Homework Help: Chemistry

Posted by Jarrod on Thursday, November 4, 2010 at 1:30am.

Hydrogen and methanol (CH3OH) have both been proposed as alternatives to hydrocarbon fuels.

a) Write balanced chemical equations for the complete combustion of hydrogen and the complete combustion of gaseous methanol.

Combustion reaction for hydrogen:

2H2(g)+ O2 --> 2H2O(l)

Combustion reaction for methanol:

2CH3OH(g) + 3O2(g) --> 2CO2(g) + 4H20(l)

b) Now use standard enthalpies of formation to calculate Delta H reaction for each of these reactions.

Delta H reaction for hydrogen combustion= -285.8 Delta H

[(2*0)+(1*0)] - [(-285.8)] = -285.8

Delta H reaction for methanol combustion = -1135 Delta H

[(2*-393.5)+(4*-187.8)] - [(2*-201.0)+(3*0)]=
-1537-(-402)=-1135 Delta H

c) Finally, calculate the amount of heat released per kilogram of the fuel.

I am having troubles with this one. What I am thinking is that I use the Delta H of what I got above which is in kJ/mol so for hydrogen -285.8kJ/mol and since this is hydrogen gas I figured that a molar mass would be appropriate since it has grams and moles but when I looked at it harder I don't see how I am going to get rid of the kJ with a molar mass. There must be a step I am missing?

Chemistry - DrBob222, Thursday, November 4, 2010 at 2:57pm
For hydrogen combustion above. You have calculated that heat released is 285.8 (you don't write a unit; I presume this is kJ/mol) so we go with -285.8 kJ/4 g hydrogen. We want heat 1 kg (1000 g).
-285.8 kJ x (1000 g/4 g) = ??

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