Question

Explain the following observations: (a) \(\mathrm{HCl}\) is a stronger acid than \(\mathrm{H}_{2} \mathrm{~S} ;\) (b) \(\mathrm{H}_{3} \mathrm{PO}_{4}\) is a stronger acid than \(\mathrm{H}_{3} \mathrm{As} \mathrm{O}_{4}\); (c) \(\mathrm{HBrO}_{3}\) is a stronger acid than \(\mathrm{HBr} \mathrm{O}_{2}\); (d) \(\mathrm{H}_{2} \mathrm{C}_{2} \mathrm{O}_{4}\) is a stronger acid than \(\mathrm{HC}_{2} \mathrm{O}_{4}^{-} ;(\mathrm{e})\) benzoic acid \(\left(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COOH}\right)\) is a stronger acid than phenol \(\left(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OH}\right)\) ).

Short answer

In summary: (a) HCl is a stronger acid than H2S due to the higher electronegativity of chlorine compared to sulfur. (b) H3PO4 is a stronger acid than H3AsO4 because phosphorus is smaller and more electronegative than arsenic. (c) HBrO3 is a stronger acid than HBrO2 because it has more oxygen atoms, which helps stabilize the negative charge of the conjugate base. (d) H2C2O4 is a stronger acid than HC2O4^- since oxalic acid can donate two protons and forms a more stable conjugate base after losing its first proton. (e) Benzoic acid (C6H5COOH) is a stronger acid than phenol (C6H5OH) because the carboxylic acid group in benzoic acid can stabilize the negative charge on the conjugate base more effectively than the hydroxyl group in phenol.

Step-by-step solution

(a) HCl is a stronger acid than H2S

This can be explained by considering the electronegativity of the elements involved. Chlorine is more electronegative than sulfur, so it attracts electrons more strongly than sulfur. As a result, the bond between hydrogen and chlorine is more polar than the bond between hydrogen and sulfur. When HCl donates a proton, the electrons in the H-Cl bond are more likely to remain with the Cl atom, forming a stable conjugate base (Cl-). For H2S, the electrons in the H-S bond don't have as strong of a tendency to remain with sulfur when donating a proton, making it a weaker acid.

(b) H3PO4 is a stronger acid than H3AsO4

Both H3PO4 and H3AsO4 are oxyacids with similar structures. In this case, we can explain their acid strength difference by considering the sizes and electronegativities of phosphorus (P) and arsenic (As) atoms. Phosphorus is smaller and more electronegative than arsenic, which causes the O-H bonds in H3PO4 to be more polar and easier to break to release a proton (H+). As a result, H3PO4 will donate a proton more readily and be a stronger acid than H3AsO4.

(c) HBrO3 is a stronger acid than HBrO2

Both HBrO3 and HBrO2 are oxyacids of bromine with different numbers of oxygen atoms attached to the central bromine atom. In general, the more oxygen atoms an oxyacid has, the more likely it is to donate a proton and act as a stronger acid. This is because oxygen atoms are electronegative and help to stabilize the negative charge on the conjugate base once a proton is donated. In this case, HBrO3 has more oxygen atoms than HBrO2, resulting in a stronger acid.

(d) H2C2O4 is a stronger acid than HC2O4^-

Both H2C2O4 (oxalic acid) and HC2O4^- (monohydrogen oxalate ion) can donate a proton; however, H2C2O4 can donate two protons, while HC2O4^- can only donate one proton. As the oxalic acid molecule loses its first proton, it forms a more stable conjugate base (HC2O4^-) due to the spreading of the negative charge across both oxygen atoms involved in a resonance structure, aside from forming a more stable conjugate base than the oxalate ion (its second step). Thus, H2C2O4 is a stronger acid than HC2O4^-.

(e) Benzoic acid (C6H5COOH) is a stronger acid than phenol (C6H5OH)

Benzoic acid and phenol are both organic acids derived from benzene. The difference lies in the functional groups attached to the benzene ring: benzoic acid has a carboxylic acid group (-COOH) while phenol has a hydroxyl group (-OH). Carboxylic acid groups are generally more acidic than hydroxyl groups due to the presence of a carbonyl (C=O) bond in the carboxylic acid group. This carbonyl bond helps to stabilize the negative charge on the conjugate base (C6H5COO-) through resonance, making it easier to donate a proton than phenol, which forms less stable phenoxide ion (C6H5O-) upon losing a proton. Therefore, benzoic acid is a stronger acid than phenol.

