A neutralization reaction is a chemical reaction between an acid and a base that forms a salt and water as products. In other words, the reaction neutralizes the acid and base. A neutralization reaction is a type of double replacement reaction in which the reactants are an acid and a base.
The acid releases a hydrogen ion in water, while the base releases a hydroxide. The hydrogen and hydroxide combine and form water, while the anion from the acid and cation from the base form an ionic compound that is a salt. Neutralization only occurs if both the acid and the base are at least partially soluble in water. The salt that is a product of the reaction may be soluble or form a precipitate.
If a strong acid and strong base completely react, the result is an aqueous solution with neutral pH (pH ~7.0). However, many neutralization reactions result in solutions with lower or higher pH values.
Acid-Base Neutralization Reaction
In general, a neutralization reaction is a type of double replacement reaction between an acid and a base (alkali). The general form of the reaction is an acid and a base react and form a salt and water:
acid + base → salt and water
For example:
HCl + NaOH → NaCl + H2O
Alternatively, you can write the reaction as an ionic equation:
H+(aq) + Cl–(aq) + Na+(aq) + OH–(aq) → Na+(aq) + Cl–(aq) + H2O(l)
Cancelling out the spectator ions, the net ionic equation is:
H+(aq) + OH–(aq) → H2O(l)
Note the reaction arrow points to the right, indicating the reaction only proceeds in the forward direction.
The general form of the reaction based on the Brønsted–Lowry acid–base theory is:
AH + B → A + BH
Strong Acid – Strong Base Neutralization
Strong acids and strong bases fully dissociate in water and react, forming a neutral salt in a neutral aqueous solution. The final pH value is approximately 7, although it can be slightly higher or lower, depending on temperature.
Here are the most common strong acids and bases. When they react with each other, expect a final pH value of 7. Reacting most other acids and bases yields a pH other than 7.
| Strong Acids | Strong Bases |
|---|---|
| HCl | LiOH |
| HBr | NaOH |
| HI | KOH |
| HCIO4 | RbOH |
| HNO3 | CsOH |
| Ca(OH)2 | |
| Sr(OH)2 | |
| Ba(OH)2 |
Sulfuric acid (H2SO4) behaves a differently from other strong acids because it has two hydrogen atoms. While sulfuric acid does fully dissociate, it does so in a step-wise manner.
Weak Acid – Weak Base Neutralization
Weak acids and weak bases do not fully dissociate in water. Instead, an equilibrium mixture forms, containing the weak acid and its conjugate base and/or the weak base and its conjugate acid.
AH + B ⇌ A− + BH+
The two species do not completely neutralize one another. The pH value of a neutralization reaction between a weak acid and weak base depends on the strength of the weak acid and base. The pH is 7 only if Ka = Kb. If the weak base is stronger (Kb > Ka), the final solution is alkaline (pH > 7). The the weak acid is stronger (Ka > Kb), the final solution is acidic (pH < 7).
Weak Acids and Strong Bases
The final pH of a neutralization reaction between a weak acid and a strong base depends on the strength of the weak acid.
AH + H2O ⇌ H3O+ + A−
Strong Acids and Weak Bases
The final pH of a neutralization reaction between a strong acid and a weak base depends on the acid dissociation constant of the base, pKa.
H3O+ + B ⇌ H2O + BH+
Predicting pH Values
The pH of a neutralization reaction depends on the characteristics of the reactants:
| Acid and Base Strength | pH |
|---|---|
| Strong Acid – Strong Base | 7 |
| Strong Acid – Weak Base | <7 |
| Weak Acid – Strong Base | >7 |
| Weak Acid – Weak Base | pH <7 if Ka>Kb pH =7 if Ka=Kb pH >7 if Ka<Kb |
The reason the pH is low reacting a strong acid with a weak base and high reacting a weak acid with a strong base is not because of the strong acid or strong base. Instead, it is because the resulting salt affects the pH. The conjugate base of a weak acid makes the solution alkaline. The conjugate acid of a weak base makes the solution acidic.
How to Predict the Products of Neutralization
Two common questions students get asked about neutralization reactions are predicting the products of the reaction and whether the final pH is acidic, alkaline, or neutral.
- Identify the reactants. Is the reaction between a strong acid and strong base or is there a weak acid or weak base? A strong acid-strong base reaction forms a salt and water and the pH is 7. If a weak acid or weak base is present, the pH depends on whether the acid or the base is stronger.
- Dissociate the acid and base into their ions. The salt is the cation from the base with the anion from the acid. The other product is water.
For example, identify the products of the reaction between hydrochloric acid (HCl) and aluminum hydroxide [Al(OH)3] and whether the final pH is acidic, neutral, or alkaline.
You know from either a chart or else memorization that hydrochloric acid is a strong acid and aluminum hydroxide is not a strong base (so it must be a weak base). The pH of the reaction between a strong acid and a weak base is less than 7, so the pH is acidic.
Hydrochloric acid dissociates into H+ and Cl–, while aluminum hydroxide dissociates into Al3+ and 3 OH–. The salt combines the cation from the base (Al3+) with the anion from the acid (Cl–), forming AlCl3.
Balance the equation assuming the number of moles of acid (H+) equals the number of moles of base (OH–):
3HCl + Al(OH)3 → AlCl3 + H2O
Examples of Neutralization Reactions in Everyday Life
Neutralizations reactions are common in everyday life.
- Antacid tables contain a weak base (such as sodium bicarbonate) that neutralizes excess gastric acid (HCl) in the stomach.
- The human digestive system also neutralizes stomach acid. The pancreas produces a bicarbonate antacid that allows nutrient absorption in the intestines.
- Weak bases, such as sodium bicarbonate, safely neutralize acid spills and burns.
- Toothpaste contains a weak base that neutralizes acids released by bacteria in the mouth.
- Calcium hydroxide or limestone (calcium carbonate) are soil treatments that raise pH and aid plant growth.
- Fertilizer production involves neutralizing either nitric acid or sulfuric acid with ammonia, forming ammonium nitrate or ammonium sulfate.
- Scrubbers neutralize acids formed from coal combustion.
- A titration is a neutralization reaction that identifies the concentration of an unknown solution.
References
- Trummal, Aleksander; Lipping, Lauri; Kaljurand, Ivari; Koppel, Ilmar A.; Leito, Ivo (2016). “Acidity of Strong Acids in Water and Dimethyl Sulfoxide”. The Journal of Physical Chemistry A. 120 (20): 3663–3669. doi:10.1021/acs.jpca.6b02253
- Skoog, D.A; West, D.M.; Holler, J.F.; Crouch, S.R. (2004). Fundamentals of Analytical Chemistry (8th ed.). Thomson Brooks/Cole. ISBN 0-03-035523-0.
- Snoeyink, V.L.; Jenkins, D. (1980). Aquatic Chemistry: Chemical Equilibria and Rates in Natural Waters. New York: Wiley. ISBN 0-471-51185-4.
- Zumdahl, Steven S. (2009). Chemical Principles (6th ed.). New York: Houghton Mifflin Company.