Nickel(II) hydroxide
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| Template:Longitem | Ni(OH)2 |
| Molar mass | 92.724 g/mol (anhydrous) 110.72 g/mol (monohydrate) |
| Appearance | green crystals |
| Density | 4.10 g/cm3 |
| Melting point | Template:Chembox CalcTemperatures |
| Template:Longitem | 5.48Template:E[2] |
| Template:Longitem | +4500.0·10−6 cm3/mol |
| Template:Longitem | hexagonal, hP3 |
| Template:Longitem | P3m1, No. 164 |
| Template:Longitem | a = 0.3117 nm, b = 0.3117 nm, c = 0.4595 nm α = 90°, β = 90°, γ = 120°
|
| Template:Longitem | 79 J·mol−1·K−1[3] |
| Template:Longitem | −538 kJ·mol−1[3] |
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Nickel(II) hydroxide is the inorganic compound with the formula Ni(OH)2. It is a lime-green solid that dissolves with decomposition in ammonia and amines and is attacked by acids. It is electroactive, being converted to the Ni(III) oxy-hydroxide, leading to widespread applications in rechargeable batteries.[4]
Properties
Nickel(II) hydroxide has two well-characterized polymorphs, α and β. The α structure consists of Ni(OH)2 layers with intercalated anions or water.[5][6] The β form adopts a hexagonal close-packed structure of Ni2+ and OH− ions.[5][6] In the presence of water, the α polymorph typically recrystallizes to the β form.[5][7] In addition to the α and β polymorphs, several γ nickel hydroxides have been described, distinguished by crystal structures with much larger inter-sheet distances.[5]
The mineral form of Ni(OH)2, theophrastite, was first identified in the Vermion region of northern Greece, in 1980. It is found naturally as a translucent emerald-green crystal formed in thin sheets near the boundaries of idocrase or chlorite crystals.[8] A nickel-magnesium variant of the mineral, Template:Chem2 had been previously discovered at Hagdale on the island of Unst in Scotland.[9]
Reactions
Nickel(II) hydroxide is frequently used in electrical car batteries.[6] Specifically, Ni(OH)2 readily oxidizes to nickel oxyhydroxide, NiOOH, in combination with a reduction reaction, often of a metal hydride (reaction 1 and 2).[10][11]
Reaction 1 Template:Chem2
Reaction 2 Template:Chem2
Net Reaction (in H2O) Template:Chem2
Of the two polymorphs, α-Ni(OH)2 has a higher theoretical capacity and thus is generally considered to be preferable in electrochemical applications. However, it transforms to β-Ni(OH)2 in alkaline solutions, leading to many investigations into the possibility of stabilized α-Ni(OH)2 electrodes for industrial applications.[7]
Synthesis
The synthesis entails treating aqueous solutions of nickel(II) salts with potassium hydroxide. When the same reaction is conducted in the presence of bromine, the product is Template:Chem2.[12]
Toxicity
The Ni2+ ion is a carcinogen when inhaled.
See also
- List of minerals named after people
- Nickel–cadmium battery
- Nickel–hydrogen battery
- Nickel–metal hydride battery
- Nickel–iron battery
References
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