General Characteristic of Compounds of Alkali Metals for IIT JEE Exam
IIT JEE Chemistry Properties of Oxides and Hydroxides of Alkali Metals
Alkali metals are the elements that belong to the first group of the Periodic Table of Modern chemistry. It contains elements – lithium < sodium < potassium < rubidium < caesium < francium (in their increasing atomic number and atomic mass). These elements form various types of compounds that are discussed below along with their general characteristics and properties:
Oxides and Hydroxides of Alkali Metals
All alkali metals, oxides, peroxides and superoxides dissolve readily in water to produce corresponding hydroxides that are strong alkali. For example:
Since they react with H2O forming H2O2 and O2 respectively, peroxides and superoxides also act as oxidizing agents. All alkaline metals ' hydroxides are white crystalline solids. They are strongest of all bases and dissolve readily in water where a lot of heat is evolved.
Basic Strength of Alkali Metal Hydroxides
As we move down the group Li to Cs, the basic strength of these hydroxides increases. Due to their low ionization energies, the hydroxides of alkali metals act as strong bases that decrease down the group.
The decrease in ionizing energy results in the weakening of the bond between metal and hydroxide ion and M – O bond in M – O – H, which can easily break M+ and OH–
This results in the solution's increased concentration of hydroxyl ions, i.e. increased basic characters.
Solubility and Stability of Alkali Metal Hydroxides
All these hydroxides, except for lithium hydroxide, are highly water soluble and thermally stable. 2LiOH +Δ → Li2O + H2O
Formation of Salts with Acids
The highly basic reaction of alkali metals hydroxides with all acids results in the formation of salts.
Under appropriate conditions, the alkali metals combine directly with halogens forming halides of the general formula MX.
These halides can also be prepared on metal oxides, hydroxides, or carbonate by the action of aqueous halogen acids (HX). All these halides are colourless crystalline solids with high melting points and highly negative standard heat of formation. Amongst these, fluorine halide has the highest enthalpy of formation while iodine halide has the lowest enthalpy.
Fluorides are therefore the most stable while the least stable are iodides. In terms of polarization effects, lattice energy and ion hydration, trends in melting points, boiling points and solubility of alkali metals halides can be understood.
Polarization Effects of Alkali Metal compounds
When a cation approaches an anion, the electron cloud of the anion is attracted towards the cation and hence gets distorted.
This effect is called polarization. The power of the cation to polarize the anion is called its polarizing power and the tendency of the anion to get polarized is called its polarizability. The higher the polarization produced, the more ionic character decreases or the covalent character increases the concentration of the electrons between the two atoms. In general, any compound's covalent character depends on the following factors.
Size of the Cations
The smaller is the cation is, the higher is its polarizing power and the covalent character. As the size of cation increases, the covalent character decreases.
Therefore, LiCl has the largest size while CsCl has the smallest, making the former more covalent than the latter.
Size of the Anion
Larger the anion, greater is its polarizability. This explains the reason behind lithium iodide being more covalent as compared
to lithium fluoride.
Charge of the Ion
If the charge on the cation is large, its polarizing power increases and hence its covalent character increases. Due to this reason, aluminium chloride is more covalent than sodium chloride.
Similarly, when the charge on an anion is large, its polarizing capacity increase, which makes it more covalent. Due to this reason, Sodium phosphate is more covalent in nature than sodium chloride. Thus, as the anion load decreases, the covalent character decreases.
Electronic configuration of the Cation
If the charge and the size of any two cations are the same, the cation having the configuration similar to noble gases i.e. having 18 electrons in the outermost shell, has more polarizing power than a cation with noble gas configuration i.e. having 8 electrons. CuCl, for instance, is more covalent than NaCl.
The amount of energy needed to separate one mole of solid ionic compound into its gaseous ions is defined as lattice energy. The energy of the lattices is obviously greater, the melting point of the alkali metals is higher and its solubility in water is lower.
