# Why is NaH a strong base?

Apr 15, 2018

Because it produces $N a O H$ and ${H}_{2}$ when placed in water.

#### Explanation:

In the Bronsted-Lowry definition, bases are proton acceptors.
To be a strong base, the substance needs to basically completely dissociate in an aqueous solution to give high $\text{pH}$.

This is the balanced equation of what happens when $N a H$ solid is placed into water:

$N a H \left(a q\right) + {H}_{2} O \left(l\right) \to N a O H \left(a q\right) + {H}_{2} \left(g\right)$

$N a O H$, as you may already know, is another very strong base that basically completely dissociates in an aqueous solution to form $N {a}^{+}$ and $O {H}^{-}$ ions.

So, another way to write our equation is this:

$N a H \left(a q\right) + {H}_{2} O \left(l\right) \to N {a}^{+} \left(a q\right) + O {H}^{-} \left(a q\right) + {H}_{2} \left(g\right)$

The ${H}^{-}$ in $N a H$ accepts an ${H}^{+}$ ion from water to form ${H}_{2}$ gas, making it a Bronsted-Lowry base.

If we were going by the Arrhenius definition of acids and bases, $N a H$ would be a base not because it dissociates to give $O {H}^{-}$ directly from its chemical structure, but because it results in $\left[O {H}^{-}\right]$ increasing upon dissociation.

This reaction happens with a large equilibrium constant, so we can say that $N a H$ almost completely dissociates when placed into an aqueous solution. This makes it a strong base.

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You might be wondering why this reaction doesn't happen instead, which would make $N a H$ an acid:

$N a H \left(a q\right) + {H}_{2} O \left(l\right) \to N {a}^{-} \left(a q\right) + {H}_{3} {O}^{+} \left(a q\right)$

This reaction doesn't happen because sodium has a lower electronegativity than hydrogen.

For example, $H C l$ can form ${H}_{3} {O}^{+}$ and $C {l}^{-}$ ions in an aqueous solution.

$H C l$ can do this because hydrogen is less electronegative than chlorine. Electrons is drawn toward chlorine. So, ${H}^{+}$ is easily pulled off of $H C l$ to form ${H}_{3} {O}^{+}$.

But $N a H$ has again, hydrogen more electronegative than hydrogen, so we more-or-less have $N {a}^{+}$ cation and a ${H}^{-}$ anion, a consequence of electrons being drawn toward hydrogen.

So, instead of an ${H}^{+}$ adding onto water to form ${H}_{3} {O}^{+}$, the electrons go with $H$ to form ${H}^{-}$ ion and form ${H}_{2}$ gas by stealing an ${H}^{+}$ from water.