Sorry if this doesn't make sense I just got off a night shift and i'm very tired.
I was wondering about the difference between heat and enthalpy.
Enthalpy is considered a state function but heat isn't. How come?
The formula for the enthalpy change seemed to just replace heat (q) in q=delta U - P*deltaV
U = internal energy
V = volume
P= pressure
state function
ProChem - 4-8-2009 at 07:14
in wikipedia...In thermodynamics, a state function, state quantity, or a function of state, is a property of a system that depends only on the current
state of the system, not on the way in which the system acquired that state.len1 - 4-8-2009 at 08:03
in wikipedia...In thermodynamics, a state function, state quantity, or a function of state, is a property of a system that depends only on the current
state of the system, not on the way in which the system acquired that state.
These standard textbook cliches clarify nothing.
Lets make it simple. The enthalpy of a body is how much 'heat' it contains. Its mainly a fundtion of temperature. How can I get a body to that
temperature? I can torch it, or I can bang it with a hammer. Say I do a combination, and get my body to 100C. How much of this was done by
torching? you dont know, its not a state function.
But one thing you know, the heat you added q, plus the work you did ON it -pdV got it to a constant state 100C. So U = q-pdVMagpie - 4-8-2009 at 08:04
I commend you for worrying about thermodynamics even when dead tired.
In simplistic terms enthalpy is a measure of the energy state of matter. Heat is energy in transition. Energy is energy, and different forms of it
can often (or always?) be related mathematically.watson.fawkes - 4-8-2009 at 09:04
Enthalpy is considered a state function but heat isn't. How come?
The word "heat" in itself is a little
confusing. What it means here is "heat flow". Thermodynamics is generally done with open systems, those with an interaction with their environment.
What this means is that there's a boundary, an inside (the "system") and an outside (the "environment"). When the word "state" is used, it means some
state of the system, that is, the state of whatever it is that's inside the boundary. Enthalpy, as a property of the state of the system (the inside),
is a form of potential energy. Heat flow is a measure of the energy that crosses the boundary.
I can't fathom why enthalpy isn't identified as a kind of potential energy in every thermodynamics text, but it's not. Enthalpy is the sum of two
different kinds of potential energy, the "internal" energy (again, a bewilderingly nonspecific term), which is essentially the potential energy in
chemical bonds and the like (including solvation energies, etc.) and the mechanical energy of pressure and volume. Another relevant energy, the heat
energy, is associated with temperature and heat capacity. This heat energy is not the same as the heat flow in the original question. Actual systems
see heat flow both as a result of temperature differences (flow from the heat energy of the system) as well as enthalpy changes.ProChem - 4-8-2009 at 09:07
len1. your answer would have been trashed in a pchem exam. you did not define a state function. I referred to wikipedia to encourage a look at the
monograph on thermodynamics and state functions because there is no simple answer. one must understand the concept and explain in thermodynamic
terms.
[Edited on 4-8-2009 by ProChem]len1 - 4-8-2009 at 12:18
Is that right, youd fail me? Fortunately in reality its the other way around. My comment was directed at the statement however, which is not yours.
