Problem 88 - Chemistry The Central Science In Si Units

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 \(\mathrm{H}_{2}(g) . \mathrm{NaAlH}_{4}\) pos- sesses 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?

Step-by-Step Solution

Answer

(a) Balanced equation: \(\mathrm{NaAlH}_4 (s) \rightarrow \mathrm{NaH} (s) + \mathrm{Al} (s) + \mathrm{\frac{3}{2} H_2} (g)\). (b) Most electronegative: H, least: Na. (c) Polyatomic anion: \([\mathrm{AlH}_4]^-\); draw Al at the center bonded to 4 H atoms. (d) Formal charge on H in \([\mathrm{AlH}_4]^-\) is 0.

Step 1: Write the Balanced Equation

The decomposition of sodium aluminum hydride (\(\mathrm{NaAlH}_4\)) releases hydrogen gas. It follows the reaction: \[\mathrm{NaAlH}_4 (s) \rightarrow \mathrm{NaH} (s) + \mathrm{Al} (s) + \mathrm{\frac{3}{2} H_2} (g)\]. This equation is balanced with respect to mass and charge.

Step 2: Identify Most and Least Electronegative Elements

In \(\mathrm{NaAlH}_4\), hydrogen (H) is the most electronegative element, as it is typically more electronegative than sodium (Na) and aluminum (Al), while sodium (Na) is the least electronegative since it is a metal and located further to the left in the periodic table.

Step 3: Determine and Draw the Polyatomic Anion

To determine the anion, consider electronegativity. The polyatomic anion is likely \([\mathrm{AlH}_4]^-\) since aluminum (Al) bonds with hydrogen to form a complex anion. The Lewis structure involves Al at the center with single bonds to four H atoms, surrounded by a negative charge due to gaining an electron, thereby balancing the compound's charge.

Step 4: Calculate Formal Charge on Hydrogen

The formula for formal charge is: \(\text{Formal charge} = \text{valence electrons} - (\text{non-bonding electrons} + \frac{1}{2} \times \text{bonding electrons})\). Hydrogen typically has one valence electron and shares a pair entirely with Al, thus: \(1 - (0 + \frac{1}{2} \times 2) = 0\). Therefore, the formal charge on hydrogen in \([\mathrm{AlH}_4]^-\) is 0.

Key Concepts

Chemical Bonds

Chemical bonds are the glue holding atoms together in molecules and complexes. In a compound like \( \mathrm{NaAlH_4} \), these bonds can be either ionic or covalent. Ionic bonds are formed when one atom donates an electron to another, creating ions. These oppositely charged ions are held together by electrostatic attraction. For instance, in \( \mathrm{NaAlH_4} \), sodium (Na) forms an ionic bond with the \([\mathrm{AlH}_4]^–\) anion.
The polyatomic anion \([\mathrm{AlH}_4]^–\) is held together by covalent bonds. This involves sharing electrons between atoms. In \([\mathrm{AlH}_4]^–\), aluminum forms single covalent bonds with hydrogen atoms, creating a stable arrangement. The presence of both ionic and covalent bonds in \( \mathrm{NaAlH_4} \) allows it to efficiently store hydrogen.

Electronegativity

Electronegativity is a measure of how strongly an atom can attract electrons in a bond. In the context of \( \mathrm{NaAlH_4} \), understanding electronegativity helps identify bond types and predict molecular behavior.
Hydrogen, being more electronegative than sodium and aluminum, tends to attract electrons more strongly. Unlike sodium, which sits further left and lower on the periodic table, giving it a lower electronegativity. This means sodium easily loses electrons, forming positive ions.

• Hydrogen: Most electronegative in \( \mathrm{NaAlH_4} \)
• Sodium: Least electronegative

Recognizing these differences in electronegativity helps predict the formation of the \([\mathrm{AlH}_4]^–\) anion, as aluminum's greater tendency to hold onto electrons over sodium aids in this bond formation.

Lewis Structure

The Lewis structure is a simplified representation of the arrangement of atoms in a molecule, showing how they bond and accommodate electrons. For the \([\mathrm{AlH}_4]^–\) anion, constructing the Lewis structure begins with placing aluminum (Al) at the center since it's the most likely to form multiple bonds due to its size and valence.
Aluminum connects to the four hydrogen atoms, forming single covalent bonds with each. The overall negative charge on \([\mathrm{AlH}_4]^–\) indicates an extra electron in the structure, balancing the compound's formal charge. It is often represented by placing this charge outside the brackets enclosing the structure.
When drawing, ensure Al is bonded to H with lines representing shared pairs of electrons. This simple depiction helps in understanding how electrons are distributed in the anion and verifies that the formal charge on each hydrogen remains \(0\). It shows a clear picture of how the hydride functions and maintains stability within the atomic structure.