[reed]

Acetylene is more acidic than ethene and ethane, however, the bond disasociation energy for the hydrogen in acetylene is greater than the hydrogens in the other two chemicals. Typically a weaker bonded hydrogen indicates higher acidity, and this seems to be the exception where the stronger bond is more acidic. How does one explain this?

Thanks

[Corribus]

First, keep in mind that most tabulated BDEs assume a homolytic cleavage of the bond. Dissociation of an acid is a heterolytic cleavage. The energy it takes to homolytically cleave a bond is different from that required for heterolytic cleavage, especially considering.... Second, acidity is a solution-phase process, and therefore involves, among other things, the ability of the product ions to be solvated (enthalpy and entropy). Such effects typically have no relation to the (homolytic) BDEs. Therefore, while BDEs can be used to understand many things, correlations to the relative favorability of chemical processes are not always good.

[reed]

makes perfect sense, thank you.

[Vidya]

Acetylene is more acidic than ethene and ethane, however, the bond disasociation energy for the hydrogen in acetylene is greater than the hydrogens in the other two chemicals. Typically a weaker bonded hydrogen indicates higher acidity, and this seems to be the exception where the stronger bond is more acidic. How does one explain this?

Thanks

Acidity  depends on two factors 1) Electronegativity(EN) 2) Bond enthalpy
Down the group in the Periodic table size is the dominating factor so BE becomes dominating in deciding the acidity of the H
Along the period EN is the dominating factor so it becomes the main criteria for comparing acidity .Now we are talking about C-H bonds with different hybridizations ..sp carbon are more EN then sp^2 and sp^3  carbon atoms ..H bonded to sp C is more acidic .