Use bond energies to calculate the heat of reaction

Question

David · Accepted Answer

Bond energy for the 
 bonds required                 kJ/mole
C=O                                   745 
N-H                                    393 
O-H                                    460 
C-N                                     276
Heat of the reaction = Sum of energy of products - Sum of energy of the reactants 
                                 = ( [CO2] + 2 * [NH3] ) – ( [urea] + [H2O] )
                                 = ( 2 * [C=O] + 2 * [3 * N-H]) – ( [2 * 2 * N-H] + [2 * C-N] + [C=O] + [2 * 460]
                                 = ( 2 * [745] + 6 * [393] ) – ( 4 *[393] + 2 * [276] + [745] + [2 * 460]
                                 = ( 1490 + 2358 ) – (1572 + 552 + 745 + 920)
                                 = (3848 ) – (3789)
                                 = - 59 kJ
    Heat of the reaction =  - 59 kJ indicating that it is an exothermic reaction

Use bond energies to calculate the heat of reaction

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