Neutralization Reactions — Definition & Overview

(Video) Neutralization Reactions

By: Professor Dave Explains

In an aqueous solution acids hydrolyze (or react with water) to form hydronium and bases form hydroxide. The amount of hydronium versus hydroxide gives insight to how acidic or basic the solution is. Water itself is neutral, but when it does auto-ionization it forms hydronium and hydroxide. The hydronium and hydroxide from water form in a equal ratio, which means the solution still remains neutral.

Solutions are neutral when hydronium and hydroxide are equal to each other. This can occur when strong acids and strong bases react together. Protons from the acid and hydroxide from the base react to form water. While the other ions in solution form a salt. The salt and water together are neutral which is why it is called a neutralization reaction.

Neutralizations reactions only occur with strong acids and strong bases. Also the hydronium/proton to hydroxide ratio must be the same. An example is hydrochloric acid ($HCl$) mixed with sodium hydroxide ($NaOH$). Together these make sodium chloride ($NaCl$) and water.

A reaction with a weak base/acid with strong acid/base does not give a neutral solution. These types of solutions will cause a weakly acidic or basic solution. Weak acids and weak bases could create a neutral, acidic, or basic solution. It depends on the compounds and conditions of the reactions.

The video concludes with practice problems.

We've been studying acids and bases! So far, we know the difference between strong and weak acids and bases. We can calculate their pH. Strong acids are easy because they fully dissociate. So the pH of a strong acid corresponds to the solution's molar concentration. Finding the pH of weak acids is challenging. They're in equilibrium. So we have to use equilibrium calculations. But, with an ICE table, we can find pH and product concentrations.

So, we've covered a lot of topics. But we haven't looked at any chemical reactions! What happens when we mix an acid and a base? With a weak acid or base, it's a little complicated. The most straightforward acid-base reaction is neutralization. Neutralization is the reaction between strong acids and bases. Typically, they are Arrhenius acids and bases. So, the acid's hydrogen ion reacts with the base's hydroxide ion. The result is water. The conjugates produce an ionic salt. Sometimes, we have Bronstead-Lowery or Lewis acid and bases.

Neutralization Reaction Examples

Let's look at an example of a neutralization reaction. We'll react hydrochloric acid with sodium hydroxide.

$HCl_{(aq)} + NaOH_{(aq)} \rightarrow H_{2}O_{(l)} + NaCl_{(aq)}$

First, we should notice that the mole ratio is 1:1. So, this is a rudimentary solution stoichiometry problem. If we add equal concentrations of acid and base, we neutralize them. Now, our pH is 7.0. What happens if we add different concentrations? We can subtract the two and calculate pH or pOH.

We can also write the net ionic equation for this reaction.

$H^{+} + \cancel{Cl^{-}} + \cancel{Na^{+}} + OH^{-} \rightarrow H_{2}O + \cancel{Na^{+}} + \cancel{Cl^{-}}$

We see that the sodium and chloride ions are spectator ions. Our net equation is the formation of water.

$H^{+} + OH^{-} \rightarrow H_{2}O$

Why doesn't NaCl affect the pH? Both sodium and chloride ions are conjugates of a strong acid or base. So, they are unbelievably weak. Neither is strong enough to affect the pH. Furthermore, they cancel each other.

Let's examine a different neutralization reaction. The acid is 500.0 ml of 0.50M hydroiodic acid. Our base is 500.0ml of 0.35M sodium hydroxide. What is the final pH?

First, we need to write our balanced chemical equation.

$HI_{(aq)} + NaOH_{(aq)} \rightarrow H_{2}O_{(l)} + NaI_{(aq)}$

Now, let's calculate the final hydrogen ion concentration. Notice that our final volume is 1.0L because we mixed two 500.0 ml solutions.

$[H^{+}]= \frac{(0.50\frac{mol}{L}\times 0.5L)-(0.35\frac{mols}{L}\times 0.5L)}{1.0L}=\frac{0.15mols}{L}$

$pH= -log{(0.15)}=0.82$

Why Not Weak Acids and Bases?

Are you wondering why neutralizations only happen between strong acids and bases? Let's investigate some reactions with weak acids and bases.

Strong Acid with a Weak Base

Previously, we examined the basicity of amines. Let's look at a reaction between trimethylamine and hydrochloric acid. Trimethylamine is a weak Lewis base. Its basicity comes from nitrogen's electron lone pair.

$(CH_{3})_{3}N_{(aq)} + HCl_{(aq)} \rightarrow (CH_{3})_{3}NH_{(aq)}^{+} + Cl^{-}_{(aq)}$

Again, the halogen ions are conjugates of the strong acids. So, they don't affect pH. But, the trimethyl-ammonium ion is the conjugate acid of a weak base. So, it's a strong acid. Our pH will be acidic!

Strong Base with a Weak Acid

Let's look at the reaction of acetic acid with sodium hydroxide.

$CH_{3}COOH_{(aq)} + NaOH_{(aq)} \rightarrow H_{2}O_{(l)} + CH_{3}COONa_{(aq)}$

Acetic acid is a weak acid. But, with a strong base, it will completely deprotonate. The resulting ionic compound is sodium acetate. But, there's a problem! Do you remember our qualitative solubility rules? Acetates are always soluble. But, the acetate ion is the conjugate of a weak acid. So, it's a strong base! Acetate will react with water to form hydroxide. So, our final solution's pH is above 7.0.

Weak Acid with a Weak Base

The most complicated situation is the reaction between weak acids and weak bases. The problem is that both conjugates are strong. You might assume the result is a neutral solution. Unfortunately, it's not that simple. We have to compare the ionization constants or dissociation constants between the two.

• If $K_{a}=K_{b}$ then the solution is neutral. In this case, each ion is reacting with water. The acid forms hydronium. The base generates hydroxide. But, both occur at equal rates. So, they balance. An example is acetate and ammonium ions.

• If $K_{a}>K_{b}$ then the solution is acidic. Here, the acid is stronger than the base. So, there's more hydronium. Our pH will be less than 7.0.

• If $K_{a}<K_{b}$ then the solution is basic. The base is stronger than the acid. So, there's more hydroxide. Our pH will be greater than 7.0.

But, how do we determine the pH of the reactions between weak acid and bases? Unfortunately, we still have some topics to cover before we get there. But, after a few more explanations, we'll be able to do it! If you want to learn more, look at our lessons on hydrolysis, acid-base properties of salt solutions, and the common ion effect.