Hydride ion, \(\mathrm{H}\), is an exceptionally strong base, reacting with water to produce lots of hydroxide ion and \(\mathrm{H}_{2}\) gas. The \(K_{\mathrm{eq}}\) for this reaction is huge. (a) Write the balanced equation for the reaction between hydride and water. (b) Explain why \(\mathrm{H}_{2}\) gas forms. (Hint: Use the Bronsted-Lowry definition of base.)

When discussing acids and bases in chemistry, it's essential to understand the Brønsted-Lowry theory. This theory broadens the scope beyond the substances that produce hydroxide ions in water. A Brønsted-Lowry base is defined as a substance that can accept a hydrogen ion (or a proton), which is exactly what a hydride ion ((H⁻)) does when it reacts with water.

In the reaction between the hydride ion and water, the hydride ion is accepting a proton from the water molecule. This is because water has a polar nature; it has a partial positive charge near the hydrogen atoms, making it able to release a proton, acting as an acid. Conversely, the hydride ion, being negatively charged, is highly eager to accept this proton, thereby acting as a strong base. The result of this acceptance of a proton is the formation of hydrogen gas ((H₂)) and a hydroxide ion ((OH⁻)), with the latter being the conjugate base of water.

Balancing chemical equations is like ensuring both sides of a scale have equal weights. For a reaction to adhere to the law of conservation of mass, each element must have the same number of atoms on both the reactant and product sides of a chemical equation. The balance gives us a clear picture of the stoichiometry of the reaction—meaning the ratio of reactants to products.

In our example, the hydride ion ((H⁻)) reacts with water ((H₂O)) to give hydroxide ion ((OH⁻)) and hydrogen gas ((H₂)). To balance this, one must ensure that the number of hydrogen atoms and the charge are the same on both sides. Indeed, we see two hydrogen atoms on each side, and the overall charge balances out—one negative charge from the hydride on the left, and on the right, the neutral molecules of (H₂) and (OH⁻) cancel each other out.

The formation of hydrogen gas ((H₂)) is a common observation in many chemical reactions involving metals and acids or strong bases reacting with water. In our scenario, the reason for hydrogen gas formation lies in the nature of the hydride ion ((H⁻)). This ion is highly reactive due to its extra electron, making it an excellent candidate for donating that electron in a bid to gain stability, which often involves the formation of molecular hydrogen.

Furthermore, the interaction with water releases hydrogen gas as two hydrogen ions ((H⁺)) combine. This gas formation can be visually impressive, often bubbling out of the liquid in a reaction mixture. It's crucial to understand the tendencies of hydride ions to predict and safely conduct reactions that result in the generation of hydrogen gas.

Every reversible chemical reaction reaches a state of equilibrium, where the rates of the forward and reverse reactions are equal, resulting in no net change in the concentrations of reactants and products. This state can be quantified using the chemical equilibrium constant ((Keq)), which is a ratio of the concentrations of the products to the reactants, each raised to the power of their respective coefficients in the balanced chemical equation.

For a strong base like the hydride ion, which reacts with water, the (Keq) is very large. A large (Keq) indicates that at equilibrium, the concentration of products (hydroxide ions and hydrogen gas) is much greater than the concentration of the reactants (hydride ions and water molecules). This is another way of saying the reaction strongly favors the production of products, and in the case of hydride ions, this means a robust formation of (H₂) and (OH⁻). Understanding (Keq) helps predict the position of equilibrium and the extent of reaction, which is paramount in chemical synthesis and various industrial applications.