March 28, 2024, 09:24:36 PM
Forum Rules: Read This Before Posting


Topic: Hess law insane problem  (Read 9187 times)

0 Members and 1 Guest are viewing this topic.

Offline Rutherford

  • Sr. Member
  • *****
  • Posts: 1868
  • Mole Snacks: +60/-29
  • Gender: Male
Hess law insane problem
« on: April 11, 2013, 02:15:44 PM »
Determine ΔH for the reaction: 4CO + 8H2 ---> 3CH4 + CO2 + 2H2O given the following data: (attached).

By what logic can I solve this problem when more than one possible ways of solving it exist, but only one is correct?

Offline curiouscat

  • Chemist
  • Sr. Member
  • *
  • Posts: 3006
  • Mole Snacks: +121/-35
Re: Hess law insane problem
« Reply #1 on: April 11, 2013, 02:18:17 PM »
By what logic can I solve this problem when more than one possible ways of solving it exist, but only one is correct?

If there's two valid ways they must both give the same answer.

Offline curiouscat

  • Chemist
  • Sr. Member
  • *
  • Posts: 3006
  • Mole Snacks: +121/-35
Re: Hess law insane problem
« Reply #2 on: April 11, 2013, 02:21:47 PM »
One rigorous way would be assume coefficients a,b,c,d and e. One for each reaction.

And then balance out each of the 5 species in your target equation.

5 equations in 5 unknowns. Solve.

Offline Rutherford

  • Sr. Member
  • *****
  • Posts: 1868
  • Mole Snacks: +60/-29
  • Gender: Male
Re: Hess law insane problem
« Reply #3 on: April 11, 2013, 02:51:49 PM »
There are more than one answer. If I started, e.g. by multiplying (b) by 2 and then deducting 2*(a) from it, I can adjust the other equations to get the target equation. If I started by multiplying (b) by 5 and then adding (a), I can adjust the other equations to get the target equation. There are, I think infinite number of ways to get the target equation, but the values for ΔH would be always different. What logic could help me here, or is this a hole in Hess' law?

Offline curiouscat

  • Chemist
  • Sr. Member
  • *
  • Posts: 3006
  • Mole Snacks: +121/-35
Re: Hess law insane problem
« Reply #4 on: April 11, 2013, 02:53:54 PM »
There are more than one answer. If I started, e.g. by multiplying (b) by 2 and then deducting 2*(a) from it, I can adjust the other equations to get the target equation. If I started by multiplying (b) by 5 and then adding (a), I can adjust the other equations to get the target equation. There are, I think infinite number of ways to get the target equation, but the values for ΔH would be always different. What logic could help me here, or is this a hole in Hess' law?

ΔH should come the same each way.

Out of these infinite ways you think there are show me just two.

Offline Rutherford

  • Sr. Member
  • *****
  • Posts: 1868
  • Mole Snacks: +60/-29
  • Gender: Male
Re: Hess law insane problem
« Reply #5 on: April 11, 2013, 03:52:30 PM »
1° ΔH=-2(a)+2(b)+4(c)+2(d)-(e)
2° ΔH=-(a)+3(b)+6(c)+(d)-2(e)...

*The letters represent the equations.

Offline curiouscat

  • Chemist
  • Sr. Member
  • *
  • Posts: 3006
  • Mole Snacks: +121/-35
Re: Hess law insane problem
« Reply #6 on: April 11, 2013, 03:56:57 PM »
1° ΔH=-2(a)+2(b)+4(c)+2(d)-(e)
2° ΔH=-(a)+3(b)+6(c)+(d)-2(e)...

*The letters represent the equations.

Did you calculate numerical values? Are we  sure they don't end up the same?

Offline curiouscat

  • Chemist
  • Sr. Member
  • *
  • Posts: 3006
  • Mole Snacks: +121/-35
Re: Hess law insane problem
« Reply #7 on: April 11, 2013, 04:00:20 PM »
Ok, fine I did the heavy lifting for you. I get -747.2 both ways.

So, what's wrong? I don't see any problems.