Hydration energy is the amount of energy released in combination with water by one mole of gaseous ions to form hydrated ions. The ions' hydration energy is higher the compound's solubility in water.
The extent of hydration also depends on the ion size. The smaller the size of the ion, the more hydrated it is and its hydrated ionic radius is higher. The melting point and solubility of alkali metal halides in water or organic solvent can be explained from the above arguments.
The ultimate solubility of a compound in water is determined by a delicate balance between lattice enthalpy and hydration enthalpy. LiF's low solubility (0.27 g/100 g H2O) is due to its high lattice energy (-1005 KJmol-1), while CsI's low solubility (44g/100 g H2O) is due to lower hydration energy of both ions (-670 KJ/mol). The solubility of the most of alkali metal halides except those of fluorides decreases on descending the group since the decrease in hydration energy is more than the corresponding decrease in the lattice energy.
Due to small size and high electronegativity, lithium halides except LiF are predominantly covalent and hence are soluble in covalent solvents such as alcohol, acetone, ethyl acetate, LiCl is also soluble in pyridine. In contrast, being ionic in nature, NaCl is insoluble in all these solvents.
Because of Li+ ion's high hydration energy, lithium halides are soluble in water with the exception of LiF, which due to its high lattice energy is sparsely soluble.
In the order of fluoride > chloride > bromide > iodide, the melting point decreases for the same alkali metal ion as the size of the halide ion increases.
The melting point of lithium halides for the same halide ion is lower than those of the corresponding sodium halides and then decreases as we move from Na to Cs down the group.
LiCl's low melting point (887 K) compared to NaCl is probably due to the covalent nature of LiCl and the ionic nature of NaCl.
Salts of Oxoacids
Because the alkali metals are highly electropositive, their hydroxides are very strong bases, forming salts with all oxocids (H2SO4, H2CO3, HNO3, H3PO4 and HNO2). They are generally water-soluble and heat-stable. Alkali metals carbonates (M2CO3) are remarkably stable up to 1273 K, above which they melt first and then decompose into oxides. However, Li2CO3 is significantly less stable and easily decomposes to form lithium dioxide and carbon dioxide.
Li2CO3 → Li2O + CO2
This is probably due to the large difference in size between lithium and carbonate ions, making the crystal lattice unstable.
Alkali metals are also strongly basic and form solid bicarbonates. While NH4HCO3 bicarbonate does exist in solution, no other metals form solid bicarbonates. Everything exists as solid as well. However, lithium is not solid.
All carbonates and bicarbonates are water - soluble and their solubility increases rapidly when the group comes down. This is because when moving down the group, lattice energies decrease faster than their hydration energies.
Some other Compounds of Alkali Metals
Sodium Bicarbonate (NaHCO3)
A concentrated sodium carbonate solution absorbs CO2 to produce sparsely soluble bicarbonate of sodium. When heated between 250 °C and 300 °C, it is transformed into pure anhydrous sodium carbonate that can be used to standardize acids.
Potassium Bicarbonate (KHCO3)
It is produced by absorbing CO2 in moist potassium carbonate and then in a porous plate by drying the product. KHCO3 is similar to NaHCO3, but much more water - soluble. Due to hydrolysis, the solution is strongly alkaline.
Sodium Chloride (NaCl)
It is also called common salt as rock salt or halite occurs abundantly in nature. The most abundant source is marine water where sodium chloride occurs in the range of 2.6 – 2.9 percent. In large shallow pits, the sea water is exposed to the sun and air. Gradual water evaporation leads to salt crystallization. Purification is done by dissolving the salt in minimum water volume and filtering to remove insoluble impurities if necessary. The solution is then saturated with a dry hydrogen chloride current separating crystals of pure sodium chloride.
Potassium Chloride (KCl)
KCl is made of fused carnallite – almost pure KCl is separated from the melt, leaving behind fused MgCl2. It is a solid water soluble colourless cubic crystal. Its solubility increases proportionally with increase in the temperature.