HW-Solns-Chap-6-1413

# We want to burn 100 mol isooctane so multiplier 100

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(b) The original expression burns 2 mol isooctane. We want to burn 100. mol isooctane. So, Multiplier 100. mol 2 mol 50.0 50.0 [2 C 8 H 18 ( ) + 25 O 2 (g) 16 CO 2 (g) + 18 H 2 O( )] H° = 50.0 (– 10,992 kJ) 100. C 8 H 18 ( ) + 1250 O 2 (g) 800. CO 2 (g) + 900. H 2 O( ) H° = –5.50 10 5 kJ (c) The original expression burns 2 mol isooctane. We want to burn 1.00 mol isooctane. So, Multiplier 1.00 mol 2 mol 0.500 0.500 [2 C 8 H 18 ( ) + 25 O 2 (g) 16 CO 2 (g) + 18 H 2 O( )] H° = 0.500 (– 10,992 kJ) 1.00 C 8 H 18 ( ) + 12.5 O 2 (g) 8.00 CO 2 (g) + 9.00 H 2 O( ) H° = –5.50 10 3 kJ Reasonable Answer Check: It makes sense that when more moles are involved, the H° is larger, and when fewer moles are involved, the H° is smaller. 41 . Answer: –1.45 10 3 kJ/mol Strategy and Explanation: Given a chemical equation for the combustion of a fuel, the mass of fuel burned, and the thermal energy evolved at constant pressure for the reaction, determine the molar enthalpy of combustion of the fuel. Molar enthalpy change ( H°) is identical to the thermal energy released at constant pressure per mol of substance. So, convert the mass into moles, then divide the thermal energy by the moles to get the molar enthalpy of combustion. The balanced thermochemical expression tells us that exactly 1 mol C 2 H 5 OH burns to produce heat energy. q = –3.62 kJ (It is negative since heat energy is evolved, rather than absorbed.) n 0.115 g C 2 H 5 OH 1 mol C 2 H 5 OH 46.0682 g C 2 H 5 OH 0.00250 mol C 2 H 5 OH

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