Based on the provided reference, the enthalpy of neutralization for a weak monoprotic acid (HA) in a 1 M solution with a strong base is -55.95 KJ/mol.
Understanding Neutralization Enthalpy
Neutralization is a chemical reaction where an acid and a base react quantitatively with each other. The enthalpy of neutralization ($\Delta H_{neut}$) is the heat energy released or absorbed during this reaction under standard conditions.
Strong Acid vs. Weak Acid Neutralization
The enthalpy of neutralization differs between strong acid-strong base reactions and weak acid-strong base reactions.
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Strong Acid - Strong Base: When a strong acid reacts with a strong base, the reaction is essentially the combination of hydrogen ions (H⁺) from the acid and hydroxide ions (OH⁻) from the base to form water (H₂O). Both the strong acid and strong base are completely dissociated in solution.
H⁺(aq) + OH⁻(aq) → H₂O(l)
According to the reference, the enthalpy of neutralization for a strong monobasic acid with a strong monoacidic base is -57.3 KJ/mol. This value is highly exothermic, meaning it releases a significant amount of heat. -
Weak Acid - Strong Base: In the case of a weak acid reacting with a strong base, the weak acid is only partially dissociated in solution. The overall reaction can be viewed as involving two main steps:
- The ionization (dissociation) of the weak acid into its ions.
- The reaction of the released hydrogen ions (H⁺) with the hydroxide ions (OH⁻) from the strong base to form water.
HA(aq) + OH⁻(aq) → A⁻(aq) + H₂O(l)
The ionization of a weak acid requires energy. This energy absorbed during ionization makes the overall neutralization process less exothermic compared to the strong acid-strong base reaction where ionization energy is negligible for the strong acid.
Impact of Ionization Energy
The provided reference states that the unionized weak acid required 1.4 KJ/mol heat for its complete ionization. This positive value represents the energy needed to break the bond in the weak acid and separate the H⁺ ion.
The enthalpy change for the weak acid neutralization ($\Delta H{weak_neut}$) is approximately the sum of the energy required for ionization ($\Delta H{ionization}$) and the energy released from the formation of water from H⁺ and OH⁻ (which is the strong acid-strong base neutralization enthalpy, $\Delta H_{strong_neut}$):
$\Delta H{weak_neut} = \Delta H{ionization} + \Delta H_{strong_neut}$
Using the values from the reference:
-55.95 KJ/mol ≈ 1.4 KJ/mol + (-57.3 KJ/mol)
-55.95 KJ/mol ≈ -55.9 KJ/mol
This demonstrates why the enthalpy of neutralization for a weak acid is less negative (less heat released) than that of a strong acid. Some of the energy released from the formation of water is consumed by the ionization of the weak acid.
Comparing Enthalpies
Let's compare the values provided:
| Reaction Type | Enthalpy of Neutralization (KJ/mol) | Notes |
|---|---|---|
| Strong Acid - Strong Base | -57.3 | H⁺ + OH⁻ → H₂O |
| Weak Acid - Strong Base | -55.95 | Includes energy for weak acid ionization |
The difference (-55.95 - (-57.3)) = 1.35 KJ/mol, which is very close to the ionization energy (1.4 KJ/mol) mentioned in the reference.
In summary, while the core process of forming water from H⁺ and OH⁻ is highly exothermic, the overall enthalpy of neutralization for a weak acid is slightly less exothermic because energy is absorbed to ionize the weak acid before it can fully react.
