List these compounds in order of decreasing carbon–carbon bond strength: hcch, h2cch2, h3cch3. rank from strongest to weakest.

The strongest bond strength is of HCCH (Ethyne), then comes H₂CCH₂ (Ethene) and H₃CCH₃ (Ethane) has the least bond strength as compare to Ethyne and Ethene.

                                      HCCH > H₂CCH₂ > H₃CCH₃
               Strongest > Weakest

Explanation:
                     Ethyne has triple bond, one sigma bond and two pi bonds, Hence, a greater overlap is found between two carbon atoms which resulted in shortening the bond length (i.e 1.9 Angstrome). Also carbon carbon triple bond has highest force constant value i.e 15 ×10⁵ dyne/cm which is greater than carbon carbon double bond (10 ×10⁵ dyne/cm) and single bond (5 × 10⁵ dyne/cm). 

Hope the image of the frequency formula