Problem 67 - General Chemistry: Principles and Modern Applications

Question

Which is the stronger acid of each of the following pairs of acids? Explain your reasoning. (a)\( HBr or HI;\) (b\() HOClO or HOBr; (c) I_SCCH_CH_COOH\) or \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CCl}_{2} \mathrm{COOH}\).

Step-by-Step Solution

Answer

The stronger acids are HI, HOBr, and CH3CH2CCl2COOH.

Step 1: Compare Acidity of HBr and HI

When going down the group on the periodic table, the acid strength increases. This is due to the increase in the size of the atom which weakens the bond strength, allowing the proton (H+) to dissociate more easily. Thus, in the case of HBr and HI, HI is stronger due to its larger size.

Step 2: Compare Acidity of HOClO and HOBr

Just like the previous comparison, going down the group on the periodic table increases acid strength. Thus, between HOClO and HOBr, HOBr is the stronger acid due to bromine's larger size.

Step 3: Compare Acidity Between I_SCCH_CH_COOH and CH3CH2CCl2COOH

For organic compounds, the electron-withdrawing capacity of the atom attached to the carboxyl group (-COOH) affects the acidity. In the case of I_SCCH_CH_COOH and CH3CH2CCl2COOH, the latter has more chlorine atoms which pull electrons away from the carboxyl group, increasing acidity. Thus, CH3CH2CCl2COOH is the stronger acid.

Key Concepts

Periodic Table Trends

Understanding the periodic table is crucial to predicting and comparing the strength of acids. The periodic table is organized in such a way that elements exhibit patterns or 'trends' in their properties, including acidity, as we move across periods (horizontal rows) or down groups (vertical columns).

One key trend is that acidity increases as we move down a group. This increase in acidity is linked to the size of the atom; larger atoms have more diffuse electron clouds, which means that the attraction between the protons in the nucleus and bonding electrons is weaker. This reduced electron-nucleus attraction in larger atoms, like iodine compared to bromine, makes it easier for the compound to donate a proton (H+), thus becoming a stronger acid.

These principles apply not just to binary acids (which consist of hydrogen and one other element), but they can also influence the acidity of more complex molecules, including organically functionalized compounds like carboxylic acids.

Carboxylic Acid Acidity

Carboxylic acids are a group of organic acids characterized by the presence of a carboxyl group (-COOH). Their acidity is pivotal in organic chemistry and biological systems. A carboxylic acid's acidity can be affected by different factors, including the inductive effect caused by adjacent atoms or functional groups.

Atoms or groups that are more electronegative than the carbon atom in the carboxyl group can pull electron density away from the O-H bond, thus stabilizing the carboxylate anion that forms when the acid donates a proton. This stabilization enhances the ability of the compound to act as an acid. For instance, the presence of electronegative chlorine atoms in CH3CH2CCl2COOH makes it a stronger acid than a similar molecule with fewer electron-withdrawing groups, as they effectively increase the positive character of the H+ ion, facilitating its release.

Atomic Radius and Acidity

The atomic radius, or the size of an atom, plays an integral role in determining the strength of an acid, particularly with binary acids like HBr and HI. As the atomic radius increases, the bond between the hydrogen atom and the larger atom tends to become longer and weaker. This weakness is due to the increased distance between the atoms’ nuclei, which spreads out the bonding electrons and reduces the overall pull the nucleus has on them.

As a result, large atoms, such as iodine, are not as effective at holding onto the bonding pair of electrons in the H-X bond. This inefficiency facilitates the release of the H+ ion, making acids with larger atomic radii stronger. Therefore, when students are given pairs of binary acids and asked to compare their strengths, they should consider the relative sizes of the non-hydrogen atoms; the larger the atom, the stronger the acid it forms when bonded to hydrogen.