Chemical Forums

Please login or register.

Login with username, password and session length

Sponsored links

Pages: [1]   Go Down

Author Topic: Why is P2 gas unstable and N2 gas extremly stable  (Read 3121 times)

0 Members and 1 Guest are viewing this topic.

Wald_ron

  • Regular Member
  • ***
  • Mole Snacks: +10/-2
  • Offline Offline
  • Posts: 96
Why is P2 gas unstable and N2 gas extremly stable
« on: December 03, 2011, 03:53:29 PM »

Does anyone know why P2 gas is unstable and N2 gas is extremely stable.
Logged
I've never seen a mole in a bag of animal crackers , but I've heard they're tasty. Can I have one please :)

UG

  • Full Member
  • ****
  • Mole Snacks: +134/-15
  • Offline Offline
  • Gender: Male
  • Posts: 822
Re: Why is P2 gas unstable and N2 gas extremly stable
« Reply #1 on: December 03, 2011, 04:26:39 PM »

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.
Logged

Wald_ron

  • Regular Member
  • ***
  • Mole Snacks: +10/-2
  • Offline Offline
  • Posts: 96
Re: Why is P2 gas unstable and N2 gas extremly stable
« Reply #2 on: December 04, 2011, 06:07:31 AM »

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?
Logged
I've never seen a mole in a bag of animal crackers , but I've heard they're tasty. Can I have one please :)

UG

  • Full Member
  • ****
  • Mole Snacks: +134/-15
  • Offline Offline
  • Gender: Male
  • Posts: 822
Re: Why is P2 gas unstable and N2 gas extremly stable
« Reply #3 on: December 04, 2011, 10:25:09 AM »

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
Logged

Wald_ron

  • Regular Member
  • ***
  • Mole Snacks: +10/-2
  • Offline Offline
  • Posts: 96
Re: Why is P2 gas unstable and N2 gas extremly stable
« Reply #4 on: December 04, 2011, 11:45:58 AM »

Thank you
Logged
I've never seen a mole in a bag of animal crackers , but I've heard they're tasty. Can I have one please :)
Pages: [1]   Go Up
 

Mitch Andre Garcia's Chemical Forums 2003-Present.

Page created in 0.067 seconds with 23 queries.