1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Using bond energies to calculate energy changes

  1. Mar 9, 2008 #1
    1. The problem statement, all variables and given/known data

    Use the bond energies (in Table 4.2) to calculate the energy changes associated with each of these reactions. In each case indicate whether the overall reaction is endothermic or exothermic. Be sure to show your calculation work.

    2. Relevant equations[/b

    3. The attempt at a solution

    A. H2(g) + O2(g) -----> H2O2(g) Answer: H2 is H-H (+436) and O2 is O=O (+498). Total energy in breaking bonds is 934 kJ. H2O2 is H-O=O-H (-467,-498,-467). Total in making bonds is -1432 kJ. Therefore, the net energy change is -498kJ, with the overall reaction being exothermic.

    B. 2H2(g) + O2(g) -----> 2H2O(g) Answer: 2H2 is H-H (+436), H-H (+436). O2 is O=O (+498). Total energy in breaking bonds is +1370kJ. 2H20 is H-O-H (-467x2), H-O-H (-467x2). Total energy in making bonds is -1868kJ. Therefore, the net energy change is
    -498kJ, with the overall reaction being exothermic.

    C. 2H2(g) + CO(g) -----> CH3OH(g). Answer: H-H (+436), H-H (+436). CO is C=O (+1073). Total in breaking bonds is +1208. CH3OH is C-H (-416x3), C=O (-803), O-H (-467)

    I find it strange that I came up with exothermic for all three of these....
     
  2. jcsd
  3. Mar 9, 2008 #2

    Mapes

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Why?
     
  4. Mar 10, 2008 #3
    many more reactions are exothermic than endothermic. exothermic reaction tends to form more stable products, and hence are favoured by nature.
     
  5. Mar 10, 2008 #4
    okay, that makes sense. I thought I had done my work right, but it seems usually there are examples of both things in such problems. Thanks for explaining that to me.
     
  6. Mar 10, 2008 #5
    another question on this...did I do this correctly as far as how I broke it down: CH3OH is C-H (-416x3), C=O (-803), O-H (-467). I am doubing guessing myself!
     
  7. Mar 11, 2008 #6
    draw the lewis structure. you will see that there are 3 C - H bonds (correct), 1 O - H bond (correct) but a C - O bond instead of C = O. the C - O bond is about 360 kJ.
     
  8. Mar 11, 2008 #7
    I guess I totally drew that wrong:(Thanx for showing me the right way...
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Using bond energies to calculate energy changes
  1. Bond energy (Replies: 3)

  2. Bond Energies (Replies: 2)

  3. Bond Energies (Replies: 1)

Loading...