Self-Test 7.12A

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Alwel7041
Posts: 2
Joined: Fri Sep 20, 2013 3:00 am

Self-Test 7.12A

Postby Alwel7041 » Sun Jan 12, 2014 3:31 pm

The thermochemical equation for the combustion of cyclohexane, C6H12 is:
C6H12(l) + 9O2(g) --> 6CO2(g) + 6H2O(l)
Enthalpy Change (H)=-3920kJ at 298K.
What is the change in internal energy for the combustion of 1 mol C6H12(l) at 298K?

I understand that the equation to convert between H and U is
ChangeH=ChangeU + (ChangeMolsGas x RT)

In this equation, it seems that gas is being consumed so ChangeMolsGas=6-9=-3
When I plugged all the values into the equation, I had the right numbers but my answer was positive rather than negative.
As gas is being consumed, shouldn't the ChangeH be smaller than ChangeU?

Chem_Mod
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Re: Self-Test 7.12A

Postby Chem_Mod » Sun Jan 12, 2014 10:22 pm

What was your final answer? This question is answered correctly in the thread "calculating net formation of gas (moles)."

104277942
Posts: 161
Joined: Fri Sep 20, 2013 3:00 am

Re: Self-Test 7.12A

Postby 104277942 » Wed Jan 15, 2014 12:23 am

You should have set up your problem as

-3920000 J - (-3*8.3145 J/k mol*298 K)=

-3,912,566.837 J =

-3.91 x 10^3 kJ.

Make sure you don't skimp out on any constants they give you or your numbers will be slightly off!


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