# Urgent: Need help on Chemistry Problem - UPDATED PROBLEM ON PG 2

Discussion in 'The Lounge' started by colbystephens, Jan 30, 2006.

1. ### colbystephens1 ton statusGMOTM Winner

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Help! we're going over Hess's law. how is q=mass x specific heat x change in temp. related to the change in enthalpy of the reaction? this shouldn't be a hard concept for me - but i just can't seem to make the connection. can you help??

Last edited: Jan 30, 2006
2. ### BranndonC3/4 ton status

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no i'm dummified

3. ### daleearnhardt011/2 ton status

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Fun stuff Im doing the same exact thing in my chem class. What is the problem...

4. ### colbystephens1 ton statusGMOTM Winner

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the question is how do you relate q to delta H where delta H is the change in enthalpy of the reaction.

5. ### daleearnhardt011/2 ton status

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Hmm hate to say but Im just as confused as you are. If it was a math problem then Id be all over it but those theory questions get me...

6. ### mikey_d051 ton statusGMOTM Winner

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/\H is equal in magnitude to heat flow (q) for a reaction carried out directly while the pressure is held constant. I.E., when /\H(system) is equal to H(final) minus H(initial), and pressure is held constant, /\H(system) = q.

7. ### 87BrnRsd1/2 ton status

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I dont think its gonna help if its not in english.
-Harrison

8. ### ARAMP1Aviator Extraordinaire

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Don’t know if this will help, but here it goes.

In a chemical reaction (or in pretty much all thermodynamic analyses) the sum of the internal energy (U) and the product of pressure (p) and volume (V) occur so frequently, that “they” just call that enthalpy (H).

So basically, H=U+pV

(not sure if you are talking specific heats where "C sub v" and "C sub p" are defined for pure, simple compressible substances as partial derivatives of the funcitons u(T,v) and H(T,p) where v and p denote the variables held fixed during differentiation.)

9. ### colbystephens1 ton statusGMOTM Winner

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thanks. actually, i found what i was looking for in that delta H = q/#mol reactant. thanks though!

10. ### ARAMP1Aviator Extraordinaire

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Well, just busted into my Thermo book and it just confused me more. Are you talking incompressible or compressible substances?

The good thing is that you'll (likely) never use this stuff when you get out of school.

11. ### mikey_d051 ton statusGMOTM Winner

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Sorry, sounded like you'd already explored that one.

12. ### colbystephens1 ton statusGMOTM Winner

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ok. here's another one. "An alloy containing magnesium and another metal, that does not react with hydrochloric acid, needs to be analyzed. You are asked to determine the percent magnesium in the alloy. a) describe the procedure you would use to determine the percent magnesium in the alloy, and b) if the sample were 30% magnesium, calculate the heat evolved if a 5 gram sample were analyzed in that manner."

13. ### mikey_d051 ton statusGMOTM Winner

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Have any constraints on the procedure?

The simplest way would be to figure out the specific heat of your sample using a calorimeter. Given that, the specific heat of Mg, and the weight of your sample you could figure out what percentage of it is Mg. From that you could figure out the mass of the mystery substance and calculate the specific heat of it. That'd nail it down pretty well if your results were accurate. However, part b of your question leads me to believe you're supposed to break this alloy down chemically or through some other method. If that's the case, I can't help you. I was done with chemistry as of last semester.

14. ### colbystephens1 ton statusGMOTM Winner

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well, this is a follow up to a procedure which looked like this:

1. Mg(s) + 2H+(aq) à Mg2+(aq) + H2(g) ΔH*1
2. Mg2+(aq) + H2O(l) à MgO(s) + 2H+(aq) ΔH*2
3. H2(g) + ½O2(g) à H2O(l) ΔH*3 = -285.9 kJ (Given)

4. Mg(s) + ½O2(g) à MgO(s) ΔH*4

where, using a calorimeter we determined 1 and 2, and using hess's law we determined 4. so, i would imagine that the question i just posted should go along these lines - which is what i think you were thinking. can youhelp me determine part b? i'm having trouble - i need to catch up in class.

15. ### mikey_d051 ton statusGMOTM Winner

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I would if I knew what "heat evolved" meant. Once you've figured out the specific heat of your mystery compound, you can figure out the rate at which your alloy would gain and lose heat, but you have to know how many joules of energy are tranferred to it in the first place.

16. ### colbystephens1 ton statusGMOTM Winner

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hmmmmm. well i gotta go to class now. we'll see what happens.

17. ### colbystephens1 ton statusGMOTM Winner

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btw - heat evolved is heat produced by the rxn.

18. ### mikey_d051 ton statusGMOTM Winner

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If you have a set of experimental data I guess you could use your calorimeter constant and other parameters from your lab but if not, you should have been given more information.

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