Offline Rutherford

  • Sr. Member
  • *****
  • Posts: 1868
  • Mole Snacks: +60/-29
  • Gender: Male
Re: Hess law insane problem
« Reply #8 on: April 11, 2013, 04:23:21 PM »
Hmm, now I see that these answers are the same, but when I first attempted this, I got on two different ways two different answers, so I concluded what I did. I will check my papers to see whether I mistaken somewhere.

Offline Big-Daddy

  • Sr. Member
  • *****
  • Posts: 1177
  • Mole Snacks: +28/-94
Re: Hess law insane problem
« Reply #9 on: April 11, 2013, 04:27:30 PM »
What's insane about this problem is that it gives you 5 equations when only need 3. That's just wrong.

Rxn(desired)=4*Rxn(2)+8*Rxn(3)-3*Rxn(5)
ΔHr°=-747.1 kJ·mol-1

I'm sure your answer will come out like this however you do it.

One rigorous way would be assume coefficients a,b,c,d and e. One for each reaction.

And then balance out each of the 5 species in your target equation.

5 equations in 5 unknowns. Solve.

I'm not sure what you're doing here ... could you write maybe 2 of those equations so I can understand?

Offline curiouscat

  • Chemist
  • Sr. Member
  • *
  • Posts: 3006
  • Mole Snacks: +121/-35
Re: Hess law insane problem
« Reply #10 on: April 11, 2013, 04:31:45 PM »
What's insane about this problem is that it gives you 5 equations when only need 3. That's just wrong.

Not at all. One skill is to know what's useful data.  Extraneous data is a good distraction tool.

When I apply Hess's Law in practice the Handbook has maybe 2000 equations of which I need 4.

Offline Big-Daddy

  • Sr. Member
  • *****
  • Posts: 1177
  • Mole Snacks: +28/-94
Re: Hess law insane problem
« Reply #11 on: April 12, 2013, 06:54:29 AM »
What's insane about this problem is that it gives you 5 equations when only need 3. That's just wrong.

Not at all. One skill is to know what's useful data.  Extraneous data is a good distraction tool.

When I apply Hess's Law in practice the Handbook has maybe 2000 equations of which I need 4.

Fair enough. What's the method using simultaneous equations? I like to apply algebra to these things.

Offline Rutherford

  • Sr. Member
  • *****
  • Posts: 1868
  • Mole Snacks: +60/-29
  • Gender: Male
Re: Hess law insane problem
« Reply #12 on: April 12, 2013, 07:54:26 AM »
I'm sorry, I've actually mistaken when I attempted this for the first time. One change of sign lead me to perplexity  :-[.

Offline curiouscat

  • Chemist
  • Sr. Member
  • *
  • Posts: 3006
  • Mole Snacks: +121/-35
Re: Hess law insane problem
« Reply #13 on: April 12, 2013, 08:33:46 AM »
Fair enough. What's the method using simultaneous equations? I like to apply algebra to these things.

I haven't tried but my idea was something like:

a *(C + 0.5 O2 -CO ) +b *( CO + 0.5 O2 -CO2) + .= 4 CO + 8H2 -3 CH4 -CO2 -2 H2O

now on left side collect terms containing CO, O2, H2 etc.

Eventually the expression for CO equates to 4, for H2 8 and so on.

For O2 etc. equate to zero since they are absent in the target. You should get 7 equations in 7 unknowns (if my calculations are right!)

That ought to give you all potential solutions.

Again, I haven't tried it but since you seem so interested try it out and let me know. I hope your enthusiasm can motivate you to do the number crunching? 

Offline curiouscat

  • Chemist
  • Sr. Member
  • *
  • Posts: 3006
  • Mole Snacks: +121/-35
Re: Hess law insane problem
« Reply #14 on: April 12, 2013, 08:35:44 AM »
I'm sorry, I've actually mistaken when I attempted this for the first time. One change of sign lead me to perplexity  :-[.

It happens.  :)

Next time be more patient before calling a problem insane. You could have hurt Hess's feelings.

Sponsored Links