Enthelpy change for chemical reaction

In summary: Unfortunately, many test-makers do not follow this rule, and instead give the information per mole of the reactants. Which can be confusing.
  • #1
sgstudent
739
3
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!
 
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  • #2
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.
 
  • #3
Borek said:
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.

But usually which onr would it be? Like in the O levels? Thanks for the help Borek!
 
  • #4
Since it tells you that ΔH=-98kJ/mol, the decomposition of 2 moles of hydrogen peroxide has ΔH=-196kJ.
 
  • #5
Jasoni22 said:
Since it tells you that ΔH=-98kJ/mol, the decomposition of 2 moles of hydrogen peroxide has ΔH=-196kJ.

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!
 
  • #6
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.
 
  • #7
Borek said:
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.

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!
 
  • #8
sgstudent said:
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?

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.

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?"

That would be the correct way.
 
  • #9
Thanks for the help Borek! :smile:
 

1. What is enthalpy change for a chemical reaction?

Enthalpy change for a chemical reaction refers to the heat energy that is either released or absorbed during a chemical reaction.

2. How is enthalpy change measured?

Enthalpy change is measured in units of joules (J) or kilojoules (kJ) using a calorimeter, which is a device that measures the heat flow of a reaction.

3. Why is enthalpy change important in chemical reactions?

Enthalpy change is important because it helps us understand the energy changes that occur during a chemical reaction, which can provide insight into the stability and feasibility of a reaction.

4. How does enthalpy change affect the equilibrium of a reaction?

Enthalpy change can affect the equilibrium of a reaction by influencing the direction in which the reaction proceeds. If the enthalpy change is positive (endothermic), the reaction will favor the reactants at equilibrium. If the enthalpy change is negative (exothermic), the reaction will favor the products at equilibrium.

5. Can the enthalpy change of a reaction be predicted?

The enthalpy change of a reaction can be predicted using thermodynamic data such as bond energies and standard enthalpies of formation. However, these predictions may not always be accurate due to the complexities of chemical reactions.

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