Question

Question 4 You may want to reference (u Rages 610.613 Section 15.3 while completing this problem. Arrange each group of compounds in order of decreasing acid strength. Part A H2O, H2S, H2Se Match the words in the left column to the appropriate blanks in the sentences on the right. M certain each sentence is complete before submitting your answer. Reset Help H2Se 1. Arranged by decreasing acid strength, the series H20, H2S, H2Se would be organized as X-H bond strength decreases 2. The main reason for the observed trend is that from O to Se ion charge increases atom size increases H2O H2S electronegativity increases Submit My Answers Give Up Part B

Answer 1

Part A

1. The acidity of the hydrides of group 16 (oxygen family) will depend on the ease of proton release as below:

H₂X <=====> H⁺ + HX^- (X = O, S, Se)

The ease with which the X-H bond ruptures depends on the strength of the X-H bond. The strength of the X-H bond depends on the electronegativity of the X atom. The more electronegative the X atom is, the more strongly will it withdraw the shared electron pair of the X-H bond toward itself and therefore, stronger is the X-H bond and hence more difficult to rupture. The electronegativity of the X atom depends on the size of X atom. The smaller the size of X atom, the more close are the electrons to the nucleus and hence more electronegative is the atom. Therefore, in moving from O to S to Se, the number of electrons in the valence orbital remains the same while the atomic size increases. Therefore, the atomic size varies as

O < S < Se (atomic size increases)

The strength of the X-H bond varies as O-H > S-H > Se-H

Therefore, the Se-H bond is easily broken to give a proton and hence H₂Se is the most acidic.

1. Therefore, the order of decreasing acid strength is

H₂Se > H₂S > H₂O (ans)

2. The main reason for the observed trend is that atomic size increases from O to Se (ans).

Part B

In the given acid series, the central atom changes its identity. In acids of the type HXO₃ (X = Cl, Br, I), the ease with which the O-H bond is broken depends on the strength of the O-X bond. The strength of the O-X bond depends on the electronegativity of the X atom and hence, the atomic size of X atom. The smaller the X atom, the stronger is the bond between O and X (since both O and X atoms are small, the bonding will be better). Therefore, the smaller the X atom is, the more electronegative it is. On the other hand, as the size of X increases, the bond between O and X becomes weak because the O atom is less attracted by the X atom. In other words, the X atom is less electronegative.

The stronger the O-X bond, the weaker will be the O-H bond (since O will have a greater tendency to share the electron pair with a stronger electronegative element than with H) and hence more easily broken.

1. The order of decreasing acid strength is

HClO₃ > HBrO₃ > HIO₃ (ans)

2. The main reason for the observed trend is that electronegativity increases from I to Cl (ans).