1. Limited time only! Sign up for a free 30min personal 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!

Bond angle in methane? Have been doing for 1 hr

  1. Sep 16, 2009 #1
    Bond angle in methane? Have been doing for 1 hr!!

    1. The problem statement, all variables and given/known data
    In the methane molecule, CH4, each hydrogen atom is at a corner of a regular tetrahedron with the carbon atom at the center. In coordinates where one of the C-H bonds is in the direction of i + j + k, an adjacent C-H bond is in the i - j - k direction. Calculate the angle between these two bonds?



    2. Relevant equations
    I know this is a pyramid.



    3. The attempt at a solution
     
  2. jcsd
  3. Sep 16, 2009 #2

    rl.bhat

    User Avatar
    Homework Helper

    Re: Bond angle in methane? Have been doing for 1 hr!!

    If A and B are vectors, then the angle between them is given by
    cosθ= A.B/A*B
     
  4. Sep 16, 2009 #3
    Re: Bond angle in methane? Have been doing for 1 hr!!

    Would that mean that cos theta is equal to the (Ax+Bx) + (Ay+By) divided by the magnitudes of A and B? I think that each side of the pyramid is 1. Is this right?
     
  5. Sep 16, 2009 #4

    rl.bhat

    User Avatar
    Homework Helper

    Re: Bond angle in methane? Have been doing for 1 hr!!

    No.
    cosθ = (AxBx + AyBy + AzBz)/AB, where A and B are the magnitudes of the vectors.
     
  6. Sep 16, 2009 #5
    Re: Bond angle in methane? Have been doing for 1 hr!!

    So how would I use the method of components to find Ax and Bx, and Ay and By?? I never learned how to do Az and Bz either. I only have one fact that I think, each side is 1.
     
  7. Sep 16, 2009 #6

    rl.bhat

    User Avatar
    Homework Helper

    Re: Bond angle in methane? Have been doing for 1 hr!!

    You have expressed bonds is in the direction of i + j + k, an adjacent C-H bond is in the i - j - k direction.
    Here coefficient of i, j and k are Ax, Ay and Az. i.e. 1, 1, 1.
    Similarly find Bx, By and Bz. Magnitudes of A and B are sqrt(3)
    Now find the angle using the formula.
     
  8. Sep 16, 2009 #7
    Re: Bond angle in methane? Have been doing for 1 hr!!

    It might help to know that a tetrahedron can be embedded in a cube. I think it's far more intuitive to obtain the H-C-H angle by examining the cube.

    anim5in1.gif

    umm. that wasn't supposed to be spinning, but it's the best I could find.
     
    Last edited by a moderator: May 1, 2017
  9. Sep 16, 2009 #8
    Re: Bond angle in methane? Have been doing for 1 hr!!

    I have been trying it out and I still cannot do this
     
  10. Sep 17, 2009 #9
    Re: Bond angle in methane? Have been doing for 1 hr!!

    Consider if the cube is one unit on a side. The blue line at the top of the cube connects two hydrogen atoms. The length of the line is sqrt(2).

    The center of this line lies at the center of a cube face. The center of the cube face is 1/2 unit from the center of the cube. The carbon atom is at the center of the cube.

    Draw a picture. Does the line from the center of the cube intersect the blue line at a right angle?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Bond angle in methane? Have been doing for 1 hr
  1. Bond Angle (Replies: 1)

Loading...