Enthalpy change for chemical reaction

sgstudent

hi i'm taking GCE O levels so this might give you an idea of my level of chemistry. For a reaction like 2H2O2-->2H2O+O2 and the question tells me it has an ΔH=-98kJ/mol so does it mean that when 2 moles of hydrogen peroxide decomposes to form water and oxygen, the net amount of heat generated is 98kJ? So if i half the equation into H2O2-->H2O+1/2O2 the ΔH=-39kJ?

Thanks for the help!

Borek

You are most likely right, although details may depend on the exact wording. ΔH is usually given per mole - either so called "mole of reaction", or a mole of selected reactant/product.

sgstudent

But usually which onr would it be? Like in the O levels? Thanks for the help Borek!

Jasoni22

Since it tells you that ΔH=-98kJ/mol, the decomposition of 2 moles of hydrogen peroxide has ΔH=-196kJ.

sgstudent

I don't get it, what does the -98kj/mol even mean? I'm getting really confused here. What if they don't tell us whether it is per mole of something like they just tell you that its -98kJ/mol then what do I assume? Is it right to say if they tell you that its per mole of one of the reactants than its per mole of that reactants but if they don't than it could be as Borek said "mole of reaction". How do I tackle this issue? Thanks for the help!

Borek

I checked - it is -98kJ per mole of hydrogen peroxide decomposed.

Unfortunately you will find such ambiguous information way too often. In this case they probably mean "per mole of hydrogen peroxide decomposed", but your confusion is perfectly justified and it is not your fault. When in doubt - ask for clarification. During test you can write "I am assuming data given means -98kJ per mole of hydrogen peroxide decomposed" and follow from there - at least it will be easier for you to defend your solution if they meant something else.

sgstudent

So for example a question gives:
Methane, CH4, burns in excess oxygen to form carbon dioxide and water
CH4+2O2-->CO2+2H2O ΔH=&890 kJ/mol
Calculate the energy release when 2.4g of methanoic is burnt

In the answer that treated the per mole to be for the number of moles of CH4 when they did the mole calculation. So I'm guessing in exams they ought to make it clearer like saying: when 1 mole of methane burns completely it produces 40kJ of heat. Then they continue to write the information?

But four multiple choice questions then am I in a way in the mercy of the marker? If I were to ask the examiner for the information can I ask it like this:"I'm not very sure if they are trying to imply whether its one mole of (name of compound) being reacted or one mole of the entire equation reacted. can I assume that it refers to per mole of the entire equation?"

Thanks for the help Borek!

Borek

Yes. When it is about burning you can usually assume ΔH is given per mole of burnt substances, but in the case like

2KMnO₄ + 5H₂O₂ + 3H₂SO₄ -> 2MnSO₄ + K₂SO₄ + 5O₂ + 8H₂O

stating ΔH=xkJ/mol is meaningless.

That would be the correct way.

sgstudent

Thanks for the help Borek!