Question

Sodium hydride, \(\mathrm{NaH}\), is available commercially as a gray-white powder. It melts at \(800^{\circ} \mathrm{C}\) with decomposition. It reacts explosively with water and ignites spontaneously upon standing in moist air. (a) Write a Lewis structure for the hydride ion and for sodium hydride. Is your Lewis structure consistent with the fact that this compound is a high- melting solid? Explain. (b) When sodium hydride is added very slowly to water, it dissolves with the evolution of a gas. The resulting solution is basic to litmus. What is the gas evolved? Why has the solution become basic? (c) Write an equation for the reaction between sodium hydride and 1-butyne, \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{C} \equiv \mathrm{CH}\). Use curved arrows to show the flow of electrons in this reaction.

Short answer

Answer: The Lewis structure for sodium hydride is [Na]+ [H:]-, representing an ionic bond between Na+ and H- ions. When sodium hydride reacts with water, hydrogen gas (H2) is evolved.

Step-by-step solution

Part (a): Lewis Structure for Hydride Ion and Sodium Hydride

First, we will draw the Lewis structure for the hydride ion (H-). Hydride ion has one extra electron compared to a neutral hydrogen atom. In the Lewis structure, we represent this additional electron as a pair of dots: H:- Next, we draw the Lewis structure for sodium hydride (NaH). Sodium, being an alkali metal, has one valence electron which it can donate to form a bond with the hydride ion. So, the Lewis structure for sodium hydride can be represented as an ionic bond between Na+ and H-: [Na]+ [H:]- Yes, this Lewis structure is consistent with the fact that sodium hydride is a high-melting solid. Sodium hydride is an ionic compound formed by the transfer of an electron from sodium to hydrogen. Ionic compounds generally have high melting points due to the strong electrostatic forces between the positively and negatively charged ions.

Part (b): Gas Evolved and Basic Solution Explanation

When sodium hydride is added slowly to water, it reacts with water to form the hydronium ion (H3O+) and release hydrogen gas (H2). The balanced chemical equation for the reaction can be written as: \(\mathrm{NaH + H_2O \rightarrow NaOH + H_2}\) The evolved gas is hydrogen (H2). The solution becomes basic because sodium hydride reacts with water to form sodium hydroxide (NaOH), which is a strong base. When sodium hydroxide dissolves in water, it dissociates to form hydroxide ions (OH-) and sodium ions (Na+): \(\mathrm{NaOH \rightarrow Na^+ + OH^-}\) The presence of hydroxide ions in the solution causes it to be basic and change the color of litmus paper.

Part (c): Reaction Between Sodium Hydride and 1-butyne

Sodium hydride acts as a base and transfers the hydride ion (H-) to the acidic hydrogen of 1-butyne. Thus, the reaction between sodium hydride and 1-butyne can be written as: \(\mathrm{NaH + CH_3CH_2C \equiv CH \rightarrow CH_3CH_2C \equiv C^- + Na^+}\) To show the flow of electrons, we use curved arrows. The arrow starts from the electron pair (lone pair) of the hydride ion (H-) and points towards the acidic hydrogen of 1-butyne. Another arrow starts from the bond between carbon and the acidic hydrogen, and points towards the carbon atom forming a negative charge. H: - + \(\mathrm{CH_3CH_2C \equiv CH \xrightarrow{H- \text{ transfer}} CH_3CH_2C \equiv C^- + Na^+}\)