Hydrochloric vs Hydrobromic Acid

[Mrudula Akkinepally]

Can someone explain to me how we can determine which one of these is a stronger acid based on periodic trends? Lecture 26 mentions bond lengths but I'm still not too sure.

Thank you!

[Mrudula Akkinepally]

Lol, I just watched the lecture again. It's because the bond length between hydrogen and bromine is shorter than the one between hydrogen and chlorine. A larger bond length means a weaker bond. So in the presence of water, it's easier to break the bond between hydrogen and bromine in comparison to hydrogen and chlorine.

But I guess my question is now, why does a weaker bond in the molecule equate to a stronger acid?

[Khoa Vu 3l]

The strength of an acid (HA) is defined by how easily it dissociates into a proton (H+) and its anion (A-). Because HBr has a longer bond length than HCl and therefore weaker bond strength, HBr will more readily dissociate, making it a stronger acid than HCl by comparison.

[Ashley Ko 3I]

Hi! I agree with Khoa in that HBr is a stronger acid than HCl because it is easier to remove the H proton from HBr than from HCl. I would say in terms of periodic trends, the smaller the atomic radius or the greater the electronegativity, the weaker the acid since the H proton will be harder to remove (H proton is more strongly bonded to the other atom). Here's another example: HF is a weaker acid than HCl since F has a smaller atomic radius and is more electronegative than Cl. Hope this helps!

[Linette Choi 3L]

HBr would be stronger than HCl as it has a shorter bond length, meaning the bond would be more difficult to break. However, like Khoa stated, the strength of an acid is defined by how easily it dissociates into a proton. Thus, with HBr having a longer bond length, it would more readily give away its protons, making it a stronger acid than HCl.