Question

Enter the chemical formula of a binary molecular compound of hydrogen and a Group 7A element that can reasonably be expected to be more acidic in aqueous solution than HCl, e.g. have a larger Ka

Answer 1

A binary molecular compound of H and a group 7A element more acidic in aqueous solution than HCl could be HBr or HI.

Step-by-step explanation:

A large value of Ka means that, in the equilibrium of the aqueous dissociation of the acid, a big concentration of
`H^+` will be found. For an acid to have a big value of Ka, the bond with hydrogen should be weak.

While the electronegativity can be considered in this prediction, as an indicator that the 7A element would attract the electron of the H and form the corresponding anion, the ionic size also affects in the acidity. While F has a bigger electronegativity, it's ionic size is lower than the ionic size of the other elements in this group, such that the bond in HF is more stable than the bond in HCl, HBr or HI. Because of this same reason, with bigger ionic sizes, the bond in the correponding acids of Br and I will be weaker than the bond in HCl, being more acidic.