Question

Predict the stronger acid in each pair: (a) \(\mathrm{HCl}\) or HF; (b) \(\mathrm{H}_{3} \mathrm{PO}_{4}\) or \(\mathrm{H}_{3} \mathrm{AsO}_{4} ;\) (c) \(\mathrm{HBrO}_{3}\) or \(\mathrm{HBrO}_{2}\) (d) \(\mathrm{H}_{2} \mathrm{C}_{2} \mathrm{O}_{4}\) or \(\mathrm{HC}_{2} \mathrm{O}_{4} \overline{;} ;(\mathbf{e})\) benzoic acid \(\left(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COOH}\right)\) or phenol \(\left(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OH}\right) .\)

Short answer

The stronger acids among the given pairs are: (a) HCl (b) H₃AsO₄ (c) HBrO₃ (d) HC₂O₄⁻ (e) Benzoic acid (C₆H₅COOH)

Step-by-step solution

Evaluate Electronegativity Difference

In this case, it is helpful to compare the electronegativity difference between the central atoms, Cl and F. The general trend of the periodic table is that electronegativity increases as we move right across a period and decreases as we move down a group. Since chlorine is lower in the periodic table, it is less electronegative than fluorine.

Determine the Stronger Acid

Fluorine is more electronegative, so it holds the shared electrons more tightly. But this makes it less willing to give up a proton, and therefore, HF is a weaker acid than HCl. #b) H₃PO₄ vs H₃AsO₄#

Evaluate Atomic Size

Phosphorous and arsenic are in the same group but differ in the period they belong to. Since arsenic lies in a lower period, it is a larger atom compared to phosphorous.

Determine the Stronger Acid

A larger size atom stabilizes the negative charge on the conjugate base better. Therefore, H₃AsO₄ is a stronger acid than H₃PO₄. #c) HBrO₃ vs HBrO₂#

Evaluate the Number of Oxygen Atoms

Start by counting the number of oxygen atoms surrounding the central bromine atom. In HBrO₃, there are three oxygen atoms, whereas, in HBrO₂, there are only two oxygen atoms.

Determine the Stronger Acid

More oxygen atoms increase the electron-withdrawing nature of the molecule, which stabilizes the resulting negative charge on the conjugate base. Therefore, HBrO₃ is a stronger acid than HBrO₂. #d) H₂C₂O₄ vs HC₂O₄− #

Compare the number of acidic protons

H₂C₂O₄ has two acidic protons, whereas its counterpart HC₂O₄⁻ has only one acidic proton.

Determine the Stronger Acid

H₂C₂O₄ can donate two protons, and therefore its conjugate base has a higher charge (2-) compared to HC₂O₄⁻, which has one proton to donate and has a conjugate base with a charge of 1-. Due to the higher charge stabilization needed for the H₂C₂O₄ conjugate base, HC₂O₄⁻ is the stronger acid. #e) Benzoic acid (C₆H₅COOH) vs Phenol (C₆H₅OH)#

Examine the functional groups

Look at the functional groups of the two compounds. Benzoic acid has a carboxyl group (COOH), and phenol has a hydroxyl group (OH).

Evaluate resonance structures

For benzoic acid, the resulting negative charge from donating a proton is stabilized through resonance with the aromatic ring. In the case of phenol, the negative charge formed after donating a proton can also be stabilized through resonance, but to a lesser extent because the anion is less delocalized compared to benzoic acid.

Determine the Stronger Acid

Due to the higher stabilization of the negative charge through resonance in the carboxyl group, benzoic acid (C₆H₅COOH) is a stronger acid compared to phenol (C₆H₅OH).

Key concepts

Electronegativity

Electronegativity is a measure of an atom's ability to attract and hold onto electrons. This property is crucial in determining the strength of acids. When you look at the periodic table, electronegativity increases from left to right across a period and decreases from top to bottom in a group. This trend is necessary to understand when comparing acids such as HCl and HF.

Fluorine is the most electronegative element, meaning it tightly holds onto electrons. This tight hold on the shared electrons in HF makes it harder for the compound to give up a proton, which is a necessary function for acid strength. Consequently, HF is a weaker acid than HCl, where chlorine, with lesser electronegativity, allows for easier proton donation and thus greater acid strength. Understanding these trends in electronegativity helps predict the reactivity and strength of acids effectively.

Conjugate Base Stabilization

The strength of an acid can also be determined by how well its conjugate base stabilizes the charge once a proton is donated. This principle plays a significant role in comparing acids like H extsubscript{3}PO extsubscript{4} and H extsubscript{3}AsO extsubscript{4}.

In this example, arsenic in H extsubscript{3}AsO extsubscript{4} is larger than phosphorous in H extsubscript{3}PO extsubscript{4}, allowing the arsenate ion to better stabilize the negative charge that forms after the acid donates a proton. The ability of a larger atom to distribute the negative charge effectively across its surface makes H extsubscript{3}AsO extsubscript{4} a stronger acid. Understanding how size and electron distribution improve charge stabilization in conjugate bases helps explain why certain acids are stronger than others.

Resonance Structures

Resonance structures are a way of describing delocalized electrons within a molecule. They play a significant role in the stability of molecules and their ability to act as acids. In the example of benzoic acid (C extsubscript{6}H extsubscript{5}COOH) versus phenol (C extsubscript{6}H extsubscript{5}OH), resonance greatly influences acid strength.

When benzoic acid donates a proton, the negative charge on the oxygen is delocalized over the aromatic ring, stabilizing the resulting anion. This resonance stabilization makes benzoic acid a stronger acid.

In phenol, though there is also resonance, the extent is less as the anion isn't as effectively delocalized, resulting in weaker stabilization. Having a solid grasp of resonance helps you understand why certain molecules stabilize their charges better upon losing a proton, resulting in higher acid strength.

Functional Groups

Functional groups are specific groups of atoms within molecules that determine the characteristic reactions of those molecules. They are important in evaluating the chemical behaviors, such as acidity, of molecules like benzoic acid and phenol.

In comparing C extsubscript{6}H extsubscript{5}COOH (benzoic acid) and C extsubscript{6}H extsubscript{5}OH (phenol), the carboxyl group (COOH) of benzoic acid is a key player in its acid strength. This group tends to donate protons readily, and the negative charge left behind is stabilized by resonance, as previously discussed.

On the other hand, phenol contains a hydroxyl group (OH), which is not as effective in stabilizing the negative charge after proton donation. Having a good understanding of functional groups and how they affect molecular stability and reactivity can provide deeper insights into acid strength and behavior.