Reaction with Water
Boron's Inertness, Aluminum Amalgams, and Thallium's Strong Base.
The reactivity of Group 13 elements towards water varies significantly down the group. While some elements are completely inert due to their physical structure or protective layers, others can decompose water to liberate hydrogen gas under the right conditions.
1. Boron: The Inert Metalloid
Boron ($B$) is a typical non-metal/metalloid with a very rigid and strong crystal lattice. Because of its small size and high ionization energy, it does not lose electrons easily.
2. Aluminum: Passivation & The Amalgam Trick
Aluminum ($Al$) is a highly electropositive metal ($E^\circ = -1.66\text{ V}$). Thermodynamically, it should react violently with water to liberate Hydrogen gas. However, if you put a piece of ordinary aluminum in water, nothing happens.
Why is everyday Aluminum unreactive?
As discussed in the "Reaction with Air" section, Aluminum immediately forms a highly protective, impermeable layer of Aluminum Oxide ($Al_2O_3$) on its surface. This layer prevents water molecules from reaching the pure metal underneath.
To make Aluminum react with water, we must prevent the oxide layer from forming. This is done by rubbing the surface of Aluminum with Mercury ($Hg$), forming an Aluminum Amalgam ($Al\text{-}Hg$).
Mercury prevents the continuous oxide layer from sticking, allowing pure Aluminum to aggressively decompose cold water, forming Aluminum Hydroxide and liberating Hydrogen gas.
Note: If red-hot Aluminum is passed over steam, it forms Aluminum Oxide instead of the hydroxide: $2Al + 3H_2O_{(g)} \rightarrow Al_2O_3 + 3H_2$.
3. Gallium and Indium
Gallium ($Ga$) and Indium ($In$) are relatively less electropositive than Aluminum.
Under normal conditions, they are unreactive towards cold and hot water. They do not easily decompose water to liberate hydrogen unless specific severe conditions (or the absence of oxygen) are met, which are generally outside the scope of the standard syllabus.
4. Thallium: Forming a Strong Base
Thallium ($Tl$) is a heavy metal at the bottom of the group. Its standard reduction potential for $Tl^+/Tl$ is $-0.34\text{ V}$, making it capable of reacting with water, though it is often slow.
- When Thallium is exposed to moist air or left in water, it reacts to form Thallium(I) hydroxide ($TlOH$).
- Crucial Point: Due to the Inert Pair Effect, it forms $TlOH$ ($+1$ state) instead of $Tl(OH)_3$ ($+3$ state).
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