Two quick/dumb questions regarding thermodynamics

In summary: He is trying to understand why the enthalpy of a reaction would be equal to the difference of the enthalpies of formation. Hess's law can be used to calculate the change in enthalpy between two equilibrium states.
  • #1
davidbenari
466
18
First question is : why is ##\bigg(\frac{\partial \Delta H_{vap/fusion maybe something else} }{\partial T} \bigg)_p=\Delta Cp ## I can't wrap my head around that and nowhere other than my class can I find such an expression.

The other question is regarding the calculation of stuff (say H, S, G, A) during chemical reactions as (for example, enthalpy of a reaction):

##H_{reaction}=\Bigg(\sum \nu_i \Delta Hº\Bigg)_{products}- \Bigg(\sum \nu_i \Delta Hº\Bigg)_{reactants}##

Namely I don't get why adding Deltas ##\Delta## of formation (I'm excepting entropy here) will give the correct enthalpy of the reaction.

I have a very naive argument that defends this expression which I will give just to see what happens:

I imagine two mountains of different height. Here height is refers to the thermodynamic variable in question and its strictly a Delta with respect to the ground. If I have two states, my way to check the height between them is to substract one Delta from the other.

This sounds dumb though.

Any help would be nice. Thanks!
 
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  • #2
davidbenari said:
If I have two states, my way to check the height between them is to substract one Delta from the other.
What makes you think this method wouldn't work?
davidbenari said:
why is (∂ΔH vap/fusionmaybesomethingelseT ) pCp
What do you think it should be?
 
  • #3
Bystander said:
What makes you think this method wouldn't work?
Well if I consider enthalpy for example, it's a bit mysterious. Why would the enthalpy of a reaction be equal to the difference of the enthalpies of formation? The only way I can make sense of it is through my dumb analogy of mountains and heights.

Bystander said:
What do you think it should be?
Well, I don't know what it should be. I've finally seen a derivation on google after a lot of time, which makes sense to me at least. Namely they say:

##H_{reaction}=H_{products}-H_{reactants}## If I take the partial derivative of the LHS then its decomposed into two partial derivatives on the RHS which are just Cp's and its therefore a change in Cp.
 
  • #4
The answers to both your questions relate to the fact that the enthalpy of a system is a function of state and, in each case, you are determining the change in enthalpy between two equilibrium states. To do this, it doesn't matter what path you follow between the two equilibrium states. So you can choose any convenient route. These evaluations can be done by applying Hess's law. Are you familiar with the derivation and application of Hess's law?

In the equation you wrote for the heat of reaction, those superscript 0's means that you are doing whatever is necessary to hold the final temperature the same as the initial temperature, and you are starting out with the pure reactant gases in stoichiometric proportions and going to the pure product gases in corresponding stoichiometric proportions (all at 1 atm.)

Chet
 

1. What is thermodynamics?

Thermodynamics is the branch of science that deals with the study of energy and its transformations, particularly in relation to heat and work.

2. What is the first law of thermodynamics?

The first law of thermodynamics states that energy cannot be created or destroyed, it can only be transferred or converted from one form to another.

3. What is the second law of thermodynamics?

The second law of thermodynamics states that in any energy conversion, some energy will be lost as heat and the amount of disorder in the universe will increase.

4. What is the difference between an open and closed thermodynamic system?

An open thermodynamic system allows for the exchange of both matter and energy with its surroundings, while a closed system only allows for the exchange of energy.

5. How is thermodynamics applied in real life?

Thermodynamics has many practical applications in daily life, such as in engines, refrigerators, and air conditioners. It is also used in the study of weather and climate, as well as in chemical reactions and industrial processes.

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