Question

Acid-Base Properties of Ions and Salts Pick the stronger base from each pair. \(\begin{array}{l}{\text { a. } \mathrm{F}^{-} \text { or } \mathrm{Cl}^{-}} \\\ {\text { b. } \mathrm{NO}_{2}^{-} \text { or } \mathrm{NO}_{3}^{-}} \\\ {\text { c. } \mathrm{F}^{-} \text { or } \mathrm{ClO}^{-}}\end{array}\)

Short answer

The stronger bases are: a. F-, b. NO2-, c. F-.

Step-by-step solution

Understand the concept of acid-base properties

Acid-base properties of ions are often derived from the strength of their conjugate acids. A stronger acid will have a weaker conjugate base and vice versa. The strength of acids generally increases from left to right across a period and from top to bottom down a group in the periodic table.

Compare the conjugate acids of F- and Cl-

Compare the strength of the conjugate acids, HF and HCl. Because HCl is a much stronger acid than HF, F- is a stronger base than Cl-, since the conjugate base of a weaker acid is stronger.

Compare the conjugate acids of NO2- and NO3-

The acid strengths of HNO2 and HNO3 show that HNO3 is a stronger acid than HNO2. Therefore, NO2- is a stronger base than NO3-, because it comes from the weaker acid HNO2.

Compare the conjugate acids of F- and ClO-

Fluoride ion comes from HF which is a weak acid compared to HClO (hypochlorous acid), but HClO is not as strong as HCl. This makes it slightly more complicated, but since HF is the weaker acid compared to HClO, F- is the stronger base compared to ClO-.

Key concepts

Conjugate Acid-Base Pairs

Understanding conjugate acid-base pairs is crucial when studying the properties of ions. Imagine a seesaw where when one side goes up, the other goes down. This is much like how acids and bases work. In a water solution, when an acid donates a proton, it becomes a base, specifically, the conjugate base of the acid before the proton was lost.

For instance, hydrogen fluoride (HF) can donate a proton to become fluoride ion (F-), which is its conjugate base. On the other hand, when F- accepts a proton, it becomes HF again, now acting as the conjugate acid. A rule of thumb to remember is: the stronger the acid, the weaker its conjugate base, and vice versa. As such, a weakly acidic molecule, such as HF, has a strong conjugate base (F-). This is key to predicting the base strength in a pair, just as we did in the exercise for F- and Cl-.

Periodic Trends in Acid Strength

Periodic trends play an essential role in determining the acid strength of molecules, which directly influences the base strength of their corresponding ions. Moving from left to right across a period in the periodic table, electronegativity increases. More electronegative elements hold on to protons more tightly, making their acids stronger. As a result, their conjugate bases are typically weaker.

Down the Group Trend

Meanwhile, moving down a group, the size of atoms increases. Larger atoms with more diffuse electron clouds tend to hold the protons less tightly, making their acids weaker and their conjugate bases stronger. This understanding helps us predict that, generally, an ion like Cl- derived from HCl will be a weaker base than an ion like F- derived from HF because HF is further to the right in the same period.

Relative Base Strength Comparison

When comparing the base strength of two ions, look at the acid they come from. The weaker the acid, the stronger the base. It's basically a balance between the two. Think of it as a sibling rivalry; if one twin is really good at basketball (the acid), the other might excel at chess (the base). So, in our exercise:

• F- is a stronger base than Cl-, as HF is a weaker acid than HCl.
• NO2- is stronger than NO3-, because its conjugate acid, HNO2, is weaker than HNO3.
• F- is a stronger base than ClO-, because HF is weaker than HClO.

Remember, these comparisons always go back to looking at the strength of the associated acids. Think of it as familial traits: understanding the character of the parent acid gives you insight into the behavior of the child base.