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### Question 4J.7

Posted: Tue Feb 11, 2020 8:58 pm
When calculating the enthalpy, why are only some reactants/products used to calculate it? For example, for part (a), you only use the enthalpy of formation of H2O and H2O2, and don't include O2.

### Re: Question 4J.7

Posted: Tue Feb 11, 2020 9:08 pm
Enthalpy of formation of O2 is zero because it's in its most stable form of oxygen

### Re: Question 4J.7

Posted: Wed Feb 12, 2020 3:08 pm
Jordan Young 2J wrote:Enthalpy of formation of O2 is zero because it's in its most stable form of oxygen

How would you know that it's the most stable form? Would you have to memorize it?

### Re: Question 4J.7

Posted: Wed Feb 12, 2020 9:18 pm
VioletKo3F wrote:
Jordan Young 2J wrote:Enthalpy of formation of O2 is zero because it's in its most stable form of oxygen

How would you know that it's the most stable form? Would you have to memorize it?

Yeah, it's kind of a memorization thing. It's a gas, bonded between two identical atoms. And O2 as a gas is literally one of the simplest elements to be found in nature, easily diffusing in blood cells, lungs, through cell membranes. Also, another tip, if you try looking up the enthalpy of formation of O2 on the constants sheet they give us on the tests, it doesn't exist. So you can just assume its zero and as such is in is most stable form.

### Re: Question 4J.7

Posted: Sat Feb 15, 2020 7:18 am
Enthalpy of formation of O2 is 0 at its most stable form.

### Re: Question 4J.7

Posted: Fri Feb 28, 2020 8:21 am
Just to list a few more, H2, Cl2, Br2, and C(s, graphite) also have enthalpies of formation equal to 0.