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1 NJC H2 Chemistry Prelim Paper 2 Suggested Answers 1 (a) Due to high temperature in the car engine, N2and O2from the aircan react to form NO2. NO2can be removed from the exhaust gas with the use of a catalytic converter, it can be reduced by COto form harmless N2. (b) (i) (ii) NO2, being a radical, is very reactiveand reacts with other gases in the air and gets destroyed more readily, hence a shorter atmospheric residence time (c) (i) ΔHo= +9.2 –2(+33.2) = –57.2 kJ mol‒1ΔSo= +304 –2(+240) = –176 J mol‒1 K‒1(ii) ΔHohas a negative sign since the dimerization is a bond formation process, hence heat is given out / reaction is exothermic. ΔSohas 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 ΔHo–TΔSo= 0 –57.2 –T(–176/1000) = 0 T = 325 K (iv) When N2O4liquefies, ΔG = 0 ΔHvap–TΔSvap= 0 ΔHvap= TΔSvap= 294 x 88 = +25.9 kJ mol‒1 (iv) Using Hess’s law,= –For ΔHrxn= –57.2 –+25.9 = –83.1 kJ mol‒1For ΔSrxn= –176 –88 = –264 J mol‒1 K‒12 (a) (i) H2O2oxidises the I‒, to aqueous I2, so a brown solution/ black solid would be obtained. H2O2 +2I‒+ 2H+→ I2 + 2H2O
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