What Is Enthalpy of Neutralization? Definition, Equation & Examples
The standard enthalpy change of neutralization reaction is the enthalpy change that occurs when the solutions of an acid and an alkali react together under conditions to produce 1 mole of water. This reaction is also said to be an exothermic reaction as a high amount of energy is being given out when the neutralization reaction takes place. Enthalpy is also called the heat that is involved when a reaction takes place.
Neutralization Enthalpy of Strong Base and Strong Acid
For a strong acid and a strong base, the neutralization enthalpy is still constant. This is because both strong acids and strong bases are fully ionized in a dilute solution. Neutralization changes in enthalpy are often negative-when an acid and alkali react, heat is released
What is Standard Enthalpy Change?
As solutions of an acid and an alkali react together under normal conditions to produce 1 mole of water, the standard enthalpy change of neutralization is the enthalpy change. Note that the neutralization shift in enthalpy is always measured per mole of water produced. Neutralization alterations in enthalpy are often negative - when an acid and alkali react, heat is released. The values are often very nearly similar for reactions involving strong acids and alkalis, with values between -57 and -58 kJ mol-1. Which varies slightly depending on the combination of acid and alkali.
Neutralization Reaction
When an acid and a base react to form water and salt, a neutralization reaction requires the combination of H+ ions and OH- ions to produce water. There is a pH equal to 7 for the neutralization of a heavy acid and strong base. Neutralizing a strong acid and a weak base would have a pH of less than 7 and, conversely, the resultant pH will be greater than 7 when a strong base neutralizes a weak acid.
It means that salts are formed from equal weights of acid and base when a solution is neutralized. The amount of acid required is the amount that one mole of protons (H+) would give and the amount of base needed is the amount that one mole of protons would give (OH-). Since salts are formed from neutralization reactions of equal acid and base weight concentrations, N parts of the acid will always neutralize N parts of the base.
Why do Strong Acids that React with Strong Alkalis Produce Similar Values?
We assume that strong acids and strong alkalis in the solution are completely ionized and that the ions work independently of each other. In solution, dilute hydrochloric acid, for example, contains hydrogen ions and chloride ions. The sodium hydroxide solution in the solution consists of sodium ions and hydroxide ions. In essence, the equation for any strong acid being neutralized by a strong alkali is just a reaction to make water between hydrogen ions and hydroxide ions. The other ions present (for example, sodium and chloride) are merely spectator ions, which do not participate in the reaction.
The equation of reaction between hydrochloric acid and sodium hydroxide solution is:
NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)
But the actual happening is different:
OH−(aq) + H+(aq) → H2O(l)
If the reaction is the same in both a strong acid and strong alkali, then it is not surprising that enthalpy change is similar.
Anything about 99% of the acid is not naturally ionized in a weak acid such as acetic acid at ordinary concentrations. This implies the other enthalpy terms involved in ionizing the acid as well as the reaction between the hydrogen ions and hydroxide ions would include the enthalpy shift of neutralization. And ammonia is also present primarily as ammonia molecules in solution in a weak alkali like ammonia solution.
Again, apart from the basic form of water from hydrogen ions and hydroxide ions, there may be other enthalpy modifications involved. The calculated enthalpy shift of neutralization for reactions involving acetic acid or ammonia is a few kJ less exothermic than with solid acids and bases.
One source that provides the enthalpy shift of sodium hydroxide solution neutralization with HCl as-57.9 kJ mol-1:
NaOH(aq) + HCl(aq) → Na+(aq) + Cl−(aq) + H2O
The neutralization enthalpy change for acetic acid-neutralizing sodium hydroxide solution is -56.1 kJ mol-1:
NaOH(aq) + CH3COOH(aq) → Na+(aq) + CH3COO−(aq) + H2O
For very weak acids, such as cyanide hydrogen solution, the neutralization shift of enthalpy can be much less. The value of the hydrogen cyanide solution being neutralized by potassium hydroxide solution as -11.7 kJ mol-1, for example, is given by another source.
NaOH(aq) + HCN(aq) → Na + (aq) + CN − (aq)+H2O
Experiment to understand the Enthalpy of Neutralization of Strong Acid and Strong Base
The experiment can be conducted between a strong acid and a strong base by titration process. The temperature that is evolved while the reaction is proceeding to equilibrium is noted down and then the heat value is calculated from the same.
Precautions to be taken while Performing the Experiment for Neutralization Reaction
When performing the experiment in the lab it is necessary to take some precautions which can be provided as follows:
Due to radiation, there is some heat lost to the environment and hence the reaction flask can be a bit hot. Handling hot things must be done carefully.
The solution density must be equal to 1 g/ml.
