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

In summary, the conversation is discussing whether or not delta H is a state function. It is determined that since delta H is a sum of delta U and a path dependent function, it is not a state function. However, according to Hess Law, delta H can be the same for two different reactions, making it a state function. This may seem contradictory, but it is explained that when pressure is fixed, W becomes a state function, making H a state function as well. The conversation then moves on to discussing the different sign conventions used for work in physics and chemistry, with the conclusion that delta H has a defined value for a particular change, regardless of whether or not it is at constant pressure, making it a state function.
  • #1
sachin123
121
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
  • #2


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?
 
  • #3


oh.so even W is a state function in this case then.
so that makes H a state function in general.
Thank you Mapes
 
  • #4


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?
 
  • #5


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?
 

1. What is delta H and why is it a state function?

Delta H is the symbol used to represent the change in enthalpy of a system. It is a state function because it only depends on the initial and final states of the system, not the path taken to get there. This means that the value of delta H will be the same regardless of the process used to reach the final state.

2. How is delta H related to Hess's Law?

Hess's Law states that the overall change in enthalpy for a chemical reaction is independent of the pathway taken, as long as the initial and final conditions are the same. This concept is directly related to delta H being a state function, as it shows that the change in enthalpy is only dependent on the initial and final states, not the specific steps taken to get there.

3. Can delta H be measured directly?

No, delta H cannot be measured directly. It is a calculated value based on the difference between the enthalpy of the products and the enthalpy of the reactants in a chemical reaction. However, the enthalpy of the products and reactants can be measured experimentally and then used to calculate delta H.

4. What are some applications of delta H and Hess's Law?

Delta H and Hess's Law are commonly used in thermodynamics and chemistry to calculate and predict energy changes in chemical reactions. They are also used in industries such as manufacturing and energy production to optimize processes and determine the most efficient ways to produce products.

5. Is delta H always positive or negative?

Delta H can be either positive or negative, depending on the direction of the reaction. A positive delta H indicates an endothermic reaction, where energy is absorbed, while a negative delta H indicates an exothermic reaction, where energy is released. However, the magnitude of delta H is always the same, regardless of its sign, as it is a state function.

Similar threads

Chemistry Valence Bond Theory: Energy of a system with H and Cl atoms
    • Biology and Chemistry Homework Help
Replies
6
Views
2K
Chemistry Thermochemistry and the heat of complete combustion
    • Biology and Chemistry Homework Help
Replies
4
Views
1K
Calculation of the change in internal energy of water
    • Biology and Chemistry Homework Help
Replies
19
Views
3K
Is this correct Hess' law, thermodynamics
    • Biology and Chemistry Homework Help
Replies
22
Views
3K
Help with enthelpy change and Hess's Law
    • Biology and Chemistry Homework Help
Replies
1
Views
4K
Why is delta-H equal to q at constant pressure?
    • Biology and Chemistry Homework Help
Replies
11
Views
2K
B Irreversibility of Aristotle's law
    • Mechanics
Replies
15
Views
1K
What is the difference between internal energy and enthalpy?
    • Chemistry
Replies
12
Views
2K
I Representation of Spin 1/2 quantum state
    • Quantum Physics
2
Replies
61
Views
1K
Verify Green's Formula for a Simple DE
    • Calculus and Beyond Homework Help
Replies
0
Views
163

Hot Threads

Recent Insights

Back
Top