(a) Explain briefly why \(^{4}_{2}\mathrm{He}\) has a stable electron configuration compared to \(^{9}_{4}\mathrm{Be}\) (b) Consider the following elements:...
Question 1 Report
(a) Explain briefly why \(^{4}_{2}\mathrm{He}\) has a stable electron configuration compared to \(^{9}_{4}\mathrm{Be}\)
(b) Consider the following elements: 1H and 3Li.
(i) State the number of electrons that an atom of each element would have after forming an ionic bond.
(ii) Give a reason for each of your answers stated in (b)(i).
(c) State two factors that should be considered when siting a chemical industry.
(d) State two advantages of using a catalyst instead of high temperatures in chemical reactions.
(e) Turpentine burns in chlorine according to the following equation:
\[ \mathrm{C}_{10}\mathrm{H}_{16(l)} + 8\mathrm{Cl}_{2(g)} \to 10\mathrm{C}_{(s)} + 16\mathrm{HCl}_{(g)} \]
Calculate the mass of turpentine that would completely burn in 21.3 g of chlorine.
[Molar mass of chlorine = \(71\ \mathrm{gmol}^{-1}\); Molar mass of Turpentine = \(136\ \mathrm{gmol}^{-1}\)]
(f) What is cracking?
(g) State two factors that may influence the value of electron affinity.
(h) What are carbohydrates?
(i) State two differences between a simple sugar and starch.
(j) Write an equation to show the dissociation of each of the following acids:
(i) \(\mathrm{H}_2\mathrm{CO}_3\);
(ii) \(\mathrm{CH}_3\mathrm{COOH}\).
Answer Details
(a) Helium has the electron configuration 2 (a completely filled first shell, a stable duplet), so its outermost shell is full and it has no tendency to gain or lose electrons. Beryllium is 2,2; its outermost shell holds only 2 of the electrons needed to complete it, so it is not a stable arrangement and beryllium readily loses those electrons.
(b)(i) In forming an ionic bond (as in LiH), hydrogen gains one electron to become H- with 2 electrons, and lithium loses one electron to become Li+ with 2 electrons.
(ii) Hydrogen (1 electron) gains one electron to attain the stable duplet of helium; lithium (2,1) loses its single outer electron to attain the stable duplet of helium.
(c) Any two: availability of raw materials; availability of water; nearness to market; good transport network; availability of cheap labour and power.
(d) A catalyst lowers energy cost because a high temperature is not needed; and it is not used up (it can be reused), and it avoids decomposition of heat-sensitive products.
(e) \(C_{10}H_{16} + 8Cl_2 \rightarrow 10C + 16HCl\)
Amount of chlorine \(= \dfrac{21.3}{71} = 0.300\ \text{mol}\).
Mole ratio turpentine : chlorine \(= 1 : 8\), so amount of turpentine \(= \dfrac{0.300}{8} = 0.0375\ \text{mol}\).
Mass of turpentine \(= 0.0375 \times 136 = \mathbf{5.10\ g}\).
(f) Cracking is the breaking down of large (long-chain) hydrocarbon molecules into smaller, more useful molecules using heat and/or a catalyst.
(g) Any two: atomic size (radius); nuclear charge; shielding (screening) effect of inner electrons; existing electron configuration.
(h) Carbohydrates are organic compounds of carbon, hydrogen and oxygen in which hydrogen and oxygen are present in the ratio 2:1 (general formula \(C_x(H_2O)_y\)); they are polyhydroxy aldehydes or ketones.
(i) Any two: a simple sugar is soluble in water while starch is insoluble; a simple sugar is sweet while starch is not; a simple sugar is a small molecule while starch is a large polymer (macromolecule); a simple sugar reduces Fehling's solution while starch does not.
(j)(i) \(H_2CO_3 \rightleftharpoons 2H^+ + CO_3^{2-}\)
(ii) \(CH_3COOH \rightleftharpoons CH_3COO^- + H^+\)
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