What is the common formula for the reaction of an alkali metal with water?

What is the common formula for the reaction of an alkali metal with water?

• A. M(s) + H2O(l) -> MH(aq) + H2(g)
• B. M(s) + H2O(l) -> MOH(aq) + 1/2 H2(g)
• C. M(s) + 2 H2O(l) -> M(OH)2(aq) + H2(g)
• D. M(s) + O2(g) -> MO2(g)

Answer: (B)

When alkali metals react with water, the products are a metal hydroxide and hydrogen gas. The hydroxide carries a single negative charge, so one metal cation pairs with one hydroxide to form MOH in solution, and the leftover hydrogen from water is released as H2. That gives the per-metal equation: M(s) + H2O(l) -> MOH(aq) + 1/2 H2(g). If you balance it for two metal atoms, you get 2 M + 2 H2O -> 2 MOH + H2, which is the same relationship. This form is preferred because alkali metals form +1 cations, so their reaction with water yields a single hydroxide unit per metal. Forms like MH would imply a metal hydride, which isn’t the typical product here, and M(OH)2 would require a +2 metal charge (as with alkaline earths), not an alkali metal. Reaction with oxygen to make MO2 is a different process altogether.

When alkali metals react with water, the products are a metal hydroxide and hydrogen gas. The hydroxide carries a single negative charge, so one metal cation pairs with one hydroxide to form MOH in solution, and the leftover hydrogen from water is released as H2. That gives the per-metal equation: M(s) + H2O(l) -> MOH(aq) + 1/2 H2(g). If you balance it for two metal atoms, you get 2 M + 2 H2O -> 2 MOH + H2, which is the same relationship.

This form is preferred because alkali metals form +1 cations, so their reaction with water yields a single hydroxide unit per metal. Forms like MH would imply a metal hydride, which isn’t the typical product here, and M(OH)2 would require a +2 metal charge (as with alkaline earths), not an alkali metal. Reaction with oxygen to make MO2 is a different process altogether.