1. Dismiss Notice
  2. Hey Guest! Check out the 3 MCAT Study Plan Options listed in the 'stickies' area at the top of the forums (BoomBoom, SN2ed, and MCATJelly). Let us know which you like best.

    Also, we now offer a MCAT Test-Prep Exhibitions Forum where you can ask questions directly from the test-prep services.
    Dismiss Notice
  3. This forum is for support and discussion only. Please promote test prep materials/services (including AMAs) in the Special Offers subforum only. Thanks!
    Dismiss Notice
Dismiss Notice

Interview Feedback: Visit Interview Feedback to view and submit interview information.

Interviewing Masterclass: Free masterclass on interviewing from SDN and Medical College of Georgia

Delta Hf???

Discussion in 'MCAT: Medical College Admissions Test' started by akalin24, Apr 17, 2004.

  1. akalin24

    akalin24 Member
    10+ Year Member

    Joined:
    Nov 18, 2003
    Messages:
    58
    Likes Received:
    1
    What has the LOWEST Delta Hf..

    cyclopropane
    cyclobutane
    cyclopentane
    cyclohexane

    I think its cyclohexane.. least amount of ring strain..

    Thoughts
     
  2. pmpndaflip

    pmpndaflip Member
    5+ Year Member

    Joined:
    Apr 7, 2004
    Messages:
    65
    Likes Received:
    0
    Its cyclopropane...LOWEST delta H meaning most negative enthalpy or most exothermic...since G= H - TS the lower the delta H means lowest delta G...so which is most exothermic upon reaction and the one with the greatest ring straing is most exothermic. :thumbup:
     
  3. ASDIC

    ASDIC The 9th Flotilla
    15+ Year Member

    Joined:
    Apr 7, 2003
    Messages:
    961
    Likes Received:
    5
    Status:
    Medical Student
    no...its cyclohexane!!!

    check out this website from the Yale University Chemistry Dept:

    http://classes.its.yale.edu/chem220a/studyaids/thermo/heats-formation.html#Cycloalkanes

    cyclopropane ==> delta Hf is +12.7 kcal/mol
    cyclobutane ===> delta Hf is +6.6
    cyclopentane ==> -18.3
    cyclohexane ===> -29.5

    clearly, -29.5 is lower than 12.7. So the answer is cyclohexane due to decreased ring strain favoring a very negative exothermic reaction.
     
  4. pmpndaflip

    pmpndaflip Member
    5+ Year Member

    Joined:
    Apr 7, 2004
    Messages:
    65
    Likes Received:
    0
    doh! I already missed one! OH NO!! THE WORLD IS GONNA END :laugh:
     
  5. liverotcod

    liverotcod Lieutenant Crunch
    10+ Year Member

    Joined:
    Nov 1, 2003
    Messages:
    2,336
    Likes Received:
    5
    Status:
    Attending Physician
    I remember asking this same question of my organic instructor when we were doing that topic, and made the same argument about negative numbers being smaller than positive numbers. But really, it's a trick question either way. The "lowest" in this case, according to him, is defined as the one that gives off the least heat, so cyclopropane. It really could go either way, depending on the exact phrasing of the question.
     
  6. akalin24

    akalin24 Member
    10+ Year Member

    Joined:
    Nov 18, 2003
    Messages:
    58
    Likes Received:
    1
    Can anyone confirm the answer to this post??
     
  7. akalin24

    akalin24 Member
    10+ Year Member

    Joined:
    Nov 18, 2003
    Messages:
    58
    Likes Received:
    1
    I found this..

    Delta Hf values in (kJ/mol)

    2 ethylene 52.2625
    3 cyclopropane 53.0987
    4 cyclobutane 28.4308
    5 cyclopentane -76.9304
    6 cyclohexane -123.3395
    7 cycloheptane -117.9042
    8 cyclooctane -124.1757
    9 cyclononane -132.5377
    10 cyclodecane -154.2789
     
  8. liverotcod

    liverotcod Lieutenant Crunch
    10+ Year Member

    Joined:
    Nov 1, 2003
    Messages:
    2,336
    Likes Received:
    5
    Status:
    Attending Physician
    If the question was really the LOWEST delta Hf, then cyclohexane is correct. Aside from all of the previous discussion, it has more bonds than any of the others. It's just that the wording of the question is really important for this type of problem. A question that asked lowest delta Hf per carbon atom might have a difference answer, for example.
     

Share This Page