Question:

In which of the following reactions, the heat of neutralization will be the highest?

Updated On: Jun 7, 2022
  • HCl  and  NaOH HCl\,\,\text{and}\,\,NaOH
  • NH4OH  and  HCl N{{H}_{4}}OH\,\,\text{and}\,\,HCl
  • NaOHandCH3COOH NaOH\,\text{and}\,C{{H}_{3}}COOH
  • CH3COOHandNH4OH C{{H}_{3}}COOH\,\text{and}\,N{{H}_{4}}OH
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The Correct Option is A

Solution and Explanation

Heat of neutralisation is maximum (i.e., approximately 13.7 kcal/mole) for the neutralisation of a strong acid and a strong base. If one of them is weak, then heat of neutralisation also decreases (due to being consumed in ionization of weak electrolyte). Hence, here the heat of neutralisation of HCl HCl and NaOH NaOH will be the highest.
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Concepts Used:

Enthalpy change

Enthalpy Change refers to the difference between the heat content of the initial and final state of the reaction. Change in enthalpy can prove to be of great importance to find whether the reaction is exothermic or endothermic.

Formula for change in enthalpy is:-

dH = dU + d(PV)

The above equation can be written in the terms of initial and final states of the system which is defined below:

UF – UI = qP –p(VF – VI)

Or qP = (UF + pVF) – (UI + pVI)

Enthalpy (H) can be written as H= U + PV. Putting the value in the above equation, we obtained: 

qP = HF – HI = ∆H

Hence, change in enthalpy ∆H = qP, referred to as the heat consumed at a constant pressure by the system. At constant pressure, we can also write,

∆H = ∆U + p∆V

Standard Enthalpy of Reaction

To specify the standard enthalpy of any reaction, it is calculated when all the components participating in the reaction i.e., the reactants and the products are in their standard form. Therefore the standard enthalpy of reaction is the enthalpy change that occurs in a system when a matter is transformed by a chemical reaction under standard conditions.