Why are group I hydrides alkalis?

[jeffy]

Okay until yesterday I thought hydrogen could only make H+ ions but now I know it can make H- ions also. I know that H+ is acid but why is H- alkali?

[Gary Clark]

Originally Posted by jeffy

Okay until yesterday I thought hydrogen could only make H+ ions but now I know it can make H- ions also. I know that H+ is acid but why is H- alkali?

When a metal hydride hits water, you get MH + H₂O ---> MOH + H₂ and the metal hydroxide is an alkali.

You could think of it as the hydride ion taking a hydrogen ion from the water to leave behind the OH^-, which causes the alkalinity.

H^- + H₂O ---> H₂ + OH^-