Thermochemical equations

Sep 2017
2
0
Earth
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 −1.45×103kJmol−1

Can someone help me clarify where I have gone wrong?
THANK YOU :D
 
May 2019
1
0
Victoria
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.