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HBr is a stronger acid than HF because Br is a larger atom than F meaning the bond length between H and F is closer than H and Br. That means HBr is more likely to dissociate and a strong acid more readily dissociates in water.
The strength of an acid depends on how easily the A-H bond can be broken. The stronger the acid, the weaker the bond. So, HBr is a stronger acid than HF because the atomic radius of Br is larger than the atomic radius of F, so the bond is longer (therefore weaker) and more easily broken.
HF has a stronger bond due to the higher difference in electronegativity therefore the bond is harder to break, meaning the proton is harder to donate. Therefore, HBr is a stronger acid since it has a weaker bond and can more easily donate a proton.
Bromine is in Group 4 of the Periodic Table while Fluorine is in Group 2. Based on periodic trends and atomic mass, Bromine is a heavier atom with a much larger atomic radius. As such, the single bond in HBr is much longer than the single bond in HF. Longer bonds are weaker so hydrobromic acid has a weaker bond and it is therefore much easier for it to lose its H+ ion and behave as an acid, making it a stronger acid relative to HF.
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