Heat of solution

Heat of solution is the heat involved in the splitting of the compounds into ion and formation of compounds.  Heat is also a form of energy. Heat of solution is measured in Kj/mol, and other conversion units. Heat of solution occurs in two ways; this includes:

  • The splitting of the water molecules into hydrogen and hydroxyl ions .This process involve the lattice energy. ( lattice energy is the energy, of heat of formation of one mole of ionic crystal from widely separated gaseous ion under standard conditions). It should be noted that lattice energy cannot be determine empirically.
  • The free ions are hydrated by water molecules.

The resultant of the two steps above results to change in heat of the solution; this determines if a reaction is exothermic or endothermic.

Exothermic reaction is a reaction where heat is evolved to  the environment. The reaction vessel feels warm.

Endothermic reaction is one that heat is absorbed from the surrounding to the reaction vessel.

The conditions for the change in heat for dissolution of solute are as follows:

  • Dissolution is exothermic if the lattice energy is less than the hydration energy.
  • Dissolution is endothermic if lattice energy is greater than the hydration energy.

For hydrated salts, the heat of dissolution is endothermic.

Standard heat of solution

This is the amount of heat absorbed or librated when one mole of a substance is formed. The table below shows the standard heat solution of some compounds;

compounds Heat change type Heat change (Kj/mol)
Potassium hydroxide exothermic -54.90
Sodium hydroxide exothermic -43.00
Sodium chloride endothermic +26.3
Ammonium chloride endothermic 15.1

 

The solubility of a solid can be determined from it’s enthalpy of solution.  Endothermic processes are less soluble than exothermic processes. Also, solid dissolves exothermically at lower temperature while at higher temperature, they dissolve in endothermic process.

Standard condition for measuring heat change

Standard conditions are used as reference state of properties of substances. Heat of solution or reaction is dependent on some factors such as: pressure, temperature and nature of reactants. And in a reaction, rearrangement of molecules follows. In measuring of standard enthalpy; the reaction between hydrogen and oxygen to form liquid and gaseous water is considered. The heat changes obtained are -286kj/mol and -242kj/mol. The difference between these values is known as the latent heat of vaporization.(+44kj/mol).

Therefore, the conditions in which this reaction is carried out are at standard temperature of 298k, one atmosphere and one mol/dm3 for aqueous solutions.



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