Problem 132 - Chemistry An Atoms First Approach

Question

Place the species in each of the following groups in order of increasing base strength. Give your reasoning in each case. a. \(\mathrm{IO}_{3}^{-}, \mathrm{BrO}_{3}^{-}\) b. \(\mathrm{NO}_{2}^{-}, \mathrm{NO}_{3}^{-}\) c. \(\mathrm{OCl}^{-}, \mathrm{OI}^{-}\)

Step-by-Step Solution

Answer

The order of increasing base strength for the given groups is as follows: a. \(\mathrm{IO}_{3}^{-} < \mathrm{BrO}_{3}^{-}\) b. \(\mathrm{NO}_{3}^{-} < \mathrm{NO}_{2}^{-}\) c. \(\mathrm{OI}^{-} < \mathrm{OCl}^{-}\) This is determined by analyzing the strength of their conjugate acids using properties such as electronegativity, size of the central atom, and the presence of resonance structures.

Step 1: Find their conjugate acids

Add an \(\mathrm{H}^+\) to each oxyanion to form their respective conjugate acids: \(\mathrm{HIO}_3\) and \(\mathrm{HBrO}_3\).

Step 2: Compare the acid strengths

Both iodine (I) and bromine (Br) are in group 17, so their electronegativities are similar. However, iodine is larger than bromine, which means that \(\mathrm{HIO}_3\) has a weaker \(\mathrm{OH}\) bond and is thus a stronger acid compared to \(\mathrm{HBrO}_3\).

Step 3: Determine the order of base strength

Since \(\mathrm{HIO}_3\) is a stronger acid, \(\mathrm{IO}_3^-\) is a weaker base than \(\mathrm{BrO}_3^-\). Therefore, the order of increasing base strength is: \(\mathrm{IO}_{3}^{-} < \mathrm{BrO}_{3}^{-}\). #b. \(\mathrm{NO}_{2}^{-}, \mathrm{NO}_{3}^{-}\) We have the oxyanions \(\mathrm{NO}_2^-\) and \(\mathrm{NO}_3^-\):

Step 1: Find their conjugate acids

Add an \(\mathrm{H}^+\) to each oxyanion to form their respective conjugate acids: \(\mathrm{HNO}_2\) and \(\mathrm{HNO}_3\).

Step 2: Compare the acid strengths

Both \(\mathrm{HNO}_2\) and \(\mathrm{HNO}_3\) have the same central atom, nitrogen. However, \(\mathrm{HNO}_3\) has more oxygen atoms and resonance structures to delocalize the negative charge better, which makes it a stronger acid than \(\mathrm{HNO}_2\).

Step 3: Determine the order of base strength

Since \(\mathrm{HNO}_3\) is a stronger acid, \(\mathrm{NO}_3^-\) is a weaker base than \(\mathrm{NO}_2^-\). Therefore, the order of increasing base strength is: \(\mathrm{NO}_{3}^{-} < \mathrm{NO}_{2}^{-}\). #c. \(\mathrm{OCl}^{-}, \mathrm{OI}^{-}\) We have the oxyanions \(\mathrm{OCl}^-\) and \(\mathrm{OI}^-\):

Step 1: Find their conjugate acids

Add an \(\mathrm{H}^+\) to each oxyanion to form their respective conjugate acids: \(\mathrm{HOCl}\) and \(\mathrm{HOI}\).

Step 2: Compare the acid strengths

Both chlorine (Cl) and iodine (I) are in group 17, so their electronegativities are similar. However, iodine is larger than chlorine, which means that \(\mathrm{HOI}\) has a weaker \(\mathrm{OH}\) bond and is thus a stronger acid compared to \(\mathrm{HOCl}\).

Step 3: Determine the order of base strength

Since \(\mathrm{HOI}\) is a stronger acid, \(\mathrm{OI}^-\) is a weaker base than \(\mathrm{OCl}^-\). Therefore, the order of increasing base strength is: \(\mathrm{OI}^{-} < \mathrm{OCl}^{-}\).