Question

Rank the following acids in order of decreasing acid strength: \(\mathrm{HBrO}_{3}, \mathrm{HBrO}_{4}, \mathrm{HIO}_{3}, \mathrm{HClO}_{4-}\)

Short answer

\text{HClO}_{4}, \text{HBrO}_{4}, \text{HBrO}_{3}, \text{HIO}_{3}.

Step-by-step solution

- Identify the Acids

The acids provided are \(\text{HBrO}_{3}\), \(\text{HBrO}_{4}\), \(\text{HIO}_{3}\), and \(\text{HClO}_{4-}\). These are all oxoacids.

- Understand Oxoacid Strength

The strength of oxoacids depends largely on the number of oxygen atoms attached to the central atom and the electronegativity of the central atom. Generally, more oxygen atoms lead to stronger acids due to increased stabilization of the conjugate base.

- Analyze Each Acid

1. For \(\text{HBrO}_{3}\), there are 3 oxygen atoms attached to bromine. \(\text{HBrO}_{4}\) has 4 oxygen atoms attached to bromine. \(\text{HIO}_{3}\) has 3 oxygen atoms attached to iodine. \(\text{HClO}_{4-}\) has 4 oxygen atoms attached to chlorine but is negatively charged, suggesting it is actually the conjugate base of the acid \(\text{HClO}_{4}\).

- Rank by Number of Oxygens and Central Atom Electronegativity

Comparing acids with the same number of oxygen atoms:- \(\text{HIO}_{3}\) vs \(\text{HBrO}_{3}\): Iodine is less electronegative than bromine, so \(\text{HIO}_{3}\) is weaker than \(\text{HBrO}_{3}\).Comparing acids with different numbers of oxygen atoms:- \(\text{HBrO}_{4}\) vs \(\text{HBrO}_{3}\): \(\text{HBrO}_{4}\) has more oxygen atoms, so it's stronger.- \(\text{HClO}_{4}\) (considering \(\text{HClO}_{4-}\) as its conjugate base) vs \(\text{HBrO}_{4}\): Chlorine is more electronegative than bromine, making \(\text{HClO}_{4}\) stronger than \(\text{HBrO}_{4}\).

- Final Ranking

Putting it all together, the acids in order of decreasing strength are:1. \(\text{HClO}_{4}\)2. \(\text{HBrO}_{4}\)3. \(\text{HBrO}_{3}\)4. \(\text{HIO}_{3}\)

Key concepts

oxoacids

Oxoacids are acids that contain oxygen, hydrogen, and another element (called the central atom). They are frequently encountered in chemistry and have different acid strengths based on their structure.

For instance, in oxoacids like \(\text{HBrO}_3\), \(\text{HBrO}_4\), \(\text{HIO}_3\), and \(\text{HClO}_4\), the central atom is bonded to oxygen atoms. These oxygen atoms play a crucial role in determining the acid's strength due to their ability to stabilize the conjugate base. For oxoacids, the more oxygen atoms present, the stronger the acid usually is. This is because oxygen is a highly electronegative element that can pull electron density away from the hydrogen atom, making it easier for the hydrogen to dissociate as a proton (H⁺).

electronegativity

Electronegativity refers to the ability of an atom to attract electrons toward itself. In the context of oxoacids, electronegativity of the central atom plays a significant role in determining acid strength.

For example, chlorine (Cl) is more electronegative than bromine (Br) or iodine (I). This means that in oxoacids such as \(\text{HClO}_4\) (with chlorine) compared to \(\text{HBrO}_4\) (with bromine), \(\text{HClO}_4\) will be a stronger acid. The higher electronegativity of chlorine pulls more electron density towards itself, making it easier for the hydrogen to dissociate as a proton, thus increasing the acid strength. Therefore, the central atom's electronegativity is a key factor in comparing acids with the same number of oxygen atoms.

conjugate base stability

The strength of an acid is also determined by the stability of its conjugate base. The more stable the conjugate base, the stronger the acid.

For oxoacids, the stability of the conjugate base is enhanced by the presence of oxygen atoms, which can delocalize and redistribute the negative charge over a larger volume. This delocalization stabilizes the conjugate base.

For example, in \(\text{HClO}_4\), after donating a proton, the conjugate base \(\text{ClO}_4⁻\) is very stable due to the four oxygen atoms that can delocalize the negative charge. On the other hand, \(\text{HBrO}_3\) will have a less stable conjugate base because it has fewer oxygen atoms (only 3), resulting in less delocalization and thus a weaker acid compared to \(\text{HClO}_4\).

number of oxygen atoms

The number of oxygen atoms in an oxoacid is a significant factor influencing its strength. More oxygen atoms usually equate to a stronger acid because they help stabilize the conjugate base.

Each oxygen atom, being highly electronegative, can pull electron density away from the acidic hydrogen, making it easier for the hydrogen to be released as a proton (H⁺). For example, in \(\text{HBrO}_4\), there are four oxygen atoms as compared to three in \(\text{HBrO}_3\), making \(\text{HBrO}_4\) a stronger acid. Additionally, more oxygen atoms equate to more opportunities for delocalization of the negative charge, which further stabilizes the conjugate base.