Using the bondenthalpies tabulated below, calculate the enthalpy of reaction forthe combustion of hydrogen. Hint: The bond enthalpies areapproximate enthalpies needed to make or break a chemicalbond. 2H2(g) + O2(g) 2H2O(g) bondH(kj) H-H 436 O-H 464 O-O 138 0=0 498 Using the bondenthalpies tabulated below, calculate the enthalpy of reaction forthe combustion of hydrogen. Hint: The bond enthalpies areapproximate enthalpies needed to make or break a chemicalbond. 2H2(g) + O2(g) 2H2O(g) bondH(kj) H-H 436 O-H 464 O-O 138 0=0 498 H-H 436 O-H 464 O-O 138 0=0 498
Explanation / Answer
2H2(g) + O2(g) 2H2O(g) The enthalpy of the reaction is the energy of the bond beingbroken minus the energy of the bond being formed. In this reaction two H-H bonds and one O-O bond is broken . Two O-H bonds are formed . Hrxn = [ 2H-H + O-O ] - [ 4 O-H] = [ 2 * 436 kJ + 138 kJ ] - [ 4 * 464 kJ ] = 1010 - 1856 kJ = - 846 kJ