In
chemistry and
thermodynamics
Thermodynamics is a branch of physics that deals with heat, work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quantities is governed by the four laws o ...
, the enthalpy of neutralization () is the change in
enthalpy
Enthalpy , a property of a thermodynamic system, is the sum of the system's internal energy and the product of its pressure and volume. It is a state function used in many measurements in chemical, biological, and physical systems at a constant ...
that occurs when one
equivalent of an
acid and a
base undergo a
neutralization reaction to form
water
Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...
and a
salt
Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quanti ...
. It is a special case of the
enthalpy of reaction
The standard enthalpy of reaction (denoted \Delta_ H^\ominus or \Delta H_^\ominus) for a chemical reaction is the difference between total reactant and total product molar enthalpies, calculated for substances in their standard states. This can i ...
. It is defined as the energy released with the formation of 1 mole of water.
When a reaction is carried out under
standard conditions
Standard temperature and pressure (STP) are standard sets of conditions for experimental measurements to be established to allow comparisons to be made between different sets of data. The most used standards are those of the International Union ...
at the temperature of 298 K (25 degrees Celsius) and 1
atm of pressure and one mole of water is formed, the heat released by the reaction is called the ''standard enthalpy of neutralization'' ().
The heat () released during a reaction is
:
where is the mass of the solution, is the
specific heat capacity
In thermodynamics, the specific heat capacity (symbol ) of a substance is the heat capacity of a sample of the substance divided by the mass of the sample, also sometimes referred to as massic heat capacity. Informally, it is the amount of heat t ...
of the solution, and is the
temperature
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer.
Thermometers are calibrated in various temperature scales that historically have relied on ...
change observed during the reaction. From this, the standard enthalpy change () is obtained by division with the amount of substance (in
moles) involved.
:
When a
strong acid
Acid strength is the tendency of an acid, symbolised by the chemical formula HA, to dissociate into a proton, H+, and an anion, A-. The dissociation of a strong acid in solution is effectively complete, except in its most concentrated solutio ...
, HA, reacts with a
strong base, BOH, the reaction that occurs is
:
H+ + OH^- -> H2O
as the acid and the base are fully dissociated and neither the cation nor the anion are involved in the neutralization reaction.
The enthalpy change for this reaction is -57.62 kJ/mol at 25 °C.
For
weak acids or
bases, the heat of neutralization is pH-dependent.
In the absence of any added mineral acid or
alkali
In chemistry, an alkali (; from ar, القلوي, al-qaly, lit=ashes of the saltwort) is a basic, ionic salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as a base that dissolves in water. A solution of ...
, some heat is required for complete dissociation. The total heat evolved during neutralization will be smaller.
:e.g.
at 25°C
The
heat of ionization for this reaction is equal to (–12 + 57.3) = 45.3 kJ/mol at 25 °C.
References
{{DEFAULTSORT:Enthalpy Of Neutralization
Enthalpy
Thermochemistry
Acid–base chemistry