Formation entalphy

[ChemSiddiqui]

Hi,

 

I was pondering over this question and want you people to help me out:

 

Why couldn’t the enthalphy of formation be found directly but by taking enthalpies of combustion first?

 

I think it got to do with the time needed to measure the temperature. Any ideas!

[insane_alien]

it can be found directly but the reaction is not always spontaneous and it may not be possible to form the compound just by mixing the elements and seeing if something happens.

 

its easier to take heats of combustion and provides a common framework for getting the heats of formation for all compounds.

[ChemSiddiqui]

I see! Thank you insane_alien!

[CaptainPanic]

For the less common chemicals, it is normal that in literature the heat of combustion is listed. If you need the heat of formation, you have to calculate it from the heat of combustion.

[ChemSiddiqui]

For the less common chemicals, it is normal that in literature the heat of combustion is listed. If you need the heat of formation, you have to calculate it from the heat of combustion.

 

Thank you for your reply. But what you just said was the same thing I asked the reason for?

[CaptainPanic]

Eeh, yeah. That was not my best reply on this forum

 

So, the heat of combustion is easy to measure. (You just burn something, and measure the change in temperature). To measure the enthalpy of formation, you need to form some compound from its elements, that experiment is a lot more difficult or even impossible.

 

Does that answer the question? Here's some more info:

 

(Heat of combustion) = -(heat of formation of your compound) + (heat of formation of combustion products: often just H2O, CO2)

 

Or:

 

(heat of formation of your compound) = -(Heat of combustion) + (heat of formation of combustion products: often just H2O, CO2)

 

Make sure you get the minuses at the right place there! If something creates heat when it's burned, the heat of combustion is negative!

 

The trick is:

1. The heat of formation of the combustion products is known (wikipedia or a book).

dHf(CO2) = -3.935E5 J/mol

dHf(H2O, liquid) = -2.86E5 J/mol

 

2. The heat of combusion of your chemical is measured.

 

3. You can now calculate the heat of formation of that chemical.

 

Hope this answers the question a bit better. (I was in a bit of a rush yesterday).

[YT2095]

I have a quick question on this topic.

 

my Data book lists most all the bond enthalpies used in chem (Organic) so all the O-H and C-C and C=C type stuff is in there, and for the most part it`s pretty useful.

 

but there`s Nothing listed for Metals?

 

so How would I work out the Delta H Na+H2O ---> NaOH + H

 

Sodium isn`t listed in there.

 

do I need a better book, or is there a way to find this value?

[foodchain]

I have a quick question on this topic.

 

my Data book lists most all the bond enthalpies used in chem (Organic) so all the O-H and C-C and C=C type stuff is in there, and for the most part it`s pretty useful.

 

but there`s Nothing listed for Metals?

 

so How would I work out the Delta H Na+H2O ---> NaOH + H

 

Sodium isn`t listed in there.

 

do I need a better book, or is there a way to find this value?

 

Organometallic stuff, I think it involves carbon. Does that make steel a Organometallic compound?

[YT2095]

no, I wouldn`t have said so, esp when you consider that the iron and carbon in steel are in fact individual crystals, more akin to an Alloy than anything else I reckon.

 

Carbides on the other hand??? that`s a toughy, I don`t really know enough about the exact definition of what constitutes a proper organo-metalic compound, so I can`t answer that one for you.

 

it`s strange in a way, because my Question a couple of posts up occurred to me when I was trying to work out the Delta H of calcium carbide in water (it`s very exothermic).

and then I realised I have no listing for any of the Metal bonds to complete my calculations DOH!

[foodchain]

no, I wouldn`t have said so, esp when you consider that the iron and carbon in steel are in fact individual crystals, more akin to an Alloy than anything else I reckon.

 

Carbides on the other hand??? that`s a toughy, I don`t really know enough about the exact definition of what constitutes a proper organo-metalic compound, so I can`t answer that one for you.

 

it`s strange in a way, because my Question a couple of posts up occurred to me when I was trying to work out the Delta H of calcium carbide in water (it`s very exothermic).

and then I realised I have no listing for any of the Metal bonds to complete my calculations DOH!

 

I don’t really know what you are talking about. In steel I don’t know if the bonding is qualifying either. As for your problem I don’t understand but it sounds interesting. Is it a reaction mechanism type thing or something about apparatus or environment if that stuff is separate from reaction mechanism. To be honest I don’t hold a total understanding for what reaction mechanism is to mean.

[YT2095]

it`s no actually hard to understand, so i`ll give it a shot and try and explain

 

take Methane for example (CH4) we know it burns in O2 (oxygen in the air).

 

if we look at a Methane molecule we`ll see if has a central Carbon atom and 4 hydrogen atoms around it each making a single bond.

so we have 4 C-H bonds.

each C-H bond requires Energy to break it, this number is Delta H +

the number is Positive because it Requires you put in Energy.

