[mz]

 

Could someone please tell me which thermochemistry formula to use in the following question?

If a system does 200kJ of work and expels 400kJ of heat and the surroundings absorb 300 kJ heat and does 405kJ of work on the system, what is the remaining amount of energy left?

I am in an 8-week, accelerated Chemistry class.  We only covered thermochemistry in one class, and we didn't go into any problems where both q and w are given for the system & the surroundings.

Thanks for your help.  

[Mitch]

Energy left in the system or in the sorroundings? I guess one would just be the negative of the other though.

Plus most p-chemist usually only have "the system" on the brain.

[mz]

I don't know which one.  That's all the question has.  

[Donaldson Tan]

have u heard of the term arbituary zero?

dU = Q + W

[mz]

No, we didn't go over that.  I'll look that up in the book, and see if I can figure anything up.  Thanks.

[mz]

It's not in the book's glossary or index.  So, I'm going to assume that I'm supposed to use a different formula.  

So, now I'm back to being confused again.

[lemonoman]

There is probably no 'set' formula for this question.  It's an exercise in logic.

If a system does work or expels heat, it is losing energy.
If a system gets work done on it or is absorbing heat, it is gaining energy.

If the surroundings are doing work, or are expelling heat, the surroundings are losing energy == the system is GAINING energy
If the surroudings are getting work done on it or is absorbing heat, then the surroundings are gaining energy == the system is lowing energy.

You just have to know what energy quantities are being added and subtracted from the initial energy.

Only problem is you're not given an initial energy (in the question you gave us anyways)...so this is where 'arbitrary zero' comes in, I bet.  Try a google search, and hopefully everything will piece together.

[mz]

Thanks.  ..we did learn "absolute zero", not sure if that has anything to do with it.

I'll try to look for some other ways to solve this.

Thanks for your help.

[lemonoman]

In any case, the pointis, that if the question is EXACTLY as you give it, with no extra informatino (like an initial amount of energy)...then I don't think you can solve it at all.  It only give you changes in energy, and doesn't tell you how much you started with.

It's the equivalent of, "If I take away 3 eggs, how many do you have left?"  You have no diea...you only know you have 3 less than you started with.