Results are out! Find what you need...fast. Get quick advice or join the chat
Hey there Sign in to join this conversationNew here? Join for free

Is PCl3 polar?

Announcements Posted on
    • Thread Starter
    • 10 followers
    Offline

    ReputationRep:
    Hey guys, came across an exam question and it says that PCl3 is polar, however I thought it was non polar as it has a lone pair, a delta positive P and 3 delta negative Cl's, so the dipoles cancel each other out hence it is non polar. Can anyone explain to me why I am wrong, and why PCl3 is considered to be polar?

    Thanks!
    • 9 followers
    Offline

    ReputationRep:
    (Original post by Nator)
    Hey guys, came across an exam question and it says that PCl3 is polar, however I thought it was non polar as it has a lone pair, a delta positive P and 3 delta negative Cl's, so the dipoles cancel each other out hence it is non polar. Can anyone explain to me why I am wrong, and why PCl3 is considered to be polar?

    Thanks!
    if it is not of high symmetry like tetrahedral, octahedral of being planar, it is polar as the dipoles don't cancel out.

    it is because of this lone pair that results in the molecule adopting a pyramidal shape(analogous NH3) and if you know NH3 is polar, chemical intuition should tell you PH3 is polar.

    if for molecule like BF3 which is planar and no lone pair on B, then it is non polar.
    • Thread Starter
    • 10 followers
    Offline

    ReputationRep:
    (Original post by shengoc)
    if it is not of high symmetry like tetrahedral, octahedral of being planar, it is polar as the dipoles don't cancel out.

    it is because of this lone pair that results in the molecule adopting a pyramidal shape(analogous NH3) and if you know NH3 is polar, chemical intuition should tell you PH3 is polar.

    if for molecule like BF3 which is planar and no lone pair on B, then it is non polar.
    It's PCl3 not PH3, so surely the dipoles do cancel out?
    • 36 followers
    Offline

    ReputationRep:
    (Original post by Nator)
    It's PCl3 not PH3, so surely the dipoles do cancel out?

    It is polar as the structure is trigonal pyramidal.
    http://en.wikipedia.org/wiki/Phosphorus_trichloride
    It has a dipole moment of 0.97D.
    • Thread Starter
    • 10 followers
    Offline

    ReputationRep:
    (Original post by Seham)
    It is polar as the structure is trigonal pyramidal.
    http://en.wikipedia.org/wiki/Phosphorus_trichloride
    It has a dipole moment of 0.97D.
    So polarity is determined by shape, regardless of charge?
    • 9 followers
    Offline

    ReputationRep:
    (Original post by Nator)
    It's PCl3 not PH3, so surely the dipoles do cancel out?
    as a chemist, you should know that the shape of PCl3 is the same as PH3, similar in the sense that NCl3 has same shape as NH3. this could and should have your first educated guess, but of course special cases arises in examples like AlCl3 which could exist as a dimer.
    • 36 followers
    Offline

    ReputationRep:
    (Original post by Nator)
    So polarity is determined by shape, regardless of charge?
    Yes. Electronegativity of the atoms (and hence bond polarity) as well as shape determine if a molecule will be polar or not.
    • Thread Starter
    • 10 followers
    Offline

    ReputationRep:
    (Original post by Seham)
    Yes. Electronegativity of the atoms (and hence bond polarity) as well as shape determine if a molecule will be polar or not.
    So all pyramidal molecules are always polar, regardless of the electronegativity of the involved atoms?
    • 36 followers
    Offline

    ReputationRep:
    (Original post by Nator)
    So all pyramidal molecules are always polar, regardless of the electronegativity of the involved atoms?
    Yes. (Caveat: Unless the dipole moment can exactly equal the dipole moment in the opposite direction, but this is very unlikely)
    • Thread Starter
    • 10 followers
    Offline

    ReputationRep:
    (Original post by shengoc)
    as a chemist, you should know that the shape of PCl3 is the same as PH3, similar in the sense that NCl3 has same shape as NH3. this could and should have your first educated guess, but of course special cases arises in examples like AlCl3 which could exist as a dimer.
    Sure I understand that they have the same shape, but they have different electronegativities. For example in ammonia the N is delta negative and the lone pair is negative, whereas the H's are delta positive, so it is polar because the dipole is in one direction (negative on one side to positive at the bottom). However, PCl3 has a lone pair, and the P is delta positive and the Cl's are delta negative, so since the direction is from Negative to Positive to Negative I was always under the impression that the dipoles cancel each other out and the molecule is non-polar (my book says this principle about the direction of the dipole). Any thoughts?
    • Thread Starter
    • 10 followers
    Offline

    ReputationRep:
    (Original post by Seham)
    Yes. (Caveat: Unless the dipole moment can exactly equal the dipole moment in the opposite direction, but this is very unlikely)
    I see your point, is there any way you know if the dipole moment is equal to another of opposite direction? And also any thoughts of my long post above this one? Am really confused at the moment
    • 1 follower
    Offline