 

O2 in the air (is a double bond IIRC) like O=O, this also requires energy to break it

now to Burn Methane in O2 we make the equasion: CH4 + 2O2 = CO2 + 2H2O

so we add up the one side of the equassion in terms of Bonds to be broken

 

C-H bond (there`s 4 of them)

and O=O bonds (there`s 2 of them)

 

you look these up in a book (unless you remember all the Bond enthalpies) and you arrive at a number.

 

you then do the same to the Other side (your reaction products).

 

you then take one number away from the other.

 

the Difference is the Overall enthalpy of reaction and it will not only tell you if it`s an Exothermic reaction or not, but Also How much energy is given off

 

this is a Very basic explanation but illustrates the principal nicely I hope

[foodchain]

it`s no actually hard to understand, so i`ll give it a shot and try and explain

 

take Methane for example (CH4) we know it burns in O2 (oxygen in the air).

 

if we look at a Methane molecule we`ll see if has a central Carbon atom and 4 hydrogen atoms around it each making a single bond.

so we have 4 C-H bonds.

each C-H bond requires Energy to break it, this number is Delta H +

the number is Positive because it Requires you put in Energy.

 

O2 in the air (is a double bond IIRC) like O=O, this also requires energy to break it

now to Burn Methane in O2 we make the equasion: CH4 + 2O2 = CO2 + 2H2O

so we add up the one side of the equassion in terms of Bonds to be broken

 

C-H bond (there`s 4 of them)

and O=O bonds (there`s 2 of them)

 

you look these up in a book (unless you remember all the Bond enthalpies) and you arrive at a number.

 

you then do the same to the Other side (your reaction products).

 

you then take one number away from the other.

 

the Difference is the Overall enthalpy of reaction and it will not only tell you if it`s an Exothermic reaction or not, but Also How much energy is given off

 

this is a Very basic explanation but illustrates the principal nicely I hope

 

So its an enthalpy equation?

[YT2095]

yeah, it`s just a nice way of knowing how efficient something`s going to be in terms of reaction energy In and Energy out, without ever having to lift a test tube

 

the only problem I have is that there`s nothing listed for Metal bonds, so I can`t even say how much energy is given off if I put say 1g of Calcium metal in some water

 

I have a feeling that I might be missing something very simple OR I need a more comprehensive table.

[foodchain]

yeah, it`s just a nice way of knowing how efficient something`s going to be in terms of reaction energy In and Energy out, without ever having to lift a test tube

 

the only problem I have is that there`s nothing listed for Metal bonds, so I can`t even say how much energy is given off if I put say 1g of Calcium metal in some water

 

I have a feeling that I might be missing something very simple OR I need a more comprehensive table.

 

So you could graph something like that pretty easy huh?

 

*Is this the kind of stuff you are talking about?

http://www.jbc.org/cgi/content/abstract/282/8/5853

[YT2095]

nada, that doesn`t read like Anything I`m familiar with, looks more like Biology or something to me

[CaptainPanic]

One of the reasons that it's not listed is that these bonds are ion-ion bonds. The reaction involves an electron moving from the Na to the H. This means that not only do you break a bond, and form a bond, you also remove an electron, and add an electron.

 

The half reactions are:

2 Na --> 2 Na+ + 2e-

2H2O + 2e- --> 2OH- + H2

 

The bond energy you're looking for is called "lattice energy", and they are generally not listed in the more common books of chemistry. You'll need a book on inorganic chemistry that focuses on solids. Wikipedia has an article about it: http://en.wikipedia.org/wiki/Lattice_energy, but it's not wiki's best.

 

Hope this was useful.

 

p.s. we're now way beyond the first question as posted by ChemSiddiqui. Guess that's ok. (It's even fun).

[ChemSiddiqui]

One of the reasons that it's not listed is that these bonds are ion-ion bonds. The reaction involves an electron moving from the Na to the H. This means that not only do you break a bond, and form a bond, you also remove an electron, and add an electron.

 

The half reactions are:

2 Na --> 2 Na+ + 2e-

2H2O + 2e- --> 2OH- + H2

 

The bond energy you're looking for is called "lattice energy", and they are generally not listed in the more common books of chemistry. You'll need a book on inorganic chemistry that focuses on solids. Wikipedia has an article about it: http://en.wikipedia.org/wiki/Lattice_energy, but it's not wiki's best.

 

Hope this was useful.

 

p.s. we're now way beyond the first question as posted by ChemSiddiqui. Guess that's ok. (It's even fun).

 

Its alright. I am following every single post in this thread and actually gaining a lot.

[MulderMan]

Might be worth looking up the born-haber cycle aswell, thats what we've been using in class of late for working out ionic bond enthalpys. Much like hess cycles but with alot more specific enthalpy changes!

[ChemSiddiqui]

Well Born haber cycle will give you lattice enthalphy, formation enthalphy, atomisation, ionisation etc. That will work too yeh!