How Is the Enthalpy Balance Equation Derived for Chemical Reactions?

[MexChemE]

Hello PF! I have a very short question. We're currently doing enthalpy balances for chemical reactions in our Thermo class. We are using the simple enthalpy balance equation:
$$\Delta H_{\textrm{Reaction}}^0 = \sum \Delta H_{\textrm{Products}}^0 - \sum \Delta H_{\textrm{Reactants}}^0$$
My question is, is this equation derived from the general energy balance equation, adjusted in some way for chemical reactions?

 

[Chestermiller]

Hello PF! I have a very short question. We're currently doing enthalpy balances for chemical reactions in our Thermo class. We are using the simple enthalpy balance equation:
$$\Delta H_{\textrm{Reaction}}^0 = \sum \Delta H_{\textrm{Products}}^0 - \sum \Delta H_{\textrm{Reactants}}^0$$
My question is, is this equation derived from the general energy balance equation, adjusted in some way for chemical reactions?

No. ΔH⁰ refers to the change in enthalpy at constant temperature and 1 atm pressure, starting out with pure reactants at T and 1 atm, and ending up with pure products at T and 1 atm. To measure this, you need to dream up a process in which the reactants and products move between these states, and you measure the amount of heat you have to add so that the final temperature matches the initial temperature. That's the definition of ΔH⁰. Often, if you have tables of heats of formation that include the reactants and products participating in the particular reaction, you can determine the heat of the reaction from these.

Chet

 

[MexChemE]

So, it is not a "real" balance in the same vein as an energy or mass balance, just a useful mathematical tool?

 

[Chestermiller]

So, it is not a "real" balance in the same vein as an energy or mass balance, just a useful mathematical tool?

I wouldn't put it in those words. It's a well-defined physical quantity that is used to model heat effects in actual processes involving chemical reactions.

Chet

 

[MexChemE]

Got it. Thanks!

 

[Chestermiller]

So, it is not a "real" balance in the same vein as an energy or mass balance, just a useful mathematical tool?

I might also mention that, for a reaction mixture involving ideal gases, since the heat of mixing of ideal gases is zero, delta H zero applies directly to the mixture as well as to starting and ending with the pure products and reactants.

Chet

 

[MexChemE]

I wouldn't put it in those words. It's a well-defined physical quantity that is used to model heat effects in actual processes involving chemical reactions.

Chet

Yes, I think I didn't express myself correctly, I know ΔH⁰ is a well defined quantity. What I meant was that the enthalpy balance equation I wrote in the OP was developed exclusively for chemical reactions, independently of the general energy balance equation, is this right? Sorry, I should have watched my writing.

 

[Chestermiller]

Yes, I think I didn't express myself correctly, I know ΔH⁰ is a well defined quantity. What I meant was that the enthalpy balance equation I wrote in the OP was developed exclusively for chemical reactions, independently of the general energy balance equation, is this right? Sorry, I should have watched my writing.

Sorry, I still don't follow. Are you asking how you include chemical reactions in the general energy balance equation for a piece of equipment? The equation you wrote in the OP is the definition of the quantity known as the heat of reaction, and is not necessarily related to any specific heat balance. However, if you carry out a reaction in a closed system at constant temperature and 1 atm pressure, then the quantity in your OP equation is also the heat you have to add to hold the system at constant temperature. Don't forget that enthalpy is a physical property of each of the materials involved, and is not related to any specific process or piece of equipment.

Chet

 

[MexChemE]

Are you asking how you include chemical reactions in the general energy balance equation for a piece of equipment?

Yes, this was my original question.

The equation you wrote in the OP is the definition of the quantity known as the heat of reaction, and is not necessarily related to any specific heat balance.

Now it's crystal clear.

Don't forget that enthalpy is a physical property of each of the materials involved, and is not related to any specific process or piece of equipment.

This statement is related to Hess' Law, right?

 

[Chestermiller]

This statement is related to Hess' Law, right?

Yes. Hess' Law is how it's applied in practice.

Chet