NJC_H2_CHEM_P2_Solutions.pdf

NJC_H2_CHEM_P2_Solutions.pdf - 1 NJC H2 Chemistry Prelim...

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1 NJC H2 Chemistry Prelim Paper 2 Suggested Answers 1 (a) Due to high temperature in the car engine , N 2 and O 2 from the air can react to form NO 2 . NO 2 can be removed from the exhaust gas with the use of a catalytic converter , it can be reduced by CO to form harmless N 2 . (b) (i) (ii) NO 2 , being a radical , is very reactive and reacts with other gases in the air and gets destroyed more readily, hence a shorter atmospheric residence time (c) (i) ΔH o = +9.2 2(+33.2) = 57.2 kJ mol 1 ΔS o = +304 2(+240) = 176 J mol 1 K 1 (ii) ΔH o has a negative sign since the dimerization is a bond formation process , hence heat is given out / reaction is exothermic. ΔS o has a negative sign since the dimerization results in fewer gas particles , hence there is a decrease in the disorderliness of the system. (iii) At equilibrium, ΔG = 0 ΔH o TΔS o = 0 57.2 T( 176/1000) = 0 T = 325 K (iv) When N 2 O 4 liquefies, ΔG = 0 ΔH vap TΔS vap = 0 ΔH vap = TΔS vap = 294 x 88 = +25.9 kJ mol 1 (iv) Using Hess’s law, = For ΔH rxn = 57.2 +25.9 = 83.1 kJ mol 1 For ΔS rxn = 176 88 = 264 J mol 1 K 1 2 (a) (i) H 2 O 2 oxidises the I , to aqueous I 2 , so a brown solution / black solid would be obtained. H 2 O 2 + 2I + 2H + → I 2 + 2H 2 O
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