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Old Feb 9th 2018, 08:06 PM   #1
PrincessPeach
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Smile Thermochemical equations

Hi,

The combustion of 2.15g of methanol was found to yield 71.5kJ of heat.
Calculate the deltaH value for this value, and write the thermochemical equation.
This is what I have done so far:
2CH3OH(l) + 3O2 --> 2CO2(g) +4H2O(g)
n(CH3OH) = m/M = 3.15/32.04 = 0.098 mol
So 0.098 mol produces 71.5kJ of energy
1 mol produces 71.5kJ/0.098mol = 729.59 kJ?
So deltaH should be -729.59kJmol-1??
But the answer is −1.45103kJmol−1

Can someone help me clarify where I have gone wrong?
THANK YOU
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Old May 28th 2019, 09:03 PM   #2
mileylatrobe
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The question is the key!

2CH3OH +3O2- 2CO2 + 4H2O
To match the equation, you have to calculate the heat involved of 2 moles (64g) Not the 32 that you used. Try it and you will have the correct answer.
B/c they asked delta H of THIS reaction .
when 2 moles of methanol combusted -1.45X10(3) kJ of energy is produced . delta H refers to the equation as it is written, even though they expressed as kJ mol-1.
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