The hydrochloric acid that is the strong acid and the sodium hydroxide that is the strong base ionization is considered to be 100 per cent.
The specific heat of the solution is taken as 4.189 J/g.
The mixture that contains both HCl and NaOH must be stirred properly to get accurate results.
FAQs on Enthalpy of Neutralization of Strong Acid and Strong Base
1. What is the enthalpy of neutralization of a strong acid and a strong base?
The enthalpy of neutralization is the change in enthalpy that occurs when one mole of hydrogen ions (H⁺) from an acid reacts with one mole of hydroxide ions (OH⁻) from a base to form one mole of water under standard conditions. For any strong acid reacting with a strong base, this value is a constant, approximately -57.1 kJ/mol. The negative sign indicates that the reaction is exothermic, meaning heat is released.
2. Why is the enthalpy of neutralization constant for any strong acid-strong base reaction?
The value is constant because strong acids (like HCl, H₂SO₄) and strong bases (like NaOH, KOH) are assumed to completely ionize in dilute aqueous solutions. This means the actual reacting species are always the same, regardless of the specific acid or base used. The spectator ions (e.g., Na⁺, Cl⁻) do not participate in the reaction. Therefore, the net ionic equation is always: H⁺(aq) + OH⁻(aq) → H₂O(l). Since the fundamental reaction is the formation of one mole of water, the heat evolved remains constant.
3. Are neutralization reactions always exothermic? Explain with respect to bond formation.
Yes, the neutralization of an acid and a base is an exothermic process. This is because the reaction involves the formation of a stable water molecule from hydrogen (H⁺) and hydroxide (OH⁻) ions. The formation of the strong covalent O-H bond in the water molecule releases a significant amount of energy. This energy release is much greater than any energy absorbed, leading to a net release of heat into the surroundings.
4. How does the enthalpy of neutralization for a weak acid and strong base differ from that of a strong acid-strong base reaction?
The enthalpy of neutralization for a weak acid and a strong base is less exothermic (less negative) than the standard -57.1 kJ/mol. This is because a weak acid does not fully ionize in solution. Before neutralization can occur, some energy from the surroundings must be used to dissociate the weak acid molecules into ions. This energy requirement is known as the enthalpy of dissociation, which reduces the total amount of heat released during the subsequent neutralization reaction.
5. What happens to the enthalpy of neutralization when a weak acid reacts with a weak base?
When a weak acid reacts with a weak base, the enthalpy of neutralization is even less exothermic (its value is significantly lower than -57.1 kJ/mol). In this case, energy is required to dissociate both the weak acid and the weak base before their ions can react to form water. This dual requirement for dissociation energy means a larger portion of the potential heat is consumed internally, resulting in a much smaller net release of heat.
6. What is the net ionic equation that represents the neutralization of a strong acid like HCl and a strong base like NaOH?
The net ionic equation simplifies the reaction to only the species that chemically change. For the reaction between HCl and NaOH, the steps are:
- Overall Equation: HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
- Total Ionic Equation: H⁺(aq) + Cl⁻(aq) + Na⁺(aq) + OH⁻(aq) → Na⁺(aq) + Cl⁻(aq) + H₂O(l)
- Net Ionic Equation: By cancelling the spectator ions (Na⁺ and Cl⁻), the essential reaction is revealed: H⁺(aq) + OH⁻(aq) → H₂O(l). This equation highlights why the enthalpy is constant for all strong acid-base neutralizations.
7. What is the practical importance of studying the enthalpy of neutralization?
Understanding the enthalpy of neutralization has several important applications:
- Calorimetry: It forms the basis for experiments to determine the heat capacity of calorimeters.
- Chemical Analysis: It helps in determining the strength of unknown acids or bases by measuring the heat evolved.
- Industrial Processes: It is crucial for managing heat in large-scale chemical reactions involving acids and bases to prevent thermal runaways.
- Biological Systems: It helps explain heat generation in biochemical reactions within the body, such as managing pH balance.
8. Besides the strength of the acid and base, what other factors can influence the measured value of enthalpy of neutralization?
While the standard value is constant under ideal conditions, the measured value can be influenced by several factors:
- Concentration: The standard value of -57.1 kJ/mol is for infinitely dilute solutions. At higher concentrations, inter-ionic interactions and the enthalpy of dilution can cause deviations.
- Temperature and Pressure: Enthalpy is a state function. As per the CBSE curriculum, standard enthalpy changes are defined at a specific temperature (298 K) and pressure (1 bar). Any deviation from these standard conditions will slightly alter the measured value.
- Solvent: The entire process is based on aqueous solutions. Using a different solvent would fundamentally change the ionization and solvation energies, and thus the enthalpy of neutralization.
