Question

Which of the following statements is not correct? (1) Alkaline earth metal hydrides are all reducing agents. (2) \(\mathrm{BeH}_{2}\) and \(\mathrm{MgH}_{2}\) are covalent polymers having bridge structures. (3) They react with water and liberate one mole of \(\mathrm{H}_{2}\) for each one mole of metal hydride. (4) \(\mathrm{CaH}_{2}, \mathrm{SrH}_{2}\) and \(\mathrm{BaH}_{2}\) are ionic hydrides.

Short answer

All statements (1), (2), (3), and (4) are correct.

Step-by-step solution

- Understanding the Statements

Read each of the four statements carefully. We need to identify the one that is not correct.

- Analyze Statement 1

Statement (1): 'Alkaline earth metal hydrides are all reducing agents.' This statement is correct because alkaline earth metal hydrides such as \(\text{CaH}_2\), \(\text{SrH}_2\), and \(\text{BaH}_2\) are known to act as strong reducing agents.

- Analyze Statement 2

Statement (2): '\(\text{BeH}_2\) and \(\text{MgH}_2\) are covalent polymers having bridge structures.' This statement is correct. \(\text{BeH}_2\) and \(\text{MgH}_2\) form covalent bonds and have polymeric structures with bridging hydrogen atoms.

- Analyze Statement 3

Statement (3): 'They react with water and liberate one mole of \(\text{H}_2\) for each one mole of metal hydride.' This statement is correct. When alkaline earth metal hydrides react with water, they typically release one mole of hydrogen gas per mole of metal hydride.

- Analyze Statement 4

Statement (4): 'CaH2, SrH2, and BaH2 are ionic hydrides.' This statement is also correct because \(\text{CaH}_2\), \(\text{SrH}_2\), and \(\text{BaH}_2\) are ionic in nature, formed by the transfer of electrons from the metal to the hydrogen.

- Identify the Incorrect Statement

Since all four statements (1), (2), (3), and (4) are correct, it seems there was an error in the exercise question. Re-evaluate the statements again, if necessary.

Key concepts

Reducing Agents

Reducing agents are substances that donate electrons to another substance during a chemical reaction, reducing the other substance. Alkaline earth metal hydrides, like \(\text{CaH}_2\), \(\text{SrH}_2\), and \(\text{BaH}_2\), act as strong reducing agents. They easily donate hydrogen atoms in reactions, forming ionic bonds by giving away electrons.
Alkaline earth metal hydrides are very reactive, often used to reduce metal oxides to metals and in various organic synthesis reactions.

Covalent Polymers

\(\text{BeH}_2\) and \(\text{MgH}_2\) are examples of hydrides that form covalent polymers. In these compounds, hydrogen bridges between metal atoms, creating a polymeric structure. Unlike ionic compounds, these polymers involve shared electrons in their bonds.
Covalent bonds are stronger and more directional, contributing to the stability and unique properties of these polymeric hydrides. Understanding the bridge structure helps explain their reactivity and physical characteristics.

Metal Hydride Reactions

When metal hydrides react with water, they release hydrogen gas. This is because the hydride ions (H\(^-\)) from the metal hydride react with water to produce hydrogen gas (H\(_2\)) and hydroxide ions (OH\(^-\)).
For example, \(\text{CaH}_2 + 2\text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 + 2\text{H}_2\). Here, one mole of calcium hydride reacts with two moles of water, resulting in one mole of calcium hydroxide and two moles of hydrogen gas. This reaction is stoichiometric and consistent across different alkaline earth metal hydrides.

Ionic Hydrides

Ionic hydrides are compounds in which hydrogen forms an anion (H\(^-\)). Alkaline earth metals like calcium (Ca), strontium (Sr), and barium (Ba) form ionic hydrides (e.g., \(\text{CaH}_2\), \(\text{SrH}_2\), \(\text{BaH}_2\)) by transferring their electrons to hydrogen.
These hydrides consist of metal cations (\text{M}\(^{2+}\)) and hydride anions (H\(^-\)), resulting in a salt-like structure. They exhibit high melting points and are typically solid at room temperature. Their reactivity, including reactions with water to release hydrogen gas, is driven by their ionic character.