Chemistry Paper 2 (Essay) ,May/June 2011

Questions:		1	2	3	4	5	6	7	8		Main

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Question 8
(a)(i) Draw the energy profile diagram for the reaction; H2(g) + I2(g) → 2HI(g); DH = -13 kJ mol-1 (ii) If the concentration of HI(g) increases from 0.000 to 0.002 mol dm-3 in 80 seconds, what is the rate of the reaction? (b) (i) Give one use of each of the following compounds: I. NaHCO3; II. CaSO4; III. CaCO3. (ii) State a drying agent that can be used for each of the following gases: I. SO2; II. HCl; III. NH3 (c) (i) Write an equation for the complete combustion of carbon in oxygen. (ii) Calculate the number of moles of carbon (IV) oxide produced form the complete combustion of 2.5 g of carbon. [C = 12.0, O = 16.0 ] (iii) mention one use of I. carbon (II) oxide; II. carbon (IV) oxide. (d) An industrial raw material has the following composition by mass: Iron = 28.1%; Chlorine = 35.7% Water of crystallization = 36.2% Calculate the formula for the material. [H = 1.00, O = 16.0, Cl = 35.5, Fe = 56.0] (e) Give one example of a (i) metal that is liquid at room temperature. (ii) non-metal that is liquid at room temperature.
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OBSERVATION
This was another popular question. Most candidates did well in it. In part (a), few candidates could not draw the graph of exothermic reaction. Few did not label the axes and also failed to name the reactions and products. Most candidates got the calculation aspect but failed in the unit. In part (b), many candidates could not give the correct use of NaHCO3, CaSO4 and CaCO3 for SO2, HCl and NH3. In (c), most candidates were able to write the correct equation but did not indicate the states of the reactants and product. Many would not calculate the number of moles of CO2 produced. In (d), this part was well handled by majority of the responding candidates. Part (e), majority of the candidates were able to name a metal that is liquid at room temperature but unable to name a non-metal that is liquid at room temperature due to their poor knowledge of the graduation in physical properties of the halogen down the group. The expected answers are: (i) $Description: C:\Documents and Settings\Administrator\My Documents\SC DOC\scan0028.tif$ (ii) Rate = Change in Concentration = 0.002 – 0.000 time 80 = 0.002 80 = 2.5 x 10-5 moldm-3 sec-1 (b) (i) I. manufacture of baking powder II. manufacture of tetraoxosulphate (IV), plaster of Paris (P.O.P). III. cement making extraction of iron, solvay process, manufacture of glass, manufacture of putty. (ii) I. Conc. H2SO4; II. Conc. H2SO4; III. CaO. (c) (i) C(s) +O2(g) → CO2 reactants and products, states (ii) 12g C gives 1 mol CO2 2.5g C gives 1 x 2.5 mol 12 = 0.208 mol (iii) I. extraction of metals, manufacture of fuel, as gaseous fuel II. manufacture of fizzy drinks; fire extinguisher; cooling nuclear reactors; refrigeration. (d) Mole: Fe = 28.1 Cl = 35.7 H2O = 36.2 56 35.5 18 Mole ratio 0.5 1 2 0.5 0.5 0.5 1 2 4 (e) ∴ Fe Cl2 4H2O (e) (i) - Mercury (Hg) (ii) - Bromine (Br2)