Neutralization Reaction

Neutralization reaction is a very important class of chemical reaction in which the effect or activity of an acid is neutralized by a base and vice versa. The acid and base forms salt and water as the products of the reaction along with generation of heat. Neutralization reactions actually take place between the acidic proton from the acid and the hydroxyl group of the base to generate water( H+ and OH- react to form H2O). The concentrations of the acid and base are the determining factors for the volumes of the individual to be taken.

Acid + Base \[\rightarrow\] Salt + Water

For example, 

HCl + NaOH \[\rightarrow\] NaCl + H2O

H2SO4 + NaOH \[\rightarrow\] Na (SO4)2 + H2O

If the strengths are known, then we can find the volumes which will completely neutralize each other and vice versa. For this, we can use the formula,

Volume (acid) \[\times\] concentration (H+ ions) = volume (base) \[\times\] concentration (OH− ions)

Question

If 10 ml of 0.5M HCl neutralizes 50ml of NaOH of unknown strength. Find its strength. 

Answer:

0.5 \[\times\] 10 = M of base \[\times\] 50

By cross multiplication, M of base = \[\frac{(0.5 \times 10)}{50} \]

                            = \[\frac{5}{50} \]  = 0.1 M

Types of Neutralization Reactions

 Now, there are four types of neutralization reactions:

1. Strong acid and strong base

2. Strong acid and weak base

3. Weak acid and strong base

4. Weak acid and weak base

1. Strong Acid and Strong Base

Strong acids and strong bases dissociate entirely in aqueous conditions. For example, HCl, H2SO4 are strong acids and NaOH, KOH is a strong base.

HCl (aq) \[\rightarrow\] H+ (aq) + Cl-(aq)

NaOH (aq) \[\rightarrow\] Na+ (aq) + OH-(aq)

H+ + OH- \[\rightarrow  H_{2}O \]

Na+ + Cl-\[\rightarrow\] NaCl

The overall reaction is,

HCl + NaOH \[\rightarrow\] NaCl + H2O 

When a strong acid neutralizes a strong base, the resultant is neutral because all the H+ ions react with all the OH-ions to form water. There are no surplus H+ or OH- ions present in the resultant solution. Its pH is 7.

General reactions;

Strong Acid: HA +H2O \[\rightarrow\] A- (aq) + H3O+ (aq)

Strong Base: BOH + H2O \[\rightarrow\] B+ (aq) + OH-(aq)

2. Strong Acid and Weak Base

Weak bases don’t dissociate only to about 5-10%. Here the amount of OH- ions is very less compared to the amount of H+ ions available. Therefore, the resultant solution is acidic. Its pH lies between 3-6.

Weak Base: BOH + H2O \[\longleftrightarrow\] B+ (aq) + OH-(aq) 

(or)

          B + H2O \[\longleftrightarrow\] BH+ (aq) + OH-(aq)

Examples of weak bases include Mg (OH)2, Ca (OH)2, and Ba (OH)2. 

\[K_b = \frac{OH^-  B^+}{BOH}\]

Taking log on both sides you get:

\[Log K_b = log(\frac{OH^- B^+}{BOH} )\]

Splitting the log terms:

\[Log K_b = log OH^- + log (\frac{B^+}{BOH})\]

Since pKb = -log10 Kb and pOH = -log10 [OH-], therefore:

\[-pK_b = – pOH + log (\frac{B^+}{BOH})\]

\[P_{OH} = pK_b + log (\frac{B^+}{BOH})\]

This equation can also be written as:

pOH = pKb + \[log (\frac{salt}{base})\] 

3. Strong Base and Weak Acid

Weak acids don’t dissociate entirely in aqueous conditions. The OH- ions concentration is much higher compared to the free H+ ions. Therefore, the resultant solution is basic with a pH of around 8-11. 

Weak acid: 

AH +H2O \[\longleftrightarrow\]  A-(aq) + H3O+ (aq)

Example of weak acids include; acetic acid and all the other organic acids.

Following the law of dissociation, the acid dissociation constant Ka can be defined according to the equation as, 

\[Ka = \frac{H^+ A^-}{HA} \]

Taking log on both sides you get:

Log Ka = \[log (\frac{H^+ A^-}{HA})\]

Splitting the log terms;

\[Log Ka = log H^+ + log (\frac{A^-}{HA})\]

Since pKa = -log10 Ka and pH = -log10 [H+], therefore:

-pKa = – pH + \[ log (\frac{A^-}{HA}) \]

pH = pKa + \[ log (\frac{A^-}{HA})\]

This equation can also be written as:

pH = pKa + \[log \frac{(salt)}{(acid)}\]

4. Weak Acid and Weak Base

Here since both, acid and base, is weak neither of them dissociates completely and so neutralization does not occur.

Equilibrium is the state when the rate of formation of product is equal to the rate of formation of the reactants.

Application

1. It is used in fertilization as the crop cannot grow in acidic soil.

2. On bee sting, using baking soda to neutralize the effect of formic acid that it has released in our body.

3. When we suffer from acidity, we take antacid tablet Mg (OH)2 to neutralize the effect of HCl produced in our stomach.