Key concepts

Electronegativity

Electronegativity is the ability of an atom to attract electrons towards itself within a chemical bond. This property helps explain why some acids are stronger than others, by affecting the polarity of the bond between hydrogen and the atom to which it is bonded.
Chlorine, for instance, is more electronegative than sulfur. This higher electronegativity means the electrons in the \( ext{H-Cl}\) bond are more attracted to the chlorine, resulting in a more polar bond. This polarity favors the release of a hydrogen ion, forming a stable conjugate base, \(\text{Cl}^-\). On the other hand, sulfur in \(\text{H}_2\text{S}\) is less electronegative, making it less likely to release a hydrogen ion.Electronegativity differences explain why hydrochloric acid (\(\text{HCl}\)) is stronger than hydrogen sulfide (\(\text{H}_2\text{S}\)). When studying acids, always consider how electronegativity affects bond polarity and acid strength.

Oxyacids

Oxyacids are a type of acid consisting of hydrogen, oxygen, and usually another non-metal element. The acidity in oxyacids is influenced by several factors, including the number of oxygen atoms present.
In oxyacids like \(\text{H}_3\text{PO}_4\) and \(\text{H}_3\text{AsO}_4\), the central atom and the electronegativity play crucial roles. Phosphorus in \(\text{H}_3\text{PO}_4\) is smaller and more electronegative compared to arsenic in \(\text{H}_3\text{AsO}_4\), leading to stronger \(\text{O-H}\) bonds that break more easily to release protons. This results in \(\text{H}_3\text{PO}_4\) being a stronger acid than \(\text{H}_3\text{AsO}_4\).Additionally, more oxygen atoms in an oxyacid like \(\text{HBrO}_3\) compared to \(\text{HBrO}_2\) make it a stronger acid. More oxygen atoms help stabilize the negative charge on the conjugate base, enhancing the acidic strength. Thus, the structure and components of oxyacids are key to understanding their acid strength.

Conjugate Base Stability

The stability of a conjugate base is a vital factor in determining an acid's strength. More stable conjugate bases typically result in stronger acids.
Take oxalic acid (\(\text{H}_2\text{C}_2\text{O}_4\)) as an example. When it donates a proton, the \(\text{H}_2\text{C}_2\text{O}_4^-\) forms a stable conjugate base because the negative charge can be spread out over both oxygen atoms through resonance. This stability makes the acid stronger.
On the other hand, when \(\text{HC}_2\text{O}_4^-\) loses a proton, the base formed is less stable because it has fewer resonance structures to diffuse the charge. The higher stability of \(\text{HC}_2\text{O}_4^-\) compared to the oxalate ion renders oxalic acid more willing to donate the first proton.Understanding how easily a conjugate base forms and stabilizes is crucial when exploring the relative strengths of different acids.

Resonance Structures

Resonance structures are a concept that describes different ways of distributing electrons within a molecule. They play a critical role in stabilizing conjugate bases, therefore influencing acid strength.
In benzoic acid (\(\text{C}_6\text{H}_5\text{COOH}\)), the carboxylic acid group stabilizes its conjugate base through resonance. The negative charge resulting from the loss of a proton is shared across different atoms and bonds. This delocalization of charge makes the conjugate base (\(\text{C}_6\text{H}_5\text{COO}^-\)) more stable.
Conversely, phenol (\(\text{C}_6\text{H}_5\text{OH}\)) does not have the same extent of resonance stabilization in its conjugate base (phenoxide ion). As a result, benzoic acid can more readily lose a proton than phenol, making it a stronger acid.The presence and effectiveness of resonance structures significantly enhance the stabilization of conjugate bases, thereby affecting acid strength.

Functional Groups

Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. In acids, functional groups can greatly influence their strength.
In comparing benzoic acid (\(\text{C}_6\text{H}_5\text{COOH}\)) to phenol (\(\text{C}_6\text{H}_5\text{OH}\)), the functional group plays a significant role. Benzoic acid contains a carboxylic acid group \(\text{-COOH}\), a functional group that easily donates a proton due to resonance stabilization of the conjugate base. This makes benzoic acid a relatively strong acid.
Phenol, with a hydroxyl group (\(\text{-OH}\)), is less able to stabilize its conjugate base as effectively. The presence of the carboxylic group in benzoic acid results in greater acidity compared to phenol. Functional groups are key determinants in the chemical behavior and strength of acids, underscoring their importance in organic chemistry.