The Dissolution of Ionic Compounds in Water
In order to analyse the processes involved in the dissolution of ionic compounds in water, it is important to remember the structure of water and ionic compounds.
Structure of Water
Water consists of two hydrogen atoms and one oxygen atom. Water is a polar molecules as a result of the large difference in electronegativity between hydrogen and oxygen. Water is a covalent molecule and due to the higher electronegativity of oxygen, the electrons in the covalent bond are closer to the oxygen atom. This results in the oxygen end of the molecule being slightly negatively charged and the hydrogen atom slightly positively charged.
Water exhibits hydrogen bonding and forms ion-dipole bonds with a large range of solutes like salt.
Structure of Ionic Compounds
Ionic compounds have an ordered crystalline structure that results from the attraction between cations and anions that exists throughout the lattice. Every negative ion (anion) is surrounded by positive ions (cations) and every positive ion is surrounded by negative ions.
Dissolution of Ionic Compounds
When ionic compounds dissolve in water, the negative end of the water molecule is attracted to the cation. Negative ends of the water molecule continue to surround the cation until it is ‘hydrated’. Conversely, the positive end of the water molecule is attracted to the anion and positive ends of the water molecule continue to surround the anion until it is also ‘hydrated’.
Water forms ion-dipole bonds with a large range of solutes like salt.
Change of Enthalpy During Dissolution
There are several processes that occur during the dissolution of an ionic compound. Each process has an associated enthalpy change associated with it. The sum of these enthalpy changes will equal the total enthalpy change for the dissolution of any particular ionic compound. It also determines if the dissolution of the ionic compound is an exothermic or endothermic process.
The Steps Involved in the Dissolution of Ionic Compounds:
1. Separation of the water molecules: This is an endothermic process because energy is required to break the hydrogen bonds that exist between water molecules.
2. Dissolution of the ionic compound: This is also an endothermic process that requires energy to overcome the electrostatic attraction between the cations and anions.
3. Hydration of ions: the hydration of ions is an exothermic process as intermolecular forces are formed between the water molecules and cations/anions which releases energy in the process.
ΔHsoln = ΔH1 + ΔH2 + ΔH3
Exothermic or Endothermic Dissolution
Energy is absorbed in processes 1 and 2 (endothermic), whilst energy is released in process 3 (exothermic).
- If the sum of ΔH1 + ΔH2 is less than ΔH3 the ΔHsoln will be negative (exothermic)
- If the sum of ΔH1 + ΔH2 is greater than ΔH3 the ΔHsoln will be positive (endothermic)
A positive value for ΔHsoln does not mean that a solution will not form. Enthalpy is only one of the contributing factors. A high ΔHsoln is usually an indication that the substance is not very soluble.