    ReputationRep:
    I see where you are coming from. Imagining a pyramid shape, you're assuming that the negativity of the lone pair at the top cancels out the negativity caused by the electronegativity of the chlorines which are pointing down. This is not the case, the negativities are not equal, the lone pair or the 3 Cl atoms, one will be more negative than the other, so there will be a dipole (although I'm not completely sure in which direction).
    • Thread Starter
    • 10 followers
    Offline

    ReputationRep:
    (Original post by lukas1051)
    I see where you are coming from. Imagining a pyramid shape, you're assuming that the negativity of the lone pair at the top cancels out the negativity caused by the electronegativity of the chlorines which are pointing down. This is not the case, the negativities are not equal, the lone pair or the 3 Cl atoms, one will be more negative than the other, so there will be a dipole (although I'm not completely sure in which direction).
    I understand your point But what's confusing is in my Chemistry AS Revision guide, a molecule of NCl3 is drawn and it says there is no overall dipole
    • 36 followers
    Offline

    ReputationRep:
    (Original post by Nator)
    Sure I understand that they have the same shape, but they have different electronegativities. For example in ammonia the N is delta negative and the lone pair is negative, whereas the H's are delta positive, so it is polar because the dipole is in one direction (negative on one side to positive at the bottom). However, PCl3 has a lone pair, and the P is delta positive and the Cl's are delta negative, so since the direction is from Negative to Positive to Negative I was always under the impression that the dipoles cancel each other out and the molecule is non-polar (my book says this principle about the direction of the dipole). Any thoughts?
    They do cancel out to some extent. However as they are not equal, there will be some excess in a given direction and so the molecule will have a dipole moment and hence be polar. If the dipole moments were equal in all directions then the molecule wouldn't be polar (e.g. CCl4- 0D)

    (Original post by Nator)
    I see your point, is there any way you know if the dipole moment is equal to another of opposite direction? And also any thoughts of my long post above this one? Am really confused at the moment
    We usually assume that the dipole moments are only equal when the atoms are the same. So the dipole moment due to the lone pair is not equal to the dipole moment due the electronegative Cls.
    • 36 followers
    Offline

    ReputationRep:
    (Original post by Nator)
    I understand your point But what's confusing is in my Chemistry AS Revision guide, a molecule of NCl3 is drawn and it says there is no overall dipole
    This is incorrect. NCl3 is trigonal pyramidal and has a dipole moment of 0.6D.
    http://en.wikipedia.org/wiki/Nitrogen_trichloride
    • Thread Starter
    • 10 followers
    Offline

    ReputationRep:
    (Original post by Seham)
    This is incorrect. NCl3 is trigonal pyramidal and has a dipole moment of 0.6D.
    http://en.wikipedia.org/wiki/Nitrogen_trichloride
    So it is Polar?
    • 36 followers
    Offline

    ReputationRep:
    (Original post by Nator)
    So it is Polar?
    Yes
    • 1 follower
    Offline

    ReputationRep:
    (Original post by Nator)
    I understand your point But what's confusing is in my Chemistry AS Revision guide, a molecule of NCl3 is drawn and it says there is no overall dipole
    Well the electronegativity of a N-Cl bond will be different to that of a P-Cl bond, so maybe in the case of NCl3 they do cancel out or the dipole is small enough to not be significant :dontknow:, maybe your book is just wrong.

    I'm not really sure, I'm doing AS chemistry too so I'm sure there are people who know more than me
    • Thread Starter
    • 10 followers
    Offline

    ReputationRep:
    (Original post by lukas1051)
    Well the electronegativity of a N-Cl bond will be different to that of a P-Cl bond, so maybe in the case of NCl3 they do cancel out or the dipole is small enough to not be significant :dontknow:, maybe your book is just wrong.

    I'm not really sure, I'm doing AS chemistry too so I'm sure there are people who know more than me
    That's what my thought was And oh right I see So for molecules like CO2 and HBr it's simple to deduce if they are polar or non-polar, but for more large molecules like the one I just gave and others, what is your strategy for determining if it is polar or not?
    • Thread Starter
    • 10 followers
    Offline

    ReputationRep:
    (Original post by Seham)
    Yes
    Awesome thanks

Reply

Submit reply

Register

Thanks for posting! You just need to create an account in order to submit the post
  1. this can't be left blank
    that username has been taken, please choose another Forgotten your password?
  2. this can't be left blank
    this email is already registered. Forgotten your password?
  3. this can't be left blank

    6 characters or longer with both numbers and letters is safer

  4. this can't be left empty
    your full birthday is required
  1. By joining you agree to our Ts and Cs, privacy policy and site rules

  2. Slide to join now Processing…

Updated: May 4, 2011
New on TSR

Find out what year 11 is like

Going into year 11? Students who did it last year share what to expect.

Article updates
Reputation gems:
You get these gems as you gain rep from other members for making good contributions and giving helpful advice.