Enthalpy of Neutralization
Definition and meaning of Enthalpy of Neutralization in chemistry.
The enthalpy of neutralization is the heat released when an acid and a base react in solution to form one mole of water. For a strong acid and a strong base, this value is nearly constant at about -57 kJ per mole of water.
In more detail
Neutralization is an exothermic reaction, meaning it gives off heat. When an acid and base combine, hydrogen ions and hydroxide ions join to form water, and the energy released shows up as a temperature rise in the solution. The enthalpy of neutralization measures this heat per mole of water produced, and because it is a release of energy, the value carries a negative sign.
For reactions between a strong acid and a strong base, the value is almost always close to -57 kJ per mole, regardless of which strong acid and base are used. The reason is that strong acids and strong bases are already fully broken into ions in water.
The only real chemical change is hydrogen ions combining with hydroxide ions to form water, so the same underlying reaction produces the same heat every time. The value shifts when a weak acid or weak base is involved. A weak acid is not fully ionized to begin with, so part of the released energy must be used to finish breaking it apart before the ions can react.
This absorbs some heat, so the measured enthalpy of neutralization for a weak acid is smaller in size than -57 kJ per mole. Chemists measure this quantity with a simple calorimeter, often a well-insulated cup. By mixing known amounts of acid and base and recording the temperature change of the solution, they calculate the heat released using q equals mass times specific heat times temperature change.
Dividing that heat by the moles of water formed gives the enthalpy of neutralization, a common experiment in introductory chemistry labs.
Key facts
| Field | Physical Chemistry |
|---|---|
| Defined per | one mole of water formed |
| Sign | negative (exothermic) |
| Strong acid + strong base | about -57 kJ/mol |
| Core reaction | H<sup>+</sup> + OH<sup>-</sup> gives H2O |
| Weak acid or base | smaller magnitude |
| Measured with | calorimeter |
| Formula | q = m x c x delta T |
When a strong acid such as hydrochloric acid reacts with a strong base such as sodium hydroxide, the reaction H<sup>+</sup> + OH<sup>-</sup> gives H2O releases about 57 kJ for every mole of water formed, warming the solution.
Frequently asked questions
Why is the value nearly the same for strong acids and bases?
Strong acids and bases are already fully ionized in water, so the only reaction is hydrogen ions joining hydroxide ions to form water. That same reaction releases about the same heat every time.
Why is neutralization exothermic?
Forming the strong bonds in water releases more energy than is needed to bring the ions together. The extra energy leaves as heat, raising the temperature of the solution.
Why is the value smaller for a weak acid?
A weak acid is only partly ionized, so some released energy is used to finish breaking it apart. That absorbed energy lowers the net heat measured per mole of water.
How is enthalpy of neutralization measured?
By mixing known amounts of acid and base in an insulated calorimeter and recording the temperature change. The heat is found with q equals mass times specific heat times temperature change, then divided by moles of water.