Question

Which of the following statements regarding interstitial hydrides is false? (1) In these hydrides hydrogen is present as hydride ion. (2) They are less dense than the parent metal. (3) In these hydrides hydrogen is often absorbed reversibly by the metal. (4) Thesc hydridcs arc nonstoichiometric.

Short answer

Statement (1) is false.

Step-by-step solution

Understand Interstitial Hydrides

Interstitial hydrides are formed when hydrogen molecules occupy the interstitial spaces between the metal atoms in a lattice. They do not form ionic bonds but rather exist as hydrogen atoms within the metal lattice.

Evaluate Statement (1)

Statement (1) claims that hydrogen is present as the hydride ion (H⁻). In interstitial hydrides, hydrogen atoms are not in the form of hydride ions but exist as atoms in the interstitial spaces. Hence, statement (1) is false.

Evaluate Statement (2)

Statement (2) claims that interstitial hydrides are less dense than the parent metal. Generally, the density of interstitial hydrides is similar to or even higher than the parent metal because the hydrogen atoms do not significantly alter the metal structure's mass and volume.

Evaluate Statement (3)

Statement (3) claims that hydrogen in interstitial hydrides is often absorbed reversibly by the metal. This is true, as interstitial hydrides typically enable reversible absorption and release of hydrogen.

Evaluate Statement (4)

Statement (4) claims that interstitial hydrides are nonstoichiometric. This is true because the amount of hydrogen that can be absorbed varies based on temperature and pressure, making their composition nonstoichiometric.

Conclusion

Based on the evaluation, it is clear that statement (1) is false while the other statements (2, 3, and 4) are true.

Key concepts

Hydride Ion Presence

Understanding the presence of hydride ions is crucial in chemistry. In the context of interstitial hydrides, it is important to note that hydrogen does not exist as hydride ions (H⁻). Instead, hydrogen atoms occupy spaces in the metal's lattice. This is different from ionic hydrides, where hydrogen exists as H⁻. Interstitial hydrides are formed by transition metals, and the hydrogen atoms are situated in the spaces (interstices) between metal atoms.

Density Comparison

When talking about the density of interstitial hydrides compared to the parent metals, it is often the case that their densities are same or even higher. This occurs because the hydrogen atoms occupy very small spaces without significantly altering the metal's volume. Therefore, they add mass without a considerable increase in the overall volume. It's incorrect to assume these hydrides are less dense just because they contain additional elements (hydrogen).

Reversible Absorption

One fascinating property of interstitial hydrides is their ability to reversibly absorb and release hydrogen. This feature is particularly useful in hydrogen storage applications. Hydrogen can be taken up by the metal at high pressures or lower temperatures and released when conditions change. This reversible aspect makes interstitial hydrides valuable for energy applications, like fuel cells and rechargeable hydrogen batteries.

Nonstoichiometric Composition

Interstitial hydrides often display a nonstoichiometric composition. This means that the ratio of hydrogen to metal is not fixed; it can vary depending on external conditions like temperature and pressure. Unlike stoichiometric compounds that have a definite composition, nonstoichiometric compounds like interstitial hydrides have variable ratios. This property allows these materials to absorb different amounts of hydrogen depending on the environment.