Bond Energy Calculations

Bond energy values ARE given in the exam.C-H          435KjO=O         497KjC=O         803KjO-H          464Kj

STEP 1Energy needed to break bonds in reactants: 4* (C-H) + 2 (O=O)4*(435) + 2*(497) = 2734 Kj/mol     1740 +  994This is the energy GAINED by chemicals ENDOTHERMIC (into chemicals),

STEP 2Energy released when new bonds form in new products2*(C=O) + 4* (O-H)2* (803)  + 4*(464) = 3462 Kj/mol   1606    +     1856This is the energy given out to the surroundings from the chemicals ( e.g. Burning fuels) EXOTHERMIC

STEP 3The energy DIFFERENCE is change in heat energyH =+2,734-3462=-728Kj/mol

The exam often requires you to EXPLAIN in words why the reaction is EXOTHERMIC overall use this format:"more energy released when new bonds are made (exothermic), then was required to break bonds in the reactants (endothermic)."

Page 1