and both are giant covalent (macromolecular) structures with very high melting points.
(b)(i) Two greenhouse gases: carbon(IV) oxide (\(CO_2\)) and methane (\(CH_4\)). (Water vapour and CFCs are also acceptable.)
(ii) One effect of an increased level of greenhouse gases: global warming (a rise in the average atmospheric temperature, leading to melting of ice caps and rising sea levels).
(iii) One source of nitrogen(I) oxide, \(N_2O\): bacterial action on nitrogenous fertilizers in the soil (or from vehicle exhausts).
(iv) Effect of heat:
I. \[2KNO_3 \rightarrow 2KNO_2 + O_2\]
II. \[2AgNO_3 \rightarrow 2Ag + 2NO_2 + O_2\]
(c)(i) To obtain pure crystals of calcium chloride from its solution: evaporate the solution carefully to concentrate it to the point of crystallization, then allow it to cool so that crystals form; filter off the crystals and dry them (calcium chloride is deliquescent, so drying should be done quickly in a desiccator).
(ii) I. Ammonia is highly soluble in water because ammonia molecules are polar and form hydrogen bonds with water (and react with water to form ammonium hydroxide).
II. The boiling point of chlorine is lower than that of iodine because chlorine molecules are smaller and lighter, so the van der Waals (intermolecular) forces between them are weaker than the stronger van der Waals forces between the larger iodine molecules; less energy is needed to separate chlorine molecules.
(d) \(2NaCl + H_2SO_4 \rightarrow Na_2SO_4 + 2HCl\). From 5.85 g of NaCl:
Molar mass of \(NaCl = 23 + 35.5 = 58.5\,g\,mol^{-1}\)
\[\text{moles of } NaCl = \frac{5.85}{58.5} = 0.1\,mol\]
From the equation, 2 moles NaCl give 2 moles HCl (1:1), so moles of \(HCl = 0.1\,mol\).
\[\text{volume at s.t.p.} = 0.1 \times 22.4 = 2.24\,dm^3\]
(a)(i) Oxidation number of sulphur in \(Na_2S_2O_3\). Let it be \(x\); Na is \(+1\), O is \(-2\), and the compound is neutral:
\[2(+1) + 2x + 3(-2) = 0\]
\[2 + 2x - 6 = 0 \;\Rightarrow\; 2x = 4 \;\Rightarrow\; x = +2\]
Sulphur has an average oxidation number of +2.
(ii) The allotropes of sulphur are rhombic sulphur and monoclinic sulphur (plastic sulphur is also sometimes listed).
(iii) Two ways in which graphite and diamond are similar: both are allotropes of carbon (made up of carbon atoms only), and both are giant covalent (macromolecular) structures with very high melting points.
(b)(i) Two greenhouse gases: carbon(IV) oxide (\(CO_2\)) and methane (\(CH_4\)). (Water vapour and CFCs are also acceptable.)
(ii) One effect of an increased level of greenhouse gases: global warming (a rise in the average atmospheric temperature, leading to melting of ice caps and rising sea levels).
(iii) One source of nitrogen(I) oxide, \(N_2O\): bacterial action on nitrogenous fertilizers in the soil (or from vehicle exhausts).
(iv) Effect of heat:
I. \[2KNO_3 \rightarrow 2KNO_2 + O_2\]
II. \[2AgNO_3 \rightarrow 2Ag + 2NO_2 + O_2\]
(c)(i) To obtain pure crystals of calcium chloride from its solution: evaporate the solution carefully to concentrate it to the point of crystallization, then allow it to cool so that crystals form; filter off the crystals and dry them (calcium chloride is deliquescent, so drying should be done quickly in a desiccator).
(ii) I. Ammonia is highly soluble in water because ammonia molecules are polar and form hydrogen bonds with water (and react with water to form ammonium hydroxide).
II. The boiling point of chlorine is lower than that of iodine because chlorine molecules are smaller and lighter, so the van der Waals (intermolecular) forces between them are weaker than the stronger van der Waals forces between the larger iodine molecules; less energy is needed to separate chlorine molecules.
(d) \(2NaCl + H_2SO_4 \rightarrow Na_2SO_4 + 2HCl\). From 5.85 g of NaCl:
Molar mass of \(NaCl = 23 + 35.5 = 58.5\,g\,mol^{-1}\)
\[\text{moles of } NaCl = \frac{5.85}{58.5} = 0.1\,mol\]
From the equation, 2 moles NaCl give 2 moles HCl (1:1), so moles of \(HCl = 0.1\,mol\).
\[\text{volume at s.t.p.} = 0.1 \times 22.4 = 2.24\,dm^3\]
