C−H bond length in organic compounds
Bond strength (bond dissociation energy, BDE) refers to homolytic cleavage of a bond ($\ce{C-H}$ in this case). Heterolytic cleavage is measured by $\mathrm{p}K_\mathrm{a}$. The two processes are the numerical inverse of one another. In the case of acetylene with $\mathrm{sp}$-hybridization (50% $\mathrm{s}$-character) of carbon, the electron density of the $\ce{C-H}$ bond lies closer to carbon when compared to ethylene (33% $\mathrm{s}$-character) and ethane (25% $\mathrm{s}$-character). Thus in acetylene, it is more difficult to break the $\ce{C-H}$ bond homolytically because the carbon is &quot;unwilling&quot; to give up an electron but is happy to keep both electrons and give up a proton. Thus, acetylene has a high BDE ($\pu{133 kcal/mol}$) and lower $\mathrm{p}K_\mathrm{a}$ ($=25$) than ethylene ($\pu{110 kcal/mol}$, $\mathrm{p}K_\mathrm{a} = 44$) and, ultimately, ethane ($\pu{101 kcal/mol}$; $\mathrm{p}K_\mathrm{a} \approx 50$).