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Quick Chem Question - Corn - 2011-07-10
Why is the heat of formation of liquid water negative?
Quick Chem Question - Panacea - 2011-07-10
Something to do with exothermic and endothermic reactions, I think.
Exothermic tend to be negative, endothermic tend to be positive.
Quick Chem Question - Corn - 2011-07-10
Put it this way.
The reaction is exothermic if the heat of formation is negative. This means hydrogen and oxygen combining should release energy.
But you really don't see water spontaneously forming in midair, don't you?
Quick Chem Question - Panacea - 2011-07-10
I'm not very good with concepts.
Going to take a shot here, thinking through it logically (in my opinion):
There's always moisture in the air, the only reason we would see the water molecules is if there was a large amount of them, like in a raindrop.
That's all I can think of, really. I never questioned why it was negative.
Quick Chem Question - Spaghetti - 2011-07-11
Corn Wrote:Put it this way.
The reaction is exothermic if the heat of formation is negative. This means hydrogen and oxygen combining should release energy.
But you really don't see water spontaneously forming in midair, don't you?
If the concentration of H2(g) in the air was high enough for a large amount of H2O to visibly form then it would also be high enough for you to die the second you light a match.
You are right that it releases energy, all exothermic reactions do because it goes from a high potential energy state to a low one. Be careful though, spontaneous reactions are characterized by having a negative dG.
dG = dH - TdS
From this formula you can see that having a positive dH (endothermic reaction) can still yield a spontaneous reaction if TdS > dH
Quick Chem Question - HellenzSin - 2011-07-11
Hydrogen gas in the mid air? It's extremely rare in Earth's atmosphere. It's way too light, pretty much all H2(g) is in outer space. If there was enough concentration of it, then you'd see random explosions due to the exothermic reaction.
Quick Chem Question - Hanabira.Kage - 2011-07-11
The negative enthalpy change of formation of water is due to the fact that bond formation releases energy. Therefore, the reaction
H2 + 0.5O2 -> H2O
is exothermic, and thus has a negative dH.
According to the formula
dG = dH - TdS
which Spaghetti posted above, we can see that a small dS and T will result in dH being approximately equal to dG and thus, a good indicator of spontaneity. However, as gases, H2 and O2 have a LARGE entropy value (S). Water, on the other hand, is a liquid and as such has a much, much smaller entropy. Therefore, the dS of the aforementioned reaction is LARGE and NEGATIVE. At room temperature, T is small and positive, so therefore, TdS is also large and negative, just like dS. dH is negative, yes, but is also fairly small (-285.83 kJ/mol). Therefore,
dG = negative - LARGE negative = negative + LARGE positive = positive
As dG is positive, the reaction is non-spontaneous. Therefore, the above reaction does not occur spontaneously.
p.s. I'm still a student myself, so if I made a mistake anywhere, feel free to correct me.
H2 + 0.5O2 -> H2O
is exothermic, and thus has a negative dH.
According to the formula
dG = dH - TdS
which Spaghetti posted above, we can see that a small dS and T will result in dH being approximately equal to dG and thus, a good indicator of spontaneity. However, as gases, H2 and O2 have a LARGE entropy value (S). Water, on the other hand, is a liquid and as such has a much, much smaller entropy. Therefore, the dS of the aforementioned reaction is LARGE and NEGATIVE. At room temperature, T is small and positive, so therefore, TdS is also large and negative, just like dS. dH is negative, yes, but is also fairly small (-285.83 kJ/mol). Therefore,
dG = negative - LARGE negative = negative + LARGE positive = positive
As dG is positive, the reaction is non-spontaneous. Therefore, the above reaction does not occur spontaneously.
p.s. I'm still a student myself, so if I made a mistake anywhere, feel free to correct me.
Quick Chem Question - Shidoshi - 2011-07-11
As people have said above, there is almost no hydrogen on the air.
Also, sometimes a reaction is very exothermic and has negative gibbs energy (and thus very favorable thermodynamically), but the kinetics of the reaction are so slow that it never actually happens.
For example, diamond at 1 atm and 25°C should spontaneously become coal, since coal is more stable in that condition. However, the kinetics of the reaction are so slow that diamond lasts pretty much forever without changing.
Quick Chem Question - 2147483647 - 2011-07-11
Activation energy.