Calculating the enthalpy of reaction from bond energies
Three steps are involved:
  1. Calculate the energy needed to break the bonds in the reactant molecules, Hr.
  2. Calculate the energy released in the making of new bonds in the product molecules, Hp.
  3. Calculate the sum of the two totals, Hp + Hr.

Have a go! Use the data from the table right (top) to enter values in the boxes in the green area. Ensure you get the signs correct. Press RESULT to calculate the enthalpy of reaction  and to see how many entries were correct.

Questions image Is this reaction an exothermic or an endothermic reaction?

Have a go! Try other self assessment exercises:

N2(g) + O2(g) ® 2NO(g)

H2(g) + Cl2(g) ® 2HCl(g)

2H2(g) + O2(g) --> 2H2O(g)

Bond Bond energy
E / kJ mol-1		 Bond Bond energy
E / kJ mol-1
C-C 346 H-H 436
C-O 358 N-H 391
O-H 464 NºN 946
O=O 498 N=O 631
H-Cl 431 Cl-Cl 242
H-H 436    

 

Bond breaking Bond making

Hydrogen

Water

Moles hydrogen

Moles water

Oxygen

   

Moles oxygen

   

Enthalpy =  x + x + x

= kJ mol-1

Score =

 

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