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# enthalpy change for the combustion of methanol watch

1. A team of racing engineers found that when 8.01g of methanol was combusted 172 kJ of heat energy was released.
Reaction of the combustion of methanol:
2CH3OH(I) + 3O2(g) --> 2CO2(g) + 4H2O(I)
Calculate the enthalpy change for the combustion of methanol based on the above measurements M(CH3OH) = 32.0g mol-1
2. What do you understand to be the definition of enthalpy change of combustion?
3. (Original post by TutorsChemistry)
What do you understand to be the definition of enthalpy change of combustion?
I know that enthalpy change is the amount of heat absorbed in a reaction and the symbol is ΔH meaning delta H
4. (Original post by Student 321)
I know that enthalpy change is the amount of heat absorbed in a reaction and the symbol is ΔH meaning delta H
OK. That would be an endothermic reaction, if the energy were absorbed.
We can also release energy during a reaction, i.e. an exothermic reaction. Exothermic reactions are more common than endothermic reactions.

(A combustion, such as the reaction you are being asked about, will always be exothermic).

What units do we use for enthalpy change?
5. What units do we use for enthalpy change?[/QUOTE]

I think it's kJ?
6. (Original post by Student 321)
What units do we use for enthalpy change?
I think it's kJ?[/QUOTE]

kJ is a quantity of energy, but is not an enthalpy change.

Enthalpy changes are the energy change per mole for a substance, and are measured in kJ mol^-1.

In the example question, the enthalpy of combustion of methanol is the quantity of energy released per mole of methanol when the methanol is combusted.

You have been given the energy change for 8.01g of methanol. How many moles of methanol is 8.01g?
7. I'm not sure because I don't know how to work it out
8. (Original post by Student 321)
I'm not sure because I don't know how to work it out
Can you work out the RMM of methanol?

Moles = mass / RMM
9. (Original post by TutorsChemistry)
Can you work out the RMM of methanol?

Moles = mass / RMM
Is it 172/32.0g mol^-1
10. I'm going to suggest you go and re-read your notes and textbook on the topics of enthalpy and moles.

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Updated: November 15, 2017
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