Three steps are involved:
- Calculate the energy needed to break the bonds in the reactant molecules,
*H*_{r}.
- Calculate the energy released in the making of new bonds in the product molecules,
*H*_{p}.
- Calculate the sum of the two totals,
*H*_{p}
+ *H*_{r}.

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.

C_{8}H_{18}(g) + O_{2}(g) ®
8CO_{2}(g) + 9H_{2}O(g) D*H* = -5470.5
kJ mol^{-1}

If the sign is **positive** this represents an endothermic reaction.

2HI(g) ® H_{2}(g) + I_{2}(g)
D*H* = + 11.3 kJ mol^{-1}

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.

N_{2}(g) + O_{2}(g) ® 2NO(g)