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14th July 2017

What does the units “mol/dm3” refer to in standard enthalpy change of reaction?

Enthalpy Change of Reaction

Some of you may be thinking the enthalpy change of reaction just a general definition for any reaction but the units actually don’t refer to any specific reactants or products.

It refers to the molar quantities of all the reactants and products. For eg,

2H2 (g) + 2O2 (g) → 2H2O (l)

It means 2 moles of hydrogen reacting with 2 moles of oxygen to form 2 moles of water.

Thus, if you are to find the enthalpy change of reaction for:

H2 (g) + O2 (g) → H2O  (l)

The ΔHrxn will be half the value of the first equation.

If you are sharp, you might notice that this equation is also applicable for enthalpy change of combustion for hydrogen, enthalpy change of formation for water.

This means that some enthalpy definitions overlap and there’s nothing wrong with that 🙂

This enthalpy change of reaction is useful for equations that doesn’t neatly fall into common enthalpy definitions.

So it is important to state the appropriate equation for the particular ΔHrxn quoted!

Do remember for Energetics, it’s very important to include state symbols in your equations as different physical states has different energy levels.

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