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Topic: thermochemistry...  (Read 9407 times)

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777888

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thermochemistry...
« on: January 03, 2005, 11:06:35 PM »
1.What is enthalpy and what is the unit for it? Are enthalpy change (delta H) and amount of heat (q) the same thing?

2.Why delta H= + or - |q(surroundings)|? Does this simply mean that delta H and q(surroundings) have opposite signs?

3.What is the meaning of a "state function"?

Thanks!

Offline Mitch

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Re:thermochemistry...
« Reply #1 on: January 03, 2005, 11:11:50 PM »
A state fuction is something that is independent of path.
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Demotivator

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Re:thermochemistry...
« Reply #2 on: January 04, 2005, 12:16:16 AM »
1) enthalpy is the same as the measured heat, q,  provided that the system is under constant pressure. Units are energy units like calories or joules.

2) The total change in heat is zero by the law of conservation of energy. The reacting system either gets its heat from or releases heat to the surroundings. Either way, the sum of the reaction heat change and surrounding heat  change is zero and therefore are opposite in sign.
« Last Edit: January 04, 2005, 12:27:44 AM by Demotivator »

Offline gregpawin

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Re:thermochemistry...
« Reply #3 on: January 04, 2005, 07:55:32 AM »
Off the top of my head, enthalpy is a thermodynamic property otherwise known as internal energy...

Depending if you're an engineer or a scientist, you'll treat changes of heat as if you were the surroundings or the system, difference being only in a change in sign.
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777888

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Re:thermochemistry...
« Reply #4 on: January 04, 2005, 10:23:54 AM »
oic...so can "q" and "delta H" have negative values?

4. Can someone tell me what is calorimetry and calorimeter?

5. What is the difference between bomb calorimeter and coffee-cup calorimeter?

Thank you :)

Offline gregpawin

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Re:thermochemistry...
« Reply #5 on: January 05, 2005, 02:19:56 AM »
Actually, enthalpy H is defined as the internal energy U plus the product PV where U, P, and V are state functions... my brain is dead.

Yes, q is just a way of keeping track of heat.  Just as a bank account has positive and negative numbers at times for when you owe and when you have money.  Yes and whether or not delta H is positive or negative can also tell you whether or not the reaction requires energy to start or gives energy out.

You do calorimetry, measurement of heat, with a calorimeter, a device used to measure heat.

A coffee-cup calorimeter is a poor-man's bomb calorimeter.  A bomb calorimeter basically is filled up with 6ATM of oxygen with an iron filament that is covered with a compound of interest.  A huge voltage is put across the filament and in the presence of oxygen burns everything including the compound.  The pressure created from this bomb, in a well designed bomb calorimeter, actually puts pressure to keep its lid on tighter.  This whole thing is kept in a big tank of circulating water to keep track of heat, which is put in another container to keep stray heat from coming in or leaving the device.

A coffee-cup calorimeter is just two coffee-cups that seperate the environment from the reaction happening in the cup and involves no explosions.  Actually, the insulating power of the cups is very very good.
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Demotivator

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Re:thermochemistry...
« Reply #6 on: January 05, 2005, 10:11:01 AM »
To elaborate:
H = U + PV
The differential is
dH = dU + PdV + VdP
= (q - w) + PdV + VdP.
For a system performing only PdV type work,
w = PdV, and
 dH = q - PdV + PdV + Vdp
 dH= q + VdP.
If At constant pressure, dP = 0. Therefore,
dH = q.
the change of heat measured under a constant pressure is the enthalpy

777888

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Re:thermochemistry...
« Reply #7 on: January 05, 2005, 03:27:14 PM »
thank you!
In these calorimeter calculation questions, why do we say the compound(eg.C2H5OH) is in the SYSTEM and the calorimeter or water is in the SURROUNDINGS?

In cofee-cup calormeter calculations, for example, the rxn:
NaOH+HCl->H2O+NaCl
Are all (NaOH, HCl, H2O, NaCl) considered to be the SYSTEM? And why do we consider the water as surroundings? (there's water in the system(chemical equation) too)

Demotivator

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Re:thermochemistry...
« Reply #8 on: January 05, 2005, 03:47:42 PM »
The system is that which reacts. The surroundings is that which absorb (or give off) heat through their heat capacities.
Na and Cl are part of the surroundings. Water produced become part of the surroundings. OH and H are the system.

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