Reading Bond Lengths and Force Constants from Spectra · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Turning spectra into bond distances

Science · Chemistry · ages 19-20
Name ______________________   Date ____________

  1. A triple bond, a double bond and a single bond are compared at similar mass. Which rings highest?

    • The single bond
    • The triple bond
    • The double bond

  2. What is the first quantity you read from evenly spaced rotational lines?

    • The rotational constant from the 2B spacing
    • The force constant k
    • The number of electrons in the bond

  3. The R branch sits above the band origin and the P branch sits below it.

    Circle one:   True   False

  4. Deuteration moves a band to lower wavenumber with no change in chemistry. What does that confirm?

    • The assignment, since only the reduced mass moved
    • The force constant changed
    • The bond broke during measurement

  5. You have B for carbon monoxide and the atomic masses. What is the correct order of steps to the bond length?

    • r guessed first, then B fitted to it
    • B converted straight into the force constant
    • B to I, then I = mu r squared solved for r

  6. Why is there a gap between the P and R branches?

    • The detector skips the centre
    • The delta J = 0 jump at the origin is forbidden
    • Molecules absorb nothing near the origin

  7. Two students number the lines of one band differently and get different B values. How do you settle it?

    • Use the P and R branches and the forbidden gap to fix the numbering
    • Average the two answers
    • Pick the larger B because bigger is better

  8. A student quotes a bond length to six figures from roughly measured line centres. What is wrong?

    • Bond lengths can never be quoted numerically
    • More figures are always better
    • The line measurement limits the figures, so six overclaims the precision
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Answer key

For grown-ups. Fold this page away before handing over the rest.

Turning spectra into bond distances W1-mt_5m7J1mFw68-s1

  1. The triple bond · Higher bond order means a stiffer spring and a higher wavenumber.
  2. The rotational constant from the 2B spacing · The spacing hands you B, and everything else follows.
  3. True · R comes from +1 steps, P from -1 steps.
  4. The assignment, since only the reduced mass moved · The shift matches the reduced mass prediction exactly.
  5. B to I, then I = mu r squared solved for r · Geometry flows one way, from spacing through inertia to distance.
  6. The delta J = 0 jump at the origin is forbidden · The gap marks the line that the rules forbid.
  7. Use the P and R branches and the forbidden gap to fix the numbering · Correct numbering starts from the right landmark.
  8. The line measurement limits the figures, so six overclaims the precision · Error in the spacing feeds straight through into r.
Worksheet · LightMySky