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 \(\mathrm{H}_{2}(g) . \mathrm{NaAlH}_{4}\) 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, predict the identity of the polyatomic anion. Draw a Lewis structure for this ion. (d) What is the formal charge on hydrogen in the polyatomic ion?

Short answer

The balanced equation for the decomposition of NaAlH4 is \(NaAlH_4 \rightarrow NaH + Al + 2H_2\). The most electronegative element is hydrogen, and the least electronegative is sodium. The polyatomic anion is AlH4-, and its Lewis structure consists of a central aluminum atom surrounded by four hydrogen atoms, each sharing a covalent bond with the aluminum. The formal charge on hydrogen in the AlH4- polyatomic ion is 0.

Step-by-step solution

Write a Balanced Equation for the Decomposition of NaAlH4

We are given that the decomposition of NaAlH4 produces NaH, Al, and H2 as products. To balance the equation, we need to ensure that the number of atoms of each element on both sides of the equation is equal. The balanced equation for the decomposition is: \[NaAlH_4 \rightarrow NaH + Al + 2H_2\]

Determine Electronegativity of Elements in NaAlH4

Electronegativity describes the ability of an atom to attract shared electrons. According to the periodic table, electronegativity generally increases from left to right and bottom to top. In NaAlH4, we have 3 elements: sodium (Na), aluminum (Al), and hydrogen (H). Using the trend mentioned above, we can determine the most and least electronegative elements in NaAlH4. The most electronegative element is hydrogen, and the least electronegative is sodium.

Predict the Identity of the Polyatomic Anion and Draw a Lewis Structure

Based on the electronegativity differences, we expect the polyatomic anion to consist of aluminum and hydrogen because they are closer in electronegativity compared to sodium. Aluminum can form a tetrahedral structure with 4 hydrogen atoms with a -1 charge, resulting in the polyatomic ion AlH4-. To draw the Lewis structure for AlH4-, follow these steps: 1. Count the total number of valence electrons: Al has 3 valence electrons, and each H has 1 valence electron, making a total of 7 valence electrons. Since AlH4- has a -1 charge, add one more electron. 2. Place the central atom (Al) in the center and surround it with 4 hydrogen atoms. 3. Distribute the remaining electrons (8) as pairs in covalent bonds between Al and each H atom. Now, we have a valid Lewis structure for the AlH4- ion.

Determine the Formal Charge on Hydrogen in the Polyatomic Ion

The formal charge on an atom can be calculated using the formula: Formal charge = Valence electrons - Non-bonding electrons - (1/2 * Bonding electrons) For hydrogen in the AlH4- ion, the number of valence electrons is 1, there are no non-bonding electrons, and there is 1 bonding electron (the electron shared in the covalent bond with Al). Thus, we can calculate the formal charge on hydrogen: Formal charge = 1 - 0 - (1/2 * 1) = 0 The formal charge on hydrogen in the AlH4- polyatomic ion is 0.

Key concepts

Hydrogen Storage

Hydrogen storage is a critical component of the hydrogen economy. It's crucial to store hydrogen safely, efficiently, and compactly for use as a clean fuel. Light metal hydrides, such as sodium alanate (\(\mathrm{NaAlH}_4\)), are attractive because they can store large quantities of hydrogen by weight, making them ideal for portable energy applications.

The compound \(\mathrm{NaAlH}_4\) can decompose to release hydrogen gas, making it a promising candidate for hydrogen storage. For example, when \(\mathrm{NaAlH}_4\) decomposes, it releases

• Sodium hydride (\(\mathrm{NaH}\))
• Aluminum (\(\mathrm{Al}\))
• Hydrogen gas (\(\mathrm{H}_2\))

This decomposition expresses itself in the balanced equation: \[\mathrm{NaAlH}_4 \rightarrow \mathrm{NaH} + \mathrm{Al} + 2\mathrm{H}_2\]

This balance ensures that the number of atoms for each element remains the same across both sides of the reaction, maintaining mass conservation. Such chemical processes underscore the importance of balancing reactions when designing efficient hydrogen storage systems.

Electronegativity

Electronegativity is a measure of how strongly an atom can attract electrons from a chemical bond. It's fundamental in predicting molecule behavior, especially when analyzing which atoms within a molecule will hold a stronger bond pull.

In \(\mathrm{NaAlH}_4\), the elements include sodium (Na), aluminum (Al), and hydrogen (H). Electronegativity trends within the periodic table generally increase from left to right and decrease from top to bottom. This means:

• Hydrogen is the most electronegative element in \(\mathrm{NaAlH}_4\).
• Sodium is the least electronegative.

Understanding this hierarchy provides insight into potential bond types within the molecule and influences our interpretation of interaction forces. For instance, the bonds between Na and other elements are more ionic, given Na's lower electronegativity. Meanwhile, bonds involving H, being more electronegative, are more covalent.

Polyatomic Anions

Polyatomic anions are ions composed of two or more atoms tied together with covalent bonds, carrying a net negative charge. They are vital components of many chemical compounds and influence the structure, reactivity, and stability of molecules.

In the case of \(\mathrm{NaAlH}_4\), the compound includes the polyatomic anion \(\mathrm{AlH}_4^−\). This anion is intriguing because it combines multiple hydrogen atoms with an aluminum atom, resulting in a tetrahedral geometric structure. Aluminum forms covalent bonds with each hydrogen, and the excess electron gives the ion its net negative charge.

To understand its structure, counting the number of valence electrons is crucial. Aluminum has 3, and every hydrogen has 1 valence electron, totaling 7. The negative charge adds an extra electron, summing up to 8. These electrons create the covalent bonds that hold the \(\mathrm{AlH}_4^-\) anion together, stabilizing the compound.

Lewis Structure

Lewis structures are diagrams used to represent the valence electrons of atoms within a molecule, providing visibility of how atoms connect and bond. These structures are essential for visualizing molecular geometry and predicting the reactivity of compounds.

For the \(\mathrm{AlH}_4^-\) anion:

• The aluminum atom sits centrally.
• Four hydrogen atoms surround it.

Each bond consists of a pair of electrons shared between aluminum and each hydrogen atom. The diagram reveals how these shared electrons form covalent bonds, hinting at the tetrahedral shape due to spatial electron repulsion. The Lewis structure aids in assessing formal charges.

The formal charge calculation helps verify the structure's accuracy:

• Hydrogen has 1 valence electron, with none unshared.
• The formal charge on hydrogen is calculated as: \[FC = \text{Valence electrons} - \text{Non-bonding electrons} - \frac{1}{2} \times \text{Bonding electrons}\]\[FC = 1 - 0 - \frac{1}{2} \times 2 = 0\]

Thus, the hydrogen atoms in \(\mathrm{AlH}_4^-\) have a formal charge of zero, confirming the correct depiction of being neutral in charge.