Solubility, Reactions and nature of Ionic and Covalent Compounds – my Digital Kemistry

Effect of bonding on physical and chemical properties:

            Physical properties depend upon intermolecular forces (Hydrogen bonding, Dipole-dipole forces & London dispersion forces). While chemical properties depend upon intramolecular forces (Chemical bonds).

(i)       Solubility of Ionic and Covalent compounds:

Solubility is based on the fact “like dissolve like”. e.g. ionic compounds are more soluble in polar solvents like water. Water molecules get attached to solute particles and it is called hydration. Ionic solutes are dissolved in water due to hydration. Partial positively charged hydrogen of water molecules attracts negative ions of the solute.

While partial negatively charged oxygen of water molecules attracts positive ions of solute. Thus solute ions get separated and dissolve in the water. In solution, each cation, as well as the anion of solute, is surrounded by many water molecules. But in a solid crystalline state, only a definite number of water molecules are attached per molecule of a crystalline substance. The substance is called hydrate. e.g. in solution each copper ion is surrounded by dozens of water molecules similarly each sulphate ion is surrounded by dozens of water molecules but in a solid crystalline state, only five water molecules are attached per formula unit of CuSO4. The number of water molecules attached depends upon the charge density of the ion. More molecules of water are attached to cations due to their high charge density. The high charge density of cations is due to their smaller size or volume. Anions have a larger size or volume so their charge density is low and hence fewer water molecules are attached to them e.g. in hydrated copper sulphate(Blue vitriol) [CuSO4.5H2O]out of 5 water molecules 4 are attached to copper ion and one to sulphate ion. [Cu.4H2O]+2[SO4.H2O]-2. Some more examples of hydrates are:

Non-polar solutes are more soluble in non-polar solvents e.g. Naphthalene is more soluble in Benzene. Forces of attraction between solvent and solute are very weak So in the case of non-polar solvent and non-polar solute both have weak forces so they can separate each other and hence are soluble.

(ii)       Reactions of Ionic and Covalent Compounds:

            Ionic compounds usually do not react in a solid state however they react in an aqueous state. In an aqueous state, the reaction is very fast because ions are already separated and no bond is to be broken and only new bonds are formed so the reaction is fast e.g. reaction between AgNO3(aq) and NaCl(aq) to produce a precipitate of AgCl.

e.g.      Ag(aq) + NO3(aq)   + Na(aq) + Cl(aq)  → AgCl(s) + Na(aq) + NO3(aq)

            Reactions between covalent compounds are usually slow because these first involve the breaking of old bonds and the formation of new bonds so reactions are slow.

e.g       H — H       +      Cl   —  Cl        →        2 H — Cl

  H-H Bond breaking    Cl-Cl Bond breaking    H-Cl Bond formation

(iii)      Directional and Non-directional nature of bonds:

            Ionic bonds are non- directional while covalent bonds are directional. This is due to the fact that covalent bonds are formed due to the overlapping of orbitals which have definite directions e.g. in CH4 carbon is sp3 hybridized. These four sp3 hybrid orbitals are directed to the four corners of the tetrahedron so CH4 has tetrahedral geometry.    


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