1. Dismiss Notice
  2. Download free Tapatalk for iPhone or Tapatalk for Android for your phone and follow the SDN forums with push notifications.
    Dismiss Notice
Dismiss Notice
Visit Interview Feedback to view and submit interview information.

hydrogen bonding question

Discussion in 'MCAT Study Question Q&A' started by jamesq, May 14, 2008.

  1. jamesq

    10+ Year Member

    Joined:
    May 14, 2008
    Messages:
    131
    Likes Received:
    0
    MDApps:
    Status:
    Medical Student
    Hi,

    I just took Kaplan Fulllength 8 and question 5 is troubling me. Im not sure if I can post the question, but it boils down to which one cannot experience hydrogen bonding. Their answer is HCN, however, how can H3BO3 experience hydrogen bonding? The hydrogen is not bonded to an F O or N, its bonded to a B? Furthermore, how can HCF3 undergro hydrogen bonding, once again the rules dont seem to apply, thanks
     
  2. Note: SDN Members do not see this ad.

  3. sicboy188

    5+ Year Member

    Joined:
    Mar 9, 2008
    Messages:
    132
    Likes Received:
    1
    Status:
    Medical Student
    why cant HCN participate in h-bonding...? are you sure its an EXCEPT question?
     
  4. jamesq

    10+ Year Member

    Joined:
    May 14, 2008
    Messages:
    131
    Likes Received:
    0
    MDApps:
    Status:
    Medical Student
    Here is the Q:

    The passage lists two mechanisms for hydrogen bonding to increase the heat of vaporization of pure solutions . Which of the following molecules will be least affected by by hydrogen bondin?
     
  5. chemnerd31

    2+ Year Member

    Joined:
    Apr 9, 2008
    Messages:
    79
    Likes Received:
    1
    MDApps:
    Status:
    Pre-Medical
    The nitrogen containing compond is the lest affected by h-bonding because it is the least electronegative out of N,O and F. Also in BH3O3 the H's are bound to an O if you draw out the lewis structure the B has 3 OH groups on it.
     
  6. jamesq

    10+ Year Member

    Joined:
    May 14, 2008
    Messages:
    131
    Likes Received:
    0
    MDApps:
    Status:
    Medical Student
    thanks your explanation makes sense, but they say that HCN would be totally not be able to hydrogen bond, whereas you seem to suggest it still can, but weakly. which one is it? Thanks!
     
  7. Character

    5+ Year Member

    Joined:
    Apr 23, 2008
    Messages:
    2,467
    Likes Received:
    1
    Status:
    Other Health Professions Student
    only fon can h-bond. HCN is Hydrogen cyanide.....The H is bonded to the C...Not the N...The N is triple bonded to the C..There is no h bonding in Hydrogen Cyanide...
     
  8. BlackSails

    2+ Year Member

    Joined:
    Apr 4, 2007
    Messages:
    865
    Likes Received:
    3
    Status:
    Pre-Medical
    Sulfur and Selenium can also hydrogen bond, iirc.

    Also, depending on solvent polarity, the HCN might just dissociate into ions. pka is about 9.
     
  9. sicboy188

    5+ Year Member

    Joined:
    Mar 9, 2008
    Messages:
    132
    Likes Received:
    1
    Status:
    Medical Student
    N can be a H-bond acceptor in the HCN configuration (not a donor).

    the question is worded badly IMO.
     
  10. BlackSails

    2+ Year Member

    Joined:
    Apr 4, 2007
    Messages:
    865
    Likes Received:
    3
    Status:
    Pre-Medical
    The actual question was though: .
    B(OH)3 will definitely have more hydrogen bonding than HCN. Oxygen is more electronegative than Nitrogen and there are three times as many per mole.
     
  11. BloodySurgeon

    Moderator Emeritus Lifetime Donor Classifieds Approved 10+ Year Member

    Joined:
    Nov 19, 2006
    Messages:
    3,234
    Likes Received:
    12
    Status:
    Medical Student
    HCN will not part-take in ANY hydrogen bonding with itself. A hydrogen must be connected to Oxygen, Nitrogen, or Fluorine and it could only connect to O,N, or F. Second the O, N, or F must be connected to a less Electronegative atom or again hydrogen bonding is not possible because it would not gain a partial negative charge. In HCN, the hydrogen is connected to the carbon not O, N ,or F. Therefore the answer much be H3BO3. Remember Boron can only have three sigma bonds with no lone electrons. The hydrogen is connected to oxygen in a manner like so, B(OH)3. The hydrogen is connected to Oxygen and the oxygen is connected to a lower EN atom, boron. That is why the answer is so.
     
  12. BlackSails

    2+ Year Member

    Joined:
    Apr 4, 2007
    Messages:
    865
    Likes Received:
    3
    Status:
    Pre-Medical
    Whats the pkb of HCN/-CN at nitrogen? You might very well end up protonating some molecules at nitrogen, which could then hydrogen bond to other molecules.
     
  13. BloodySurgeon

    Moderator Emeritus Lifetime Donor Classifieds Approved 10+ Year Member

    Joined:
    Nov 19, 2006
    Messages:
    3,234
    Likes Received:
    12
    Status:
    Medical Student
    Read the question wrong... the answer is H3BO3 because it can form more H-bonds. Yea HCN can form hydrogen bonds with for example H2O but it will be much less significant than H3BO3 which can form 3 H-Bonds per molecule.

    And there is nothing wrong with my last statement because HCN cannot H-bond with itself (other HCN molecules).
     

Share This Page