Enthalpy of neutralization

In <a href="/facts/Chemistry/SrNqkGVm">chemistry</a> and <a href="/facts/Thermodynamics/q4hIx3yP">thermodynamics</a>, the enthalpy of neutralization (ΔnH) is the change in <a href="/facts/Enthalpy/bBou8lCE">enthalpy</a> that occurs when one <a href="/facts/Equivalent_(chemistry)/ksgd425g">equivalent</a> of an <a href="/facts/Acid/FySU18n1">acid</a> and a <a href="/facts/Base_(chemistry)/M9wA7qeM">base</a> undergo a <a href="/facts/Neutralization_reaction/WC1c5fj1">neutralization reaction</a> to form <a href="/facts/Water/0lkXnJPK">water</a> and a <a href="/facts/Salt_(chemistry)/lsSKwGpn">salt</a>. It is a special case of the <a href="/facts/Enthalpy_of_reaction/u5pffxVM">enthalpy of reaction</a>. It is defined as the energy released with the formation of 1 mole of water.
When a reaction is carried out under <a href="/facts/Standard_conditions/JhJv9KFJ">standard conditions</a> at the temperature of 298 K (25 °C) and 1 <a href="/facts/Bar_(unit)/wwFWTTkT">bar</a> of pressure and one mole of water is formed, the heat released by the reaction is called the standard enthalpy of neutralization (ΔnH⊖).
The heat (Q) released during a reaction is

Q
        =
        m
        
          c
          
            p
          
        
        Δ
        T
      
    
    {\displaystyle Q=mc_{p}\Delta T}

where m is the mass of the solution, cp is the <a href="/facts/Specific_heat_capacity/9P0U6YgI">specific heat capacity</a> of the solution, and ∆T is the <a href="/facts/Temperature/QyGe4gmj">temperature</a> change observed during the reaction. From this, the standard enthalpy change (∆H) is obtained by division with the amount of substance (in <a href="/facts/Mole_(chemistry)/kCMFwCc0">moles</a>) involved.

Δ
        H
        =
        −
        
          
            Q
            n
          
        
      
    
    {\displaystyle \Delta H=-{\frac {Q}{n}}}

When a <a href="/facts/Strong_acid/vJ4rj6Lu">strong acid</a>, HA, reacts with a <a href="/facts/Strong_base/M9wA7qeM">strong base</a>, BOH, the reaction that occurs is

H
            
              +
            
          
          +
          
            OH
            
              −
            
          
          ⟶
          
            H
            
              2
            
            
              
            
          
          O
        
      
    
    {\displaystyle {\ce {H+ + OH^- -> H2O}}}

as the acid and the base are fully dissociated and neither the cation B+ nor the anion A− are involved in the neutralization reaction. The enthalpy change for this reaction is -57.62 kJ/mol at 25 °C.
For <a href="/facts/Weak_acids/vJ4rj6Lu">weak acids</a> or <a href="/facts/Weak_base/FL4HstLK">bases</a>, the heat of neutralization is pH-dependent. In the absence of any added mineral acid or <a href="/facts/Alkali/s6NqPGSP">alkali</a>, some heat is required for complete dissociation. The total heat evolved during neutralization will be smaller.

e.g. 
 
 
 
 
 HCN
 +
 NaOH
 ⟶
 NaCN
 +
 
 H
 
 2
 
 
 
 
 
 O
 
 ;
  
 Δ
 H
 
 
 {\displaystyle {\ce {HCN + NaOH -> NaCN + H2O}};\ \Delta H}
 
 = -12 kJ/mol at 25 °C
The <a href="/facts/Ionization_energy/lezUajfA">heat of ionization</a> for this reaction is equal to (–12 + 57.3) = 45.3 kJ/mol at 25 °C.

Enthalpy of neutralization open-in-new

Enthalpy of neutralization