Chemical Forums
Chemistry Forums for Students => Inorganic Chemistry Forum => Topic started by: Wald_ron on December 03, 2011, 09:53:29 PM
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Does anyone know why P2 gas is unstable and N2 gas is extremely stable.
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Consider the problem using bond enthalpies. It is more favourable to form a P4 molecules than it is to form two P2 molecules. On the other hand it is more stable to have two N2 molecules than it is to form one N4 molecule.
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Consider the problem using bond enthalpies. It is more favourable to form a P4 molecules than it is to form two P2 molecules. On the other hand it is more stable to have two N2 molecules than it is to form one N4 molecule.
I considered the problem using bond enthalpies, how are you coming to this answer. The P-P bond energy (KJ/mole) is 490 kJ/mole (two sources) , Nitrogen is 942 KJ/mol which clearly explains why Nitrogen is stable as a diatomic gas, but I'm still not really clear as to
why specifically Phosphorus as a gas is unstable and so reactive.
Why do we have to heat white phosphorus to 800oC just to make P2 gas?
The only thing I can think of is related to size and that forcing two large similar charges to be close together means a triple bond and a high activation energy where as with nitrogen, which has less protons, this factor doesn't come into play.
Would this explain it's instability?
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Ok, my data is slightly different than yours but it shouldn't make any significant differences. So I have these bond enthalpies: N-N: 163 kJ mol-1, P-P: 200 kJ mol-1, N triple bond N: 945 kJ mol-1 and P triple bond P: 481 kJ mol-1
Now consider P4, there are six P-P single bonds, total bond enthalpy = 1200 kJ mol-1
Compare this to two P2 molecules, which have total bond enthalpy of 2 x 481 or using your data 2 x 490 = 980 kJ mol-1 which is significantly less than that of P4. Therefore it is more favourable to have P4 than two P2
Similar calculation for nitrogen, if there were six N-N bonds the total bond enthalpy would be 978 kJ mol-1. Two N2 molecules would have bond enthalpy 945 x 2 = 1890 kJ mol-1. Thus, it is much more stable to have two N2 molecules than one N4
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Thank you