Question

The wrong statement among the following is (1) Ionic hydrides are formed by clements of high electropositive character. (2) Phosphinc is an cxamplc of clectron-prccise hydridc. (3) Titanium hydride is a metallic hydride. (4) Mctallic hydrides on reacting with water give hydrogen.

Short answer

The wrong statement is (4).

Step-by-step solution

- Identify Ionic Hydrides

Ionic hydrides are typically formed by alkali metals and alkaline earth metals, which have a high electropositive character. They form hydrides like NaH and CaH2. Statement 1 is correct.

- Check Electronegativity and Hydride Types

Electron-precise hydrides are formed when the number of electrons around the central atom is equal to the number necessary for the formation of a stable molecule. Phosphine (PH3) is an example, as it