Answer to Question #21242 in Physical Chemistry for david kitchen

Question

If you have two strong acids such as HCl and HBr at the same concentration, why is HBr stronger considering that both are presumed to fully dissociate and will therefore both have the same concentration of H+ ions. Thanks.

Answer 1

Strength of these halogen acids in aqueous solutions increases in the order: HF < HCl < HBr < HI. In this order the H-X bond strength decreases, perceivably due to increase in the size of X (indeed your good reasoning). By the same logic, HBr bond is less stronger than HCl. Hydrofluoric acid is much weaker than the others -- H-F bond is much stronger than the O-H bond of water; also, HF molecule is stabilized more by its hydrogen bonding in aqueous solutions (most electronegative elements form stronger hydrogen bonds) and this further reduces its acid strength. Further, acid strength = dehydration energy + dissociation energy + ionization energy of H⁺ plus electron affinity of X^- plus energy of Hydration of H⁺ and X^-.The molecules should be supplied with energy for ionisation and dissociation processes.By this we understand that hydration HX ---> H⁺ (hydrated) + X^- (hydrated)and electron affinity release energy and this energy should be sufficient to offset other terms given above (ionisation and dissociation processes). Hydration energy and electron affinity of Br- (-347 and -324) is lesser than those of Cl- (-381 and -348 kJ/mol). This gives a negative total enthalpy value and negative free energy values. HBr has total enthalpy of -64 and HCl has -60 kJ/mol. More negative enthalpy and hence more negative free energy make the acid HBr dissociation more favorable.