Standard enthalpy of element in most stable form

Moderators: Chem_Mod, Chem_Admin

405318478
Posts: 90
Joined: Sat Jul 20, 2019 12:16 am

Standard enthalpy of element in most stable form

Postby 405318478 » Sun Jan 26, 2020 2:01 pm

Could someone explain why the standard enthalpy of formation of an element in its most stable form is zero?

Amanda Mei 1B
Posts: 109
Joined: Sat Aug 24, 2019 12:16 am

Re: Standard enthalpy of element in most stable form

Postby Amanda Mei 1B » Sun Jan 26, 2020 2:26 pm

Elements in their standard state have a standard enthalpy of formation of 0 because they are are not formed, they just exist. They don't require any energy to get to their most stable form if they're already in that form.

Helen Struble 2F
Posts: 97
Joined: Sat Aug 24, 2019 12:17 am

Re: Standard enthalpy of element in most stable form

Postby Helen Struble 2F » Sun Jan 26, 2020 4:17 pm

Amanda Mei 1B wrote:Elements in their standard state have a standard enthalpy of formation of 0 because they are are not formed, they just exist. They don't require any energy to get to their most stable form if they're already in that form.


This is still a little confusing to me because elements in their standard forms still have bonds. So, if two atoms of O "just exist" with no bonds between them, and then synthesize into O2, wouldn't there be an enthalpy change because bonds are formed? Isn't that the whole concept behind the Method 2 of calculating enthalpy?

Alice Chang 2H
Posts: 101
Joined: Fri Aug 30, 2019 12:18 am

Re: Standard enthalpy of element in most stable form

Postby Alice Chang 2H » Sun Jan 26, 2020 4:29 pm

I got this from https://chem.libretexts.org/Bookshelves/General_Chemistry/Map%3A_General_Chemistry_(Petrucci_et_al.)/07%3A_Thermochemistry/7.8%3A_Standard_Enthalpies_of_Formation

For example, although oxygen can exist as ozone (O3), atomic oxygen (O), and molecular oxygen (O2), O2 is the most stable form at 1 atm pressure and 25°C. Similarly, hydrogen is H2(g), not atomic hydrogen (H). Graphite and diamond are both forms of elemental carbon, but because graphite is more stable at 1 atm pressure and 25°C, the standard state of carbon is graphite (Figure 7.8.1 ). Therefore, O2(g) , H2(g) , and graphite have ΔHof values of zero.


Honestly, I'm a bit confused still but I think the idea is that at those specific temperatures and pressures, elements will be found to have enthalpy values (amount of energy required to break or form to most stable state) of 0.

Louise Lin 2B
Posts: 97
Joined: Sat Aug 17, 2019 12:15 am

Re: Standard enthalpy of element in most stable form

Postby Louise Lin 2B » Sun Jan 26, 2020 4:46 pm

from what I understand, the standard enthalpy of an element is dependent on the standard state that the element is typically found in. For example, oxygen is not typically found as just O, but it is found as O2. Thus, like a previous comment said above, the standard enthalpy of the element in its most stable form is zero because they are typically already found like that.

Vinita Saxena 2I
Posts: 58
Joined: Fri Aug 09, 2019 12:16 am

Re: Standard enthalpy of element in most stable form

Postby Vinita Saxena 2I » Sun Jan 26, 2020 6:37 pm

They are zero because the most stable form is the form they are found, which means it takes energy to change its form. This gives a baseline to start with, like how Lavelle gave the example of sea level being 0 for airplanes.


Return to “Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)”

Who is online

Users browsing this forum: No registered users and 2 guests