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Calculating enthalpy of reaction
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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.

The sign of the result is important. It is a component of a complete equation.
If the sign is negative this represents an exothermic reaction.
C8H18(g) + O2(g)    8CO2(g) + 9H2O(g) DH = -5470.5 kJ mol-1

If the sign is positive this represents an endothermic reaction.
2HI(g)     H2(g)  +  I2(g)     DH = + 11.3 kJ mol-1

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. (If you wish to see how to enter data, press 'EXAMPLE' then 'RESULT'.) Try other calculations below.

N2(g) + O2(g) 2NO(g)

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

H2(g) + Cl2(g) --> 2HCl(g)

Bond Bond energy
(kJ mol-1)
Bond Bond energy
(kJ mol-1)
C-C 346 H-H 436
C-O 358 N-H 391
O-H 46 NN 946
O=O 498 N=O 631
H-Cl 431 Cl-Cl 242
H-H 436    
Bond breaking Bond making

Hydrogen chloride

Moles hydrogen

Moles hydrogen chloride



Moles chlorine


Enthalpy =  x + x + x

= kJ mol-1

Score =


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1999 MATTER Project, The University of Liverpool

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