Standard Enthalpy of Formation (pure substance)

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Sophia Kim 1C
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Standard Enthalpy of Formation (pure substance)

Postby Sophia Kim 1C » Sat Jan 13, 2018 8:58 pm

I was just wondering why the standard enthalpy of formation of an element in its most stable form is zero? I know that this was "by definition" but I was just wondering why specifically it was 0.

Andy Liao 1B
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Re: Standard Enthalpy of Formation (pure substance)

Postby Andy Liao 1B » Sat Jan 13, 2018 9:18 pm

First and foremost, the definition of the standard enthalpy of formation of a substance is the enthalpy change when, at a pressure of 1 bar and a temperature of 25 °C, 1 mol of the substance is formed from the most stable form of its elements. The standard enthalpy of formation of an element in its most stable form is zero because there is no change involved when they are formed from themselves.

Michelle Lee 2E
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Re: Standard Enthalpy of Formation (pure substance)

Postby Michelle Lee 2E » Sat Jan 13, 2018 10:42 pm

For example:
First, think of what element(s) make up the molecule. In O2, oxygen is the one element that makes up the molecule.
Next, think of the stability of the elements that make up the molecule. For O2, oxygen's most stable form is O2. Making O2 from O2 doesn't require and enthalpy change because it is already in its stable form and, therefore, requires no added or released heat.

This is only for elements that in their most stable form like O2, F2, Cl2, C2, N2, etc.

905022356
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Re: Standard Enthalpy of Formation (pure substance)

Postby 905022356 » Sun Jan 14, 2018 12:54 pm

Remember that the standard enthalpy of formation of any substance is the change in heat that takes place when one mol of such substance is formed from its constituent elements in their standard state. O2 (g) is formed from O2(g), so, to form one mol of O2 (g), one mole of O-O bonds need to be broken, causing an enthalpy change of +X. Then, one mol of O-O bonds will form, causing an enthalpy change of -X. So, overall, the enthalpy of the reaction would be: +X-X, which yields 0. I hope this helps!


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