Problem 163 - Introductory Chemistry Atoms First

Question

If \(\mathrm{CI}^{-}\) is the conjugate base of \(\mathrm{HCl}\), why isn't an aqueous solution of \(\mathrm{NaCl}\) acidic?

Step-by-Step Solution

Answer

An aqueous solution of NaCl isn't acidic because the dissociated ions, Na+ and Cl-, do not significantly affect the concentration of hydronium ions (\(\mathrm{H_3O^+}\)). Cl- is a weak base as the conjugate base of a strong acid (HCl), and Na+ is a weak acid as the conjugate acid of a strong base (NaOH). Both have a low tendency to either accept or donate protons, and therefore, do not make the solution acidic.

Step 1: Understanding conjugate acids and bases

A conjugate acid-base pair consists of two species that differ by one proton (H+). In this case, HCl is the acid, and Cl- is its conjugate base.

Step 2: Analyzing the dissociation of NaCl in water

When NaCl is dissolved in water, it dissociates into its constituent ions: Na+ and Cl-. The equation for this process is: \[ \mathrm{NaCl} \rightarrow \mathrm{Na}^+ + \mathrm{Cl}^- \]

Step 3: Understanding the role of Cl- ions

Since Cl- is the conjugate base of a strong acid (HCl), it is a weak base, almost inert. It has a very low tendency to accept protons (H+) from water, and so it doesn't significantly affect the concentration of hydronium ions (\(\mathrm{H_3O^+}\)). Therefore, it doesn't make the solution acidic.

Step 4: Understanding the role of Na+ ions

Na+ is the conjugate acid of a strong base (NaOH). It is a weak acid, also almost inert, and has a very low tendency to donate protons (H+) to water. It doesn't have a significant effect on the concentration of hydronium ions (\(\mathrm{H_3O^+}\)) either, so it also doesn't make the solution acidic.

Step 5: Conclusion

An aqueous solution of NaCl isn't acidic because the ions formed by the dissociation of NaCl, namely Na+ and Cl-, do not significantly affect the concentration of hydronium ions (\(\mathrm{H_3O^+}\)). The Cl- ion, as the conjugate base of a strong acid, is a weak base and does not have a high enough affinity for protons to have an effect on the acidity of the solution. Likewise, the Na+ ion, as the conjugate acid of a strong base, is a weak acid and therefore does not donate many protons to make the solution acidic.

Key Concepts

Conjugate Acid-Base Pair

In acid-base chemistry, the concept of conjugate acid-base pairs is fundamental. These pairs consist of two species that differ by the presence or absence of a proton ( H^+ ). When an acid donates a proton, it transforms into its conjugate base. Conversely, when a base accepts a proton, it becomes its conjugate acid.

For example, hydrochloric acid ( HCl ) donates a proton to become the chloride ion ( Cl^- ), its conjugate base.

• HCl (acid) → Cl^- (conjugate base)

These pairs allow us to understand the reversible nature of acid-base reactions. By examining the strength of acids and bases, we can determine the behavior of their conjugates. In general, strong acids have weak conjugate bases, and strong bases have weak conjugate acids.

Dissociation in Water

The process of dissociation involves the separation of a compound into its individual ions when it dissolves in water. This is a common occurrence among salts, acids, and bases. For instance, sodium chloride ( NaCl ), when dissolved in water, dissociates into sodium ions ( Na^+ ) and chloride ions ( Cl^- ).

The reaction can be shown as follows:

• NaCl → Na^+ + Cl^-

This dissociation is vital to understand because it helps determine the ionic composition of the solution. In the case of NaCl , the dissociated ions do not interact significantly with water to change the acidity or basicity of the solution. This means that even though chloride ions are present, they don't make the water acidic.

Hydronium Ion Concentration

The concentration of hydronium ions ( H_3O^+ ) in a solution is a key determinant of its acidity. When acids dissolve in water, they typically increase the concentration of H_3O^+ ions, making the solution acidic. However, if the ions present in the solution do not change H_3O^+ levels, the solution remains neutral.

In a sodium chloride solution, although chloride ions are present, they do not significantly alter H_3O^+ concentrations. This is because Cl^- , as the conjugate base of the strong acid HCl , is a weak base and does not have a strong tendency to accept protons from water.

• Weak bases contribute minimally to changes in H_3O^+ concentration.

This absence of significant H_3O^+ concentration change means the solution is effectively neutral and not acidic.

Strong Acids and Bases

Strong acids and bases completely dissociate into their ions in water, significantly influencing the properties of the solution. For strong acids like HCl , this means a complete conversion to hydronium ions and conjugate bases.

• HCl → H^+ (or H_3O^+) + Cl^-

Strong bases, such as sodium hydroxide ( NaOH ), also dissociate completely, forming hydroxide ions and their conjugate acids.

This complete dissociation is why strong acids and bases are powerful in altering H_3O^+ or OH^- concentrations, which affects a solution's pH significantly. However, the ions formed from the dissociation of NaCl , such as Na^+ (from a strong base) and Cl^- (from a strong acid), are themselves weak and do not participate substantially in altering the solution's pH. This is key in understanding why NaCl solutions remain neutral.