Question

A major challenge in implementing the "hydrogen economy" is finding a safe, lightweight, and compact way of storing hydrogen for use as a fuel. The hydrides of light metals are attractive for hydrogen storage because they can store a high weight percentage of hydrogen in a small volume. For example, \(\mathrm{NaAlH}_{4}\) can release \(5.6 \%\) of its mass as \(\mathrm{H}_{2}\) upon decomposing to \(\mathrm{NaH}(s), \mathrm{Al}(s),\) and \(\mathrm{H}_{2}(\mathrm{~g}),\) NaAlH \(_{4}\) possesses both covalent bonds, which hold polyatomic anions together, and ionic bonds. (a) Write a balanced equation for the decomposition of \(\mathrm{NaAlH}_{4}\) (b) Which element in \(\mathrm{NaAlH}_{4}\) is the most electronegative? Which one is the least electronegative? (c) Based on electronegativity differences, what do you think is the identity of the polyatomic anion? Draw a Lewis structure for this ion.

Short answer

(a) The balanced equation for the decomposition of NaAlH4 is: NaAlH4(s) → NaH(s) + Al(s) + 2H2(g) (b) In NaAlH4, the most electronegative element is Hydrogen (H), and the least electronegative element is Sodium (Na). (c) The polyatomic anion in NaAlH4 is AlH4-, and its Lewis structure is: H | H - Al - H | H

Step-by-step solution

(a) Write a balanced equation for the decomposition of NaAlH4:

We are given that upon decomposition, NaAlH4 releases H2 as a gas and forms NaH(s) and Al(s). We can write this in chemical equation form as: NaAlH4(s) → NaH(s) + Al(s) + H2(g) Now we need to balance the equation. Begin with the hydrogen atoms, as there are four in NaAlH4 and only 2 in H2. By placing a coefficient of 2 in front of H2, we obtain: NaAlH4(s) → NaH(s) + Al(s) + 2H2(g) Now, all atoms are balanced on both sides of the equation.

(b) Determine the most and least electronegative elements in NaAlH4:

To determine the most and least electronegative elements in NaAlH4, we can refer to the periodic table, where electronegativity generally increases from left to right across a period and decreases from top to bottom within a group. In NaAlH4, we have the elements: 1. Sodium (Na) 2. Aluminum (Al) 3. Hydrogen (H) Comparing their electronegativity values, we find that Hydrogen (H) is the most electronegative element, and Sodium (Na) is the least electronegative element.

(c) Identify the polyatomic anion and draw its Lewis structure based on electronegativity differences:

Recall that an anion is a negatively charged ion. In this case, NaAlH4 contains Na as a cation with a +1 charge. Thus, we can conclude that the remaining part, AlH4, must be the polyatomic anion with a -1 charge. Now, we'll draw the Lewis structure for the AlH4- anion: 1. Count the total number of valence electrons. Al has 3 valence electrons, H has 1 valence electron, and there is an additional electron due to the -1 charge. So, we have a total of 3 + (4 * 1) + 1 = 8 valence electrons. 2. Place the least electronegative element (Al) at the center and surround it with the more electronegative elements (H). 3. Distribute the valence electrons by forming single bonds between Al and each H atom (4 bonds total). Now we have 8 - 4(2) = 0 valence electrons remaining. 4. Since no valence electrons remain, we can complete the Lewis structure of the AlH4- anion: H | H - Al - H | H

Key concepts

Metal Hydrides

Metal hydrides represent a class of compounds widely researched for hydrogen storage applications. They consist of a metal bonded to hydrogen atoms. In the context of the exercise, \(\mathrm{NaAlH}_4\) is a metal hydride composed of sodium (Na), aluminum (Al), and hydrogen (H). This specific compound is noteworthy for its capability to store and release hydrogen gas (\(\mathrm{H}_2\)), which is essential in fuel storage for hydrogen-powered vehicles.

Metal hydrides store hydrogen through a chemical reaction where hydrogen atoms bond with the metal. The storage capacity and the stability of these compounds are influenced by the type of metal and the conditions under which they are formed. The ability of metal hydrides to release hydrogen when needed makes them promising for energy storage solutions, as seen with \(\mathrm{NaAlH}_4\) decomposing to release hydrogen gas.

Electronegativity

Electronegativity is a chemical property that describes an atom's tendency to attract a shared pair of electrons towards itself in a chemical bond. It's scale runs from 0 to 4, with higher values indicating stronger attraction. In the compound \(\mathrm{NaAlH}_4\), hydrogen is the most electronegative element and sodium is the least, as observed from their positions on the periodic table.

The concept of electronegativity helps in predicting the nature of chemical bonds. When there is a large difference in electronegativity between two bonded atoms, the bond is considered ionic, where electrons are transferred. A smaller difference indicates a covalent bond, where electrons are shared. This plays a crucial role in understanding the behavior of metal hydrides, as the type of bond affects the storage capacity and release mechanism of hydrogen.

Chemical Bond Types

There are several types of chemical bonds that hold compounds together, including ionic, covalent, and metallic bonds. Ionic bonds are formed between atoms with large differences in electronegativity and involve the transfer of electrons. Covalent bonds occur when atoms with similar electronegativity share electrons. Metallic bonds are specific to metals, where electrons float freely among positively charged metal ions.

In \(\mathrm{NaAlH}_4\), both ionic and covalent bonds are present. The ionic bonds exist between the sodium ions and the \(\mathrm{AlH}_4^-\) anion, while the covalent bonds are found within the \(\mathrm{AlH}_4^-\) anion itself. Understanding the different bond types helps in predicting the physical properties and reactivity of compounds such as metal hydrides.

Lewis Structures

Lewis structures are diagrams that represent the bonding between atoms of a molecule and the lone pairs of electrons that may exist. The Lewis structure for a compound provides a visual representation of how the valence electrons are arranged among the atoms, which has implications for the molecule's geometry and reactivity.

For the \(\mathrm{AlH}_4^-\) anion in \(\mathrm{NaAlH}_4\), the Lewis structure shows aluminum in the center with four hydrogen atoms bonded to it, utilizing all eight valence electrons. The additional negative charge on the \(\mathrm{AlH}_4^-\) anion is represented by an extra lone electron. The drawing of Lewis structures is an essential skill in chemistry because it helps students visualize molecules and predict the distribution of electrons in a compound, contributing to a deeper understanding of chemical reactions and properties.