Grade 11

Advanced

7 min

Lesson 9 of 12

Neutralization and Salt Hydrolysis

Not Started

Predicting the outcomes of mixing acids and bases and the resulting properties of salts.

Why does a bee sting feel better when treated with baking soda, but a wasp sting requires vinegar? The answer lies in the hidden chemistry of salts and their ability to change the pH of water.

Learning Objectives

1.By the end of this lesson, you will be able to predict the products of neutralization and write balanced net ionic equations.
2.You will understand how to determine if a salt solution is acidic, basic, or neutral based on its parent acid and base.
3.You will be able to explain the process of salt hydrolysis using chemical equilibrium equations.

The Neutralization Dance

In its simplest form, neutralization occurs when an acid and a base react to produce water and a salt. While we often think of salts like

NaCl

as 'neutral,' the term salt actually refers to any ionic compound formed from the cation of a base and the anion of an acid. For strong acid-strong base reactions, the net ionic equation is always the same:

H^+(aq) + OH^-(aq) \rightarrow H_2O(l)

However, when weak species are involved, the salt produced can actually change the pH of the resulting solution through a process called hydrolysis.

Quick Check

What are the two primary products of a standard neutralization reaction?

Think about what happens when

H^+

meets

OH^-

and what is left over.

Answer

A salt and water.

The Parentage of Salts

To predict the pH of a salt, look at its 'parents.' A salt is the offspring of an acid and a base. If the parents are equally strong, the salt is neutral. If one parent is stronger, the salt takes on that characteristic. 1. Strong Acid + Strong Base: Neutral salt (e.g., $NaCl$ ). 2. Strong Acid + Weak Base: Acidic salt (e.g., $NH_4Cl$ ). 3. Weak Acid + Strong Base: Basic salt (e.g., $CH_3COONa$ ). This happens because the conjugate of a weak species is strong enough to react with water, whereas the conjugate of a strong species is too stable to react.

Predict if a solution of Potassium Fluoride ( $KF$ ) is acidic, basic, or neutral. 1. Identify the parents: $K^+$ comes from $KOH$ (Strong Base). $F^-$ comes from $HF$ (Weak Acid). 2. Compare strengths: Strong Base + Weak Acid = Basic Salt. 3. Conclusion: $KF$ will produce a basic solution with a $pH > 7$ .

Salt Hydrolysis: The Reaction with Water

Salt hydrolysis is the reaction of an anion or a cation (or both) from a salt with water to produce

OH^-

H_3O^+

ions. For a basic salt like Sodium Acetate (

NaCH_3COO

), the

Na^+

ion is a spectator, but the acetate ion (

CH_3COO^-

) is the conjugate base of a weak acid. It 'steals' a proton from water:

CH_3COO^-(aq) + H_2O(l) \rightleftharpoons CH_3COOH(aq) + OH^-(aq)

Because

OH^-

is produced, the solution becomes basic.

Explain why Ammonium Nitrate (

NH_4NO_3

) is acidic using equations. 1. Dissociation:

NH_4NO_3(s) \rightarrow NH_4^+(aq) + NO_3^-(aq)

. 2. Identify the active ion:

NO_3^-

is the conjugate of a strong acid (

HNO_3

), so it does nothing.

NH_4^+

is the conjugate of a weak base (

NH_3

). 3. Hydrolysis reaction:

NH_4^+(aq) + H_2O(l) \rightleftharpoons NH_3(aq) + H_3O^+(aq)

4. Result: The production of

H_3O^+

makes the solution acidic.

Quick Check

Why doesn't the $Cl^-$ ion in $NaCl$ undergo hydrolysis?

Consider the strength of the 'parent' acid.

Answer

Because it is the conjugate base of a strong acid (HCl), making it too weak to react with water.

The Ultimate Tie-Breaker

What happens when a salt is formed from a weak acid and a weak base? In this case, both the cation and the anion undergo hydrolysis. To determine the pH, we must compare the $K_a$ of the acidic cation and the $K_b$ of the basic anion. - If $K_a > K_b$ : The solution is acidic. - If $K_b > K_a$ : The solution is basic. - If $K_a \approx K_b$ : The solution is neutral.

Predict the pH of Ammonium Fluoride ( $NH_4F$ ). Given: $K_a$ for $NH_4^+$ is $5.6 \times 10^{-10}$ and $K_b$ for $F^-$ is $1.4 \times 10^{-11}$ . 1. Identify that both ions hydrolyze. 2. Compare the constants: $5.6 \times 10^{-10} (K_a) > 1.4 \times 10^{-11} (K_b)$ . 3. Since the $K_a$ of the cation is larger than the $K_b$ of the anion, the cation produces more $H_3O^+$ than the anion produces $OH^-$ . 4. Conclusion: The solution is slightly acidic.

Key Takeaways

1Neutralization produces a salt and water, but the salt is not always pH-neutral.
2Salt hydrolysis occurs when the conjugate ion of a weak acid or base reacts with water to shift the pH.
3Strong-Strong combinations result in neutral salts; Strong-Weak combinations follow the 'strong' parent's lead.
4For Weak-Weak combinations, compare $K_a$ and $K_b$ values to determine the final pH.

Quick Check

Multiple Choice

Which of the following salts will produce a basic solution when dissolved in water?

Multiple Choice

What is the net ionic equation for the reaction between $HCl$ and $KOH$ ?

True/False

The conjugate base of a strong acid is a strong base that easily undergoes hydrolysis.

What's Next?

Review Tomorrow

In 24 hours, try to explain to someone why a solution of baking soda ( $NaHCO_3$ ) is basic using the concept of 'parent' acids and bases.

Practice Activity

Look at the ingredients of your household cleaners. Identify any salts listed and try to predict if they are acidic or basic based on their chemical formulas.

Browse

Browse

Neutralization and Salt Hydrolysis

Learning Objectives

The Neutralization Dance

The Parentage of Salts

Salt Hydrolysis: The Reaction with Water

The Ultimate Tie-Breaker

Key Takeaways

Quick Check

What's Next?

Neutralization and Salt Hydrolysis

Learning Objectives

The Neutralization Dance

The Parentage of Salts

Salt Hydrolysis: The Reaction with Water

The Ultimate Tie-Breaker

Key Takeaways

Quick Check

What's Next?