Enthalpy of Formation and Enthalpy of Reaction

∆Hr is the enthalpy for the entire reaction, EXACTLY as it is written.  If you change the coefficients or states, you will also change the heat absorbed or evolved.


∆Hf is the enthalpy of formation.  It is the heat absorbed or given off for the synthesis of ONE mole of specific substance in a specific state.  Remember this is ONLY for a SYNTHESIS reaction.  Because it is always for 1 mole, the units of ∆Hf will always be kJ/mole. The ∆Hf for a free element in its state a standard conditions will always equal 0 kJ.

The wonderful thing about heat of formation is that you can use tables to look up specific substances in given states, then use the ∆Hf to find the ∆Hr.  The heat of the entire reaction is equal to the sum of all the heat from the product side minus the heat from the reactant side.  Remember that in science, ∆ means change or difference and we always use FINAL - INITIAL.

Here's an example:

First, you need a balanced reaction with states.  Remember that ∆Hf is given in kJ/mole, so you will need to multiply by the number of moles in the balanced reaction (the coefficients).  You need states to choose the correct value from the table.

The enthalpy of the reaction therefore is:

By using the tables, ∆Hr = -89.3 kJ

Try another example