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152 chemIstry Adding equation (i) and (iii), we get the Similarly, combustion of glucose gives out desired equation, 2802.0 kJ/mol of heat, for which the overall 1 equation is : C ▯raphite,s ▯▯ O 2▯▯▯CO g ; ▯ ▯ 2 C 6 12(g6▯ 6O (g)2▯ 6CO (g)▯ 2H O(1); 2  –1 for which ∆ r  = (– 393.5 + 283.0) ∆ C = – 2802.0 kJ mol –1 Our body also generates energy from food = – 110.5 kJ mol by the same overall process as combustion, In general, if enthalpy of an overall although the final products are produced after reaction A→B along one route is ∆ H andr a series of complex bio-chemical reactions ∆ H , ∆ H , ∆ H ..... representing enthalpies r 1 r 2 r 3 involving enzymes. of reactions leading to same product, B along another route, then we have Problem 5.9 ∆rH = ∆ r 1 ∆ Hr+2∆ H r.3 (5.16) The combustion of one mole of benzene It can be represented as: takes place at 298 K and 1 atm. After combustion, CO (g)2and H O (12 are A ∆rH B produced and 3267.0 kJ of heat is liberated. Calculate the standard  ∆H 1 ∆ r 3 enthalpy of formation, ∆ f of benzene. Standard enthalpies of formation of C D CO (g) and H O(l) are –393.5 kJ mol –1 ∆ H 2 2 –1 r 2 and – 285.83 kJ mol respectively. 5.5 E nthalpiesfor di ff erenttypes Solution of reactions The formation reaction of benezene is It is convenient to give name to enthalpies given by : specifying the types of reactions. 6C ▯raphite ▯▯3H g 2 ▯ ▯ l ; 6 6 ▯▯ (a) Standard Enthalpy of Combustion   ∆fH = ? ... (i) (symbol : ∆ cH ) The enthalpy of combustion of 1 mol Combustion reactions are exothermic in of benzene is : nature. These are important in industry, 15 rocketry, and other walks of life. Standard C6H 6▯▯▯ O 2 CO g ▯ H2▯ ▯ ;3 2 ▯ ▯ enthalpy of combustion is defined as the 2 ∆ H  = – 3267 kJ mol ... (ii) enthalpy change per mole (or per unit amount) C of a substance, when it undergoes combustion The enthalpy of formation of 1 mol of and all the reactants and products being CO (g) : in their standard states at the specified 2 C ▯raphite ▯▯O g2▯▯C▯ g ; 2▯ ▯ temperature.  –1 Cooking gas in cylinders contains mostly ∆ f = – 393.5 kJ mol ... (iii) butane (C 4H 10 During complete combustion The enthalpy of formation of 1 mol of of one mole of butane, 2658 kJ of heat is H 2(l) is : released. We can write the thermochemical 1 H 2▯▯▯ O 2▯▯▯H O l2; ▯ ▯ reactions for this as: 2 13 ∆ H  = – 285.83 kJ mol ... (iv) C 4 (10▯ O 2g) ▯ CO (g)2 H O(1);2 C 2 multiplying eqn. (iii) by 6 and eqn. (iv) ∆ H  = – 2658.0 kJ mol –1 by 3 we get: C Reprint 2026-27