Is delta H a state function?(Hess Law?)

  • Thread starter Thread starter sachin123
  • Start date Start date
  • Tags Tags
    Delta Law State
Click For Summary

Discussion Overview

The discussion centers on whether delta H (enthalpy change) is a state function, particularly in the context of Hess's Law. Participants explore the implications of path dependence in thermodynamic processes and the definitions of work in different scientific contexts.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • One participant argues that since delta H is defined as delta U plus work (W), and W is path dependent, delta H should not be a state function.
  • Another participant points out that Hess's Law allows for the addition of enthalpy changes from different paths, suggesting that delta H is the same regardless of the path taken, which implies it is a state function.
  • A subsequent reply questions whether fixing pressure makes W a state function, potentially altering the status of delta H.
  • Another participant suggests that if W is considered a state function under constant pressure, then H could also be a state function.
  • A different question is raised regarding the differing sign conventions for work in physics and chemistry, seeking to understand the significance of these conventions.
  • One participant mentions a text stating that delta H has a defined value for a particular change, regardless of pressure conditions, which raises further questions about the status of W as a state function.

Areas of Agreement / Disagreement

Participants express differing views on whether delta H is a state function, with some supporting the idea based on Hess's Law and others questioning it due to the path dependence of work. The discussion remains unresolved with multiple competing perspectives.

Contextual Notes

There are limitations in the discussion regarding the assumptions about work being path dependent or independent under certain conditions, and the implications of different definitions of work in physics versus chemistry.

sachin123
Messages
117
Reaction score
0
is delta H a state function??(Hess Law?)

Is delta H a state function?

We know that delta H=delta U+W (at const P)
so,
we also know that W is path dependent function.So delta H being a sum of delta U and a path dependent function must not be a state function.(point 1)

But there is this Hess Law which allows you to add 2 chemical reactions along with their enthalpy changes.
Lets say water is formed from H2 and O2.lets say water is again formed from H2 and O2 but in a different path.according to Hess law,delta H is same in both cases.
Now doesn't that make delta H a state function?(point 2)

Don't point 1 and point 2 contradict?
Thank You.
 
Physics news on Phys.org


sachin123 said:
We know that delta H=delta U+W (at const P)
so,
we also know that W is path dependent function.

What if W is no longer path dependent when you fix P?
 


oh.so even W is a state function in this case then.
so that makes H a state function in general.
Thank you Mapes
 


Also may I extend this and ask,
why are different sign conventions used for 'Work' in physics and chemistry?
in physics we take W +ve when work is done by the system and correspondingly we take it -ve in chemistry.Is there any significance?
 


however i read a text saying that, 'delta H has a defined value for a particular change, regardless of whether or not it's at constant pressure'.

in this case, W is not a state function. then how is it possible?
 

Similar threads

Chemistry Calculation of thermodynamic variables for irreversible adiabatic process of ideal gas
  • · Replies 9 ·
Replies
9
Views
3K
Chemistry Valence Bond Theory: Energy of a system with H and Cl atoms
  • · Replies 6 ·
Replies
6
Views
3K
Chemistry How to derive 2nd law extremum principles for U, A, G, and H?
  • · Replies 1 ·
Replies
1
Views
3K
Chemistry How do we use enthalpy of formation in this calculation?
  • · Replies 9 ·
Replies
9
Views
2K
Chemistry Derivations in adiabatic process for ideal gas with C_V and C_P
  • · Replies 1 ·
Replies
1
Views
2K
Chemistry Entropy in isolated composite system for irreversible process
  • · Replies 3 ·
Replies
3
Views
3K
Chemistry How to understand the energy involved in mass transfer into an open system?
  • · Replies 0 ·
Replies
0
Views
2K
Is this correct Hess' law, thermodynamics
  • · Replies 22 ·
Replies
22
Views
4K
Chemistry Thermochemistry and the heat of complete combustion
  • · Replies 4 ·
Replies
4
Views
2K
Chemistry A few questions on an irreversible adiabatic expansion of an ideal gas
  • · Replies 1 ·
Replies
1
Views
3K