Compound having lowest melting point licl cscl rbcl kcl
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For highly ionic compound such as the NaCl, KCl and RbCl, lattice energy is directly proportional to melting point. Since lattice energy is proportional to charge-to-size ratio, Na+ would have the highest charge-to-size ratio since it is the smallest among the 3 (Na+, K+ and Rb+).
As for LiCl, it is an ionic compound with high degree of covalent character due to the polarising power of the Li+ (high charge-to-size ratio due to small ionic radius of Li+). The covalent character would reduce the melting point of LiCl as the lattice is affected. So even though Li+ has a higher charge-to-size ratio than Na+, its melting point is lower due to high covalent character. Similar trend is observed for Na2O, MgO and Al2O3 whereby the melting point of Al2O3 is lower than MgO due to high degree of covalent character (high polarising power of Al3+).
As for LiCl, it is an ionic compound with high degree of covalent character due to the polarising power of the Li+ (high charge-to-size ratio due to small ionic radius of Li+). The covalent character would reduce the melting point of LiCl as the lattice is affected. So even though Li+ has a higher charge-to-size ratio than Na+, its melting point is lower due to high covalent character. Similar trend is observed for Na2O, MgO and Al2O3 whereby the melting point of Al2O3 is lower than MgO due to high degree of covalent character (high polarising power of Al3+).
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Explanation:
The hydration energy of alkali metals decreases with increase in ionoc size.hence smaller the ion more is the extent of hydration.
LiCl> Nacl> Kcl> Rbcl>Cscl
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