Question

Arrange each group of compounds in order of decreasing acid strength. Explain your reasoning- (a) \(\mathrm{H}_{2} \mathrm{O}, \mathrm{H}_{2} \mathrm{~S}, \mathrm{H}_{\mathrm{S}} \mathrm{Se}\) (b) \(\mathrm{HClO}_{3}, \mathrm{HBrO}_{3}, \mathrm{HIO}_{3}\) (c) \(\mathrm{PH}_{3}, \mathrm{H}_{2} \mathrm{~S}, \mathrm{HCl}\)

Short answer

(a) \(H_2Se > H_2S > H_2O\); (b) \(HClO_3 > HBrO_3 > HIO_3\); (c) \(HCl > H_2S > PH_3\).

Step-by-step solution

Understanding Acid Strength in Group (a)

For group (a), the strength of binary acids like \(H_2O\), \(H_2S\), and \(H_2Se\) typically increases as you move down a group in the periodic table. This is because the bond strength between hydrogen and the other element decreases due to an increase in atomic size, making it easier for the hydrogen ion to dissociate in solution. Hence, \(H_2Se\) being the heaviest, is the strongest acid, followed by \(H_2S\), and \(H_2O\) is the weakest.

Order Compounds in Group (a)

Arrange the compounds in decreasing acid strength: \(H_2Se > H_2S > H_2O\).

Understanding Acid Strength in Group (b)

In group (b), the acids \(HClO_3\), \(HBrO_3\), and \(HIO_3\) are oxyacids. The acidity of oxyacids generally decreases with decreasing electronegativity of the central atom. Since chlorine is the most electronegative, \(HClO_3\) is the strongest acid, followed by \(HBrO_3\) with bromine, and \(HIO_3\) is the weakest with iodine.

Order Compounds in Group (b)

Arrange the compounds in decreasing acid strength: \(HClO_3 > HBrO_3 > HIO_3\).

Understanding Acid Strength in Group (c)

For group (c), the acids \(PH_3\), \(H_2S\), and \(HCl\) are binary acids. Acidity increases across the period as electronegativity increases. Chlorine is the most electronegative, so \(HCl\) is the strongest acid, followed by \(H_2S\), and \(PH_3\) is the weakest.

Order Compounds in Group (c)

Arrange the compounds in decreasing acid strength: \(HCl > H_2S > PH_3\).

Key concepts

Binary Acids

Binary acids are compounds consisting of hydrogen and one other element. Understanding their strength is essential for predicting chemical behavior. These acids include examples such as HCl, H₂S, and H₂Se.

Determining Acid Strength
The strength of binary acids increases down a group in the periodic table. This happens because the atomic size of the non-hydrogen element increases, reducing the bond strength with hydrogen. This makes it easier for the hydrogen ion (H⁺) to dissociate from the acid in water.

• For H₂O, H₂S, and H₂Se, you see increasing acidity due to increasing atomic size from oxygen to selenium.
• For HCl, H₂S, and PH₃, similar reasoning applies, but as these elements move across a period to chlorine, electronegativity becomes the dominant factor, making HCl the strongest.

This principle helps explain the acid strengths in various contexts, aiding in understanding chemical reactions and properties.

Oxyacids

Oxyacids are acids that contain hydrogen, oxygen, and another element (the central atom). Their acidity is influenced by the electronegativity of the central atom and the number of oxygen atoms.

Influencing Factors
The strength of an oxyacid often increases with the electronegativity of the central atom. This is because a more electronegative central atom pulls electron density away, making it easier for the hydrogen ion to dissociate.

• In the case of HClO₃, HBrO₃, and HIO₃, chlorine's high electronegativity makes HClO₃ the strongest acid, followed by bromine and then iodine.
• A larger number of oxygen atoms can also enhance acidity by stabilizing the negative charge after hydrogen dissociation.

Knowing these factors aids in predicting the reactivity and behavior of oxyacids in various chemical processes.

Periodic Trends in Acidity

Periodic trends provide valuable insights into the properties of acids and help predict their behavior. Acidity trends often follow the periodic table's organization, emphasizing the roles of atomic size and electronegativity.

Key Trends

• Down a Group: Increases in acidity are due to larger atomic sizes, which weaken the bond with hydrogen, enhancing ionization. This is prominent in binary acids like H₂Se being stronger than H₂S and H₂O.
• Across a Period: Acidity typically increases with greater electronegativity. For binary acids like HCl being stronger than H₂S and PH₃, the trend illustrates the importance of electronegativity.

These periodic trends offer a clear understanding of how different elements affect acid strength, serving as a guide for predicting and explaining chemical behaviors.