The alkali metals form salt-like hydrides by the direct synthesis at elevated temperature. The thermal stability of these hydrides decrease in which of the following orders?
The alkali metals form salt-like hydrides by the direct synthesis at elevated temperature. The thermal stability of these hydrides decrease in which of the following orders?
The correct option is D LiH>NaH>KH>RbH>CsH
Going down the IA group the size of the cation increase as follows
Li+<Na+<K+<Rb+<Cs+
Smaller anions forms stable compound with smaller cations.
So, the thermal stability of the hydrides decrease down the groups since the size of the cation increase down the group. Hence, going down the group they can be easily decomposed due to less thermal stability.
Therefore, the decreasing order of thermal stability of group IA hydride is
LiH>NaH>KH>RbH>CsH
Order of thermal stability of 1st and 2nd group hydrides:
1st group - LiH > NaH > KH > RbH > CsH
2nd group - CaH2>SrH2>BaH2
Electrolysis of a solution of saline hydride in molten alkali halide produces H2 at anode.
2H−Anode−−−−→H2(g)+2e−
Saline hydrides react explosively with water.
NaH(s)+H2O(aq)⟶NaOH(aq)+H2(g
Uses of Alkali metal hydrides:
a. For making LiAlH4,NaBH4, etc,.
b. For the removal of last traces of water from organic compounds.
Metallic / interstitial hydrides:
a. Hydrides formed by d and f block elements.
e.g. 3 to 5 groups ScH2,YH2,YH3,LaH2,LaH2,TiH2,rH2,HfH2,VH,VH2,NbH,NbH2,TaH
Metals of group 7, 8 and 9 do not form hydrides. This region of the periodic table from group 7 to group 9 is known as the hydride gap.
The f-block metals form hydrides of limiting compositions of MH2 and MH3
ZrHx(1.30≤x≤1.75),TiHx(1.8≤x≤2.0)
Most of these hydrides are good conductors of electricity in solid state.
Uses:
To store hydrogen, especially in cars working on fuel cells
Going down the IA group the size of the cation increase as follows
Li+<Na+<K+<Rb+<Cs+
Smaller anions forms stable compound with smaller cations.
So, the thermal stability of the hydrides decrease down the groups since the size of the cation increase down the group. Hence, going down the group they can be easily decomposed due to less thermal stability.
Therefore, the decreasing order of thermal stability of group IA hydride is
LiH>NaH>KH>RbH>CsH
Order of thermal stability of 1st and 2nd group hydrides:
1st group - LiH > NaH > KH > RbH > CsH
2nd group - CaH2>SrH2>BaH2
Electrolysis of a solution of saline hydride in molten alkali halide produces H2 at anode.
2H−Anode−−−−→H2(g)+2e−
Saline hydrides react explosively with water.
NaH(s)+H2O(aq)⟶NaOH(aq)+H2(g
Uses of Alkali metal hydrides:
a. For making LiAlH4,NaBH4, etc,.
b. For the removal of last traces of water from organic compounds.
Metallic / interstitial hydrides:
a. Hydrides formed by d and f block elements.
e.g. 3 to 5 groups ScH2,YH2,YH3,LaH2,LaH2,TiH2,rH2,HfH2,VH,VH2,NbH,NbH2,TaH
Metals of group 7, 8 and 9 do not form hydrides. This region of the periodic table from group 7 to group 9 is known as the hydride gap.
The f-block metals form hydrides of limiting compositions of MH2 and MH3
ZrHx(1.30≤x≤1.75),TiHx(1.8≤x≤2.0)
Most of these hydrides are good conductors of electricity in solid state.
Uses:
To store hydrogen, especially in cars working on fuel cells
