Acid-base Properties of Salt Solutions — Overview & Calculations

Previously, we studied neutralization and hydrolysis. When we react a strong acid and base, we produce water and an ionic salt. So, our pH is neutral. But, reactions with weak acids or bases generate conjugates. The conjugates are a strong acid or base. So, they affect the solution's pH. But, what happens when we add a salt? Let's find out.

Adding Ionic Salts

Let's start with the most straightforward scenario. What happens when we add an ionic salt? The pH remains neutral! Why? Because the two ions are very weak! As an example, let's consider sodium chloride.

The sodium ion is technically a Lewis acid. That's because it can accept electrons. But, it only has a single positive charge. So, it's unlikely it takes an electron pair. Moreso, its valence shell is a full octet. So, it's stable. Also, consider the acidity trend of the periodic table with low electronegativity. So, it should have low acidity. We can confirm by examining its dissociation constant. The $pK_{a}$ is 14.8!

Chloride ions are the conjugate base of hydrochloric acid. So, they are unbelievably weak bases. Neither sodium nor chloride ions affect our pH.

Adding a Conjugate Base

What happens when we add a salt made from a conjugate base? The pH rises, or pOH lowers. As an example, consider calcium carbonate. It's the main ingredient in antacids. It's also responsible for the buffer capacities of blood and the ocean. Let's write a net ionic equation. Start by writing the equation for the dissociation of calcium carbonate. Then, we know the self-ionization of water generates hydronium. So we can react the carbonate ion with hydronium.

$CaCO_{3} + H_{2}O \rightarrow Ca^{2+} + CO_{3}^{2-}$

$\cancel{Ca^{2+} }+ CO_{3}^{2-} + H_{3}O^{+} \rightleftharpoons \cancel{Ca^{2+}} + H_{2}O + HCO_{3}^{-}$

In the second reaction, calcium ions are both a reactant and a product. So, they act as a spectator ion. The change in pH happens because of the carbonate ion. It consumes hydronium and yields bicarbonate and water.

Adding a Conjugate Acid

What happens when we add a conjugate acid? The pH increases because we consume hydroxide. Consider ammonium chloride. Again, chloride is the conjugate base of a strong acid. So, it's a weak base. But, ammonium is the conjugate acid of an amine. The change in pH will happen because of the ammonium ion. Let's write the balanced chemical equation.

$NH_{4}^{+} + OH^{-} \rightarrow NH_{3} + H_{2}O$

The ammonium consumed hydroxide. So, our solution will become more acidic.

Rules for How Cations and Anions Affect pH

1. An anion that's the conjugate base of a strong acid does not affect pH.

2. An anion that is the conjugate base of a weak acid raises the pH.

3. A cation that is the conjugate acid of a weak base, lowers the pH.

4. The cations of the alkali and large alkaline-earth metals do not change pH. They will always be spectator ions.

5. Other metal ions, like transition metals, lower pH. They act as Lewis acids.

6. A solution with a conjugate acid and base will depend on the $K_{a}$ and $K_{b}$ values.

(Video) Acidic, Basic, and Neutral Salts - Ionic Compounds

by The Organic Chemistry Tutor

In this video, The Organic Chemistry Tutor explains which ionic salts are acidic, basic, or neutral. To figure it out, you'll determine the conjugate ions of strong and weak acids and bases. He'll show you many examples. So, when you finish this video, you'll have mastered this topic.

(Video) 16.6 Acidity and Basicity of Salts | General Chemistry

by Chad's Prep

This video provides more depth on why some salts are acidic, basic, or neutral. Chad also presents some different approaches to the practice problems. If you need more help with determining the acid-base characteristics of salts, this is an excellent video.