Question

Explain why acid strength increases in the order HIO < \(\mathrm{HBrO}<\mathrm{HClO}\)

Short answer

Acid strength increases with the halogen's electronegativity: i.e., Iodine < Bromine < Chlorine.

Step-by-step solution

Understanding Acid Strength

Acid strength refers to the ability of an acid to donate a proton. A stronger acid more readily donates a proton compared to a weaker acid. The strength of an acid is often influenced by the stability of the conjugate base that forms once the proton is donated.

Examine Halogen's Electronegativity

The acids in question are hypohalous acids where the acid strength is influenced by the electronegativity of the halogen (iodine, bromine, chlorine) attached to the oxygen. Electronegativity increases as we move from iodine to chlorine on the periodic table. Chlorine is the most electronegative, followed by bromine, and then iodine.

Correlate Electronegativity with Acid Strength

Higher electronegativity of the halogen leads to greater stabilization of the conjugate base by drawing electron density towards itself, making it easier for the acid to donate the proton. Thus, an increase in electronegativity leads to an increase in acid strength.

Apply to Given Acids

For the acids, HIO, HBrO, and HClO, chlorine is the most electronegative, followed by bromine, and then iodine. Hence, HClO has the strongest acid strength followed by HBrO, and HIO is the weakest.

Key concepts

Conjugate Base Stability

Conjugate base stability is a key factor in understanding acid strength. When an acid donates a proton, it transforms into its conjugate base. The stability of this conjugate base determines how readily the acid can donate its proton.

• If the conjugate base is highly stable, it will not reassimilate the proton easily, hence supporting the acid in being stronger.
• Conversely, if the conjugate base is unstable, it is more likely to quickly grab a proton back, which means the acid is weaker.

The stability of a conjugate base is enhanced by the ability of the remaining atoms to hold onto the negative charge that results from the proton loss. This can be achieved through various means such as resonance or electronegativity. In the case of hypohalous acids like HIO, HBrO, and HClO, the halogen attached plays a crucial role. The more electronegative the halogen, the better it stabilizes the negative charge, thus increasing the stability of the conjugate base and the acid strength. This is why acids with more electronegative halogens, such as chlorine, tend to be stronger than those with less electronegative halogens like iodine.

Proton Donation

Proton donation is the hallmark of acid behavior. The essence of an acid is its ability to donate protons (H+) to other substances. The efficiency and speed of this donation process heavily dictate the strength of the acid.

• Stronger acids donate protons more readily and completely.
• Weaker acids hold onto their protons with more reluctance and do not donate them as easily.

The propensity for proton donation relates to the overall molecular structure and the stability received after the proton is released. In the case of HIO, HBrO, and HClO, the ability to donate a proton effectively correlates with the electronegativity of the halogen attached. Higher electronegativity helps in stabilizing the negative charge post-proton release, facilitating easier and more complete proton donation, thus enhancing acid strength.

Electronegativity of Halogens

The electronegativity of halogens is crucial in determining acid strength. Electronegativity is the tendency of an atom to attract electrons towards itself. Within the group of halogens like iodine, bromine, and chlorine, there is a spectrum of electronegativity.

• Iodine, being the least electronegative, offers the weakest stabilization ability to the remaining structure once the H+ ion is lost.
• Bromine has a moderate level of electronegativity, offering better stabilization than iodine but less than chlorine.
• Chlorine, being the most electronegative, strongly attract electrons and stabilize the conjugate base effectively.

This hierarchy of electronegativity explains why HClO (containing chlorine) is a stronger acid than HBrO and why HBrO is stronger than HIO. By making the resulting conjugate base more stable, the electronegative halogen promotes stronger acid behavior, as seen with HClO compared to HIO, confirming the order of increasing acid strength: HIO < HBrO < HClO.