Self-Test 7.12A

Moderators: Chem_Mod, Chem_Admin

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?

Posts: 19515
Joined: Thu Aug 04, 2011 1:53 pm
Has upvoted: 880 times

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)."

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!

Return to “Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)”

Who is online

Users browsing this forum: No registered users and 4 guests