Which Is Stronger: HBrO4 or HIO4, and HBr or HI?

The two comparisons follow different periodic trends. HBrO4 is stronger than HIO4, while HI is stronger than HBr. The first comparison is between oxyacids; the second is between hydrogen halides, and the factors that control their acidity are different.

HBrO4 versus HIO4

Both acids have four oxygen atoms, and the central atom has oxidation state +7 in each. For oxyacids with the same number of oxygen atoms and the same structure, acidity generally increases with the electronegativity of the central atom. Bromine is more electronegative than iodine, so the trend predicts:

$$\mathrm{HBrO_4 > HIO_4}$$

The reason is largely inductive. The central atom draws electron density through the oxygen atoms, helping make it easier for the O–H bond to release a proton. After deprotonation, this electron-withdrawing effect also favors the conjugate base. Since Br is more electronegative than I, it favors acidity more strongly in this matched pair.

The acid-dissociation reactions show the conjugate bases being compared:

$$\begin{aligned} \mathrm{HBrO_4 + H_2O} &\rightleftharpoons \mathrm{H_3O^+ + BrO_4^-} \\ \mathrm{HIO_4 + H_2O} &\rightleftharpoons \mathrm{H_3O^+ + IO_4^-} \end{aligned}$$

This is a qualitative trend, not a rule to apply blindly to any two oxyacids. The number of oxygen atoms, molecular structure, and solvent can also affect acidity. Here, however, the formulas are directly comparable. HIO4 is metaperiodic acid; it should not be confused with orthoperiodic acid, H5IO6, which has a different formula.

HBr versus HI

For hydrogen halides, acidity increases down Group 17:

$$\mathrm{HI > HBr}$$

The H–I bond is longer and weaker than the H–Br bond. It therefore takes less energy to break the H–I bond during proton donation. The larger iodide ion, I−, can also accommodate negative charge more readily than the smaller bromide ion, Br−. These effects outweigh the fact that the H–Br bond is more polar.

A common mistake is to assume that a more polar H–X bond must always make a stronger acid. Bond polarity matters, but for hydrogen halides the decrease in bond strength down the group is the main trend. Thus HI is the stronger acid, even though iodine is less electronegative than bromine.

Why the trends differ

In the oxyacid pair, the proton is attached to oxygen, and the central atom influences acidity through the oxygen framework. Comparing matching oxyacids, the more electronegative central atom favors acid dissociation. In the hydrogen-halide pair, the proton is bonded directly to the halogen, so the strength of that H–X bond is central to the comparison. Moving from Br to I weakens the bond, making HI the stronger acid.

These are comparisons within each pair. They do not, by themselves, establish a complete strength ranking across all four acids.

What happens in water?

Both HBr and HI are strong acids in dilute aqueous solution: they ionize essentially completely, producing hydronium ions and halide ions. As a result, measuring their pH at the same dilute concentration will not clearly reveal their relative intrinsic strengths. This is an example of water’s leveling effect: acids stronger than hydronium are represented in solution mainly by hydronium.

When comparing acids generally, the acid-dissociation constant Ka is a measure of dissociation strength; a larger Ka means a stronger acid. Equivalently, a lower pKa means a stronger acid, where pKa = −log10(Ka). Numerical values and apparent rankings can depend on the solvent, so use the stated medium when interpreting acid-strength data.