Question

Which compound, KBr or KF, is expected to behave as a weak base? Why?

Short answer

The KF compound is expected to behave as a weak base because F- (its anion) has a weaker conjugate acid (HF), hence making F- a stronger base compared to Br-.

Step-by-step solution

Understand what makes a compound a weak base

A weak base does not ionize fully in water and will have a conjugate acid that is a weak acid. The conjugate acid of a weak base will not fully donate its proton (H+) in a solution, allowing the base (Br- or F-) to accept a proton and act as a weak base.

Determine the conjugate acids for the anions in the compounds

The conjugate acid of Br- is HBr and the conjugate acid of F- is HF. To identify which anions will act as a weak base when they are part of a compound, we should compare the strength of these conjugate acids.

Compare the strength of conjugate acids

HBr is a stronger acid than HF. This means that the bromide ion (Br-) is a weaker base than the fluoride ion (F-). A weaker acid has a stronger conjugate base and a stronger acid has a weaker conjugate base. Therefore, F- is a stronger base, making KF a stronger base as a whole.

Key concepts

Acid-Base Equilibrium

In chemistry, acid-base equilibrium is a dynamic and reversible state where the rates of the forward and reverse reactions are equal, resulting in no net change in the concentrations of the acid, base, conjugate base, and conjugate acid. This principle is critical when discussing weak bases, as they only partially ionize in solution.

For example, in the exercise provided, we are considering the ionization of a weak base in water. When KBr or KF dissolves in water, the anions Br- or F- can accept protons (H+) from water molecules, forming their respective conjugate acids, HBr or HF. However, because weak bases do not fully ionize, at equilibrium, significant amounts of both the base and its conjugate acid coexist in the solution. The position of the equilibrium, therefore, tells us how strong or weak the base is. This is because the position indicates the relative amounts of products and reactants present at equilibrium.

For a weak base such as F-, the equilibrium lies more to the left, meaning there are more reactants (KF and HOH) and fewer products (KF and OH-). This suggests that F- does not readily pick up a proton, but it is still stronger than Br-, as explained in the exercise.

Conjugate Acids and Bases

The concept of conjugate acids and bases is key to understanding acid-base reactions. In an acid-base reaction, when a base accepts a proton, it becomes its conjugate acid, and when an acid donates a proton, it becomes its conjugate base.

Considering the provided exercise, we look at KBr and KF as sources of the anions Br- and F- respectively. When Br- gains a proton, it forms the conjugate acid HBr, while F- forms HF when it gains a proton. The strength of the conjugate acids is inversely related to the strength of the base from which they are derived. The stronger the conjugate acid, the weaker the original base, and vice versa.

Importance in Predicting Base Strength

A crucial application of this concept is in predicting the relative strength of weak bases. Knowing the strength of the conjugate acid can lead us to infer the basicity of the original compound. As in our example, HF is a weaker acid than HBr, meaning its conjugate base F- is a stronger base compared to Br-, and thus KF is a stronger base than KBr.

Ionization of Weak Bases

The ionization of weak bases is a process characterized by the incomplete release of hydroxide ions (OH-) when the base dissolves in water. This partial ionization is what classifies a substance as a weak base - different from a strong base that ionizes completely.

Ionization of weak bases is best described using an equilibrium expression known as the base ionization constant (Kb). The smaller the Kb value, the weaker the base. In this context, F- is a stronger base than Br- because it is the conjugate base of the weaker acid HF.

Base Ionization in Water

When a salt like KBr or KF dissolves in water, the anion (Br- or F-) can accept a proton from a water molecule. This interaction forms hydroxide ions (OH-) and the conjugate acid (HBr or HF). The equilibrium expression for this reaction tells us how readily the anion acts as a base. As Br- is the conjugate base of the strong acid HBr, its tendency to accept protons is lower than that of F-, making KF the compound expected to behave as the weaker base of the two.