Standard Enthalpy of NeutralizationneutHo - Enthalpy of Neutralization, Dissociation of acid and base, their strength, Practice problems and FAQs

Matches between strong teams, irrespective of the game draw a huge crowd. Within the strong team, there will be better understanding, and unison of activities giving their best output. Similarly, when two extroverted people become friends. Is easy for both of them to understand each other. But when either of them is an introvert or both are introverts, it becomes complicated to understand each other because introverts always hesitate to share their feelings.

A similar case happens in neutralization, when a strong acid and a strong base react, their reaction called a neutralization reaction is really simple and the energy involved (enthalpy of neutralization) is always the same as ―57.1 KJmol^-1 , irrespective of the chemical nature of the acid and base.

But, if one of them is weak or both acid and base are weak, the enthalpy of neutralization becomes less than ―57.1 KJmol^-1. Let’s discuss why this happen.

Table of Contents

Standard Enthalpy of NeutralizationneutHo
Dissociation of acid and base
Strength of acids and bases
Practice Problems
Frequently Asked Questions

Standard Enthalpy of

An acid reacts with a base to form a salt and water. The reaction is exothermic. The heat energy liberated during this process of neutralization is called enthalpy of neutralization, which is always negative..

Let’s take an example to understand the enthalpy of neutralization. A beaker filled with HCl is attached to a thermometer. NaOH is taken in a test tube and is added slowly into the HCl solution in the beaker. HCl and NaOH will undergo a neutralization reaction and the heat is released in this process that increases the thermometer reading.

The base NaOH(aq), the acid HCl(aq), and the salt formed are completely dissociated in aqueous solution resulting in ions as-

The H₂O formed in the neutralization reaction will remain unionized (i.e., undissociated).

So, ultimately neutralization of an acid and a base is a reaction between hydrogen ion and hydroxide ion to form an undissociated water molecule. T

Standard enthalpy of neutralization is the amount of heat released when one mole of H⁺in dilute solution combines with one mole of OH^-to give rise to undissociated water at 1 bar pressure and 298 K. It can also be defined as the amount of heat released in the formation of one mole of water when an acid is neutralized by a base at 1 bar and 298 K

Neutralization reaction is given as

H⁺(aq)(1mol)+OH^-(aq)(1mol)H₂O(l)(1 mol)

Standard enthalpy of neutralization in this process = ―57.1 kJmol^-1

= ―13.7 kcalmol^-1

Since the strong acid and strong base are almost 100% ionized, the energy liberated is solely due to the reaction between Hydrogen ion and hydroxide ion and will be maximum.

SO irrespective of the acid Hydrochloric sulphuric or nitric acid or sodium hydroxide, potassium hydroxide, or calcium hydroxide the standard heat of neutralization is maximum of ―57.1 kJmol^-1

Dissociation of acid and base

Acids, such as HCl, HNO₃ and CH₃COOH, produce H⁺ ions when they dissociate. It can be written as follows:

will constitute the anionic part (A-).

On dissociation, bases such as NaOH, KOH, NH4OH, and others produce OH- ions. It can be written as follows:

will constitute the cationic part (B+).

Strength of acids and bases

Strong acid and weak acid

A strong acid, such as HCl, will entirely dissociate into its constituent ions H⁺ and Cl^-. Thus, acids that dissociate completely into their constituent ions are known as strong acids. However, acids like CH₃COOH will show partial dissociation (i.e., incomplete dissociation) into their constituent ions CH₃COO^- and H⁺. Thus, acids that dissociate partially into their constituent ions are known as weak acids.

Strong base and weak base

A strong bases like NaOH will dissociate entirely into their constituent ions Na⁺ & OH-. Those bases that completely dissociate into their constituent ions are known as strong bases. Bases like NH₄OH will not completely dissociate into their constituent ions NH₄⁺ & OH^- and are known as weak bases.

Consider the reaction between a strong acid (HCl) and a strong base (NaOH).

The weak species are not completely ionized in solutions. Some of the heat liberated during the initial neutralization reaction is consumed in ionizing more of the unionized weak species. This heat is known as the enthalpy of ionization. So, what will be measured will be the enthalpy of neutralization less of enthalpy of ionization.

For a better understanding, consider a neutralization reaction between a weak acid and a strong

base where the energy liberated is around ―50 kJmol^-1. The enthalpy of neutralization was expected to be approximately equal to ―57.1 k. Thus, it can be concluded that 7.1 kJmol^-1 of energy was utilized in the ionization of the weak acid. Hence, the enthalpy of ionization, in this case, is 7.1 KJmol^-1.

The enthalpy of neutralization is less than 57.1 KJmol^-1. whenever a weak acid (monobasic) or weak base (monoacidic) is involved in the neutralization reaction.

Important Points

As the magnitude of increases, the acidic strength of the acid increases.
Consider two acids being neutralized with the same base. The acid with the higher magnitude of is a stronger acid as that particular acid will require only a lesser amount of energy for ionization.
Similarly, as the magnitude of increases, the basic strength of the base increases.
In the case of a neutralization reaction between a weak acid and a weak base, the exact prediction of energy required for their ionization is not possible at this level.

Practice Problems

Q 1. The enthalpy of neutralization of HCl by NaOH is –57.1 kJmol^-1, while the enthalpy of neutralization by NH₄OH is –51.1 kJmol^-1. Calculate the NH₄OH dissociation enthalpy.

a. 14 kJmol^-1
b. 6kJmol^-1
c. 8kJmol^-1
d. 10kJmol^-1

Answer: When NaOH (strong base) neutralizes HCl (strong acid),

As a result, the dissociation enthalpy of NH₄OH is 57.1 + (–51.1) = 6 kJmol^-1

Hence, the correct option is (B).

Q 2. The enthalpy of neutralization of NaOH by HCl is –57.1kJmol^-1, while the enthalpy of neutralization by CH₃COOH is -50.6 kJmol^-1. Calculate the CH₃COOH dissociation enthalpy.

a. 9.5 kJmol^-1
b. 6 kJmol^-1
c. 8.5kJmol^-1
d. 6.5kJmol^-1

Answer: When NaOH (strong base) neutralizes HCl (strong acid),

As a result, the dissociation enthalpy of NH4OH is 57.1 + (- 50.6 ) = 6.5 kJmol^-1

Hence, the correct option is (D).

Q 3. In which of the following pair, the exact prediction of energy required for their ionization is not possible?

a. CH₃COOH & NaOH
b. HCN & NaOH
c. CH₃COOH & NH₄OH
d. HCl & NH₄OH

Answer: In the case of a neutralization reaction between a weak acid and a weak base, the exact prediction of energy required for their ionization is not possible at this level.

CH3COOH & NaOH is a pair of a weak acid and a strong base

HCN & NaOH is a pair of a weak acid and a strong base

CH₃COOH & NH₄OH is a pair of a weak acid and a weak base

HCl &NH₄OH is a pair of a strong acid and a weak base

Hence, the correct option is (C).

Q 4. In which of the following pair, the exact prediction of energy required for their ionization is not possible?

a. CH₃COOH & C₅H₅N
b. HNO₃ & NH₄OH
c. H₂SO4 & NH₄OH
d. HCN & KCN

Answer: In the case of a neutralization reaction between a weak acid and a weak base, the exact prediction of energy required for their ionization is not possible at this level.

CH₃COOH & C₅H₅N is a pair of a weak acid and a weak base

HNO₃ & NH₄OH is a pair of a strong acid and a weak base

H₂SO₄ & NH₄OH is a pair of a strong acid and a strong base

HCN & KCN is a pair of a weak acid and a strong base

Hence, the correct option is (A).

Frequently Asked Questions

Q 1. Why does the enthalpy of neutralization of a strong acid by a strong base always remain constant?
Answer: This is because unionized water molecules are usually formed by combining one gramme equivalent of H⁺ ions with OH^– ions.

Q 2. What causes the HCl and NaOH reaction being exothermic?
Answer: The reaction between HCl and NaOH is a neutralization reaction that produces NaCl + H₂O as a consequence. The ΔH is negative in this reaction because there is more bond formation than bond breaking, indicating that it is more exothermic.

Q 3. What factors influence the neutralization enthalpy?
Answer: The heat change of the neutralization process is influenced by three factors:

1. Acid and alkali concentrations
2. The basicity of acids and alkali
3. Strength of acid and alkali

Q 4. What is the nature of neutralization?
Answer: A neutralization reaction is irreversible since the products (salt and water) cannot be converted back to the reactants (acid and base). It's a chemical reaction that turns salt and water from an acid and a base.

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