Understanding enthalpy

[Ineedhopenow]

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This is my understanding of enthalpy:

The greater the negative change in enthalpy, delta H, the more stable the product. Therefore a reaction where delta H = -396 is more stable than one with delta H = -394. How is it that diamond has a delta H of -396 and graphite is -394 if graphite is more stable than diamond?

Am I thinking of enthalpy wrong?

Here's another example: liquid water has delta H = -285.8, water vapor has delta H = -241.8. This makes sense to me because liquid water is more stable than vapor at 25C, therefore liquid water has to have a greater negative delta H than water vapor. The more positive the delta H, the more energy it has. Therefore it's leas stable, right?

Also, how do you post a thread using the sdn app? I can only read and reply, but can't create new threads like this one.

 

[chiddler]

Carbon has 0 heat of enthalpy because it is the standard form of elemental carbon. Where are you getting your numbers from?

For water, you expect a more negative heat of enthalpy for liquid water yes because it is more "stable", in that more intermolecular bonds form.

 

[MedPR]

This is my understanding of enthalpy:

The greater the negative change in enthalpy, delta H, the more stable the product. Therefore a reaction where delta H = -396 is more stable than one with delta H = -394. How is it that diamond has a delta H of -396 and graphite is -394 if graphite is more stable than diamond?

Am I thinking of enthalpy wrong?

Here's another example: liquid water has delta H = -285.8, water vapor has delta H = -241.8. This makes sense to me because liquid water is more stable than vapor at 25C, therefore liquid water has to have a greater negative delta H than water vapor. The more positive the delta H, the more energy it has. Therefore it's leas stable, right?

Also, how do you post a thread using the sdn app? I can only read and reply, but can't create new threads like this one.

It really depends what you are talking about. This is a kind of touchy topic when it comes to vocabulary.

If the heat of hydrogenation of X is -100, and the heat of hydrogenation of Y is -500, X is MORE stable than Y. Y gives off more heat because it has more energy (is less stable) to begin with.

If you're talking about a reaction like X-->Y having an enthalpy of -100 and X-->Z enthalpy of -500, then Z is more stable than Y because there was a greater energy change to Z, which means Z is at a lower energy level (is more stable) than Y.

 

[Ineedhopenow]

Apparently, for the diamond/graphite example, the numbers are from heat of combustion. Whereas the water liquid/vapor example was from enthalpies of formation.

What's the difference?

 

[Ineedhopenow]

My numbers are from EK p. 58, problems 61 and 62.

 

[chiddler]

The enthalpy is the difference between bonds broken and bonds formed. So heat of combustion is how much energy is released as new bonds form with the burning reaction.

Since the end product is the same in both cases (CO2 + H2O), then what must be different is the starting product. Graphite is very stable, right? So there isn't as much energy difference in products and reactants.

Diamond is unstable so the products will be significantly more stable than the reactants.

here.

 

[MedPR]

Apparently, for the diamond/graphite example, the numbers are from heat of combustion. Whereas the water liquid/vapor example was from enthalpies of formation.

What's the difference?

More negative heat of combustion means less stable reactant because there was less energy used to combust (break) it. Less energy used = more energy leftover to be dissipated (conservation of energy). So per your example, graphite is more stable because it requires 2 more units of energy to combust than diamond does.

Stronger bonds = more stable = higher bond dissociation energy = more energy consumed = less energy leftover = less energy given off in an exothermic reaction.

More negative enthalpy of formation = forming more stable product. So looking at your example of vapor and liquid, since the liquid has the lower (more negative) enthalpy of formation value, it is more stable than the vapor.

Some people like to think of it as a mountain of energy. Would you rather run down a mountain, or up a mountain? Down, of course. Do you want stable products or unstable products? Stable, of course.

 

[MedPR]

Apparently, for the diamond/graphite example, the numbers are from heat of combustion. Whereas the water liquid/vapor example was from enthalpies of formation.

What's the difference?

More negative heat of combustion means less stable reactant because there was less energy used to combust (break) it. Less energy used = more energy leftover to be dissipated (conservation of energy).

More negative enthalpy of formation = forming more stable product. So looking at your example of vapor and liquid, since the liquid has the lower (more negative) enthalpy of formation value, it is more stable than the vapor.

Some people like to think of it as a mountain of energy. Would you rather run down a mountain, or up a mountain? Down, of course. Do you want stable products or unstable products? Stable, of course.

Another thing that might help is if you remember that negative enthalpy just means energy is a product. Energy is still a positive value. So the combustion of graphite produces 394kJ of energy, and the combustion of diamond produces 396kJ of energy. Since energy isn't created or destroyed, the 2kJ difference must have gone into fueling the reaction. Since graphite used more "fuel" it must have been less willing to undergo the reaction. If something doesn't want to react, it must be stable.

Also, these numbers say graphite is more stable, but is it really? Given no values, I would've guessed diamond is more stable. Oh well.

Edit: Not sure how I doubled posted, sorry.

 

[MedPR]

The enthalpy is the difference between bonds broken and bonds formed. So heat of combustion is how much energy is released as new bonds form with the burning reaction.

Since the end product is the same in both cases (CO2 + H2O), then what must be different is the starting product. Graphite is very stable, right? So there isn't as much energy difference in products and reactants.

Diamond is unstable so the products will be significantly more stable than the reactants.

here.

I don't think your graphs are correct. If diamond and graphite combust into the same products, the product energy levels should be the same. The reason why there is a difference in enthalpy is because the graphite starts at a lower level than the diamond.

So say the products are at 100kJ. The diamond is at 100kJ+396 and the graphite is at 100kJ+394. So your reactants are right, but your products are telling a different story 😉

 

[chiddler]

yeah, you're right. i looked at only the left side. should of looked at both.

thanks.

 

[Ineedhopenow]

Okay thanks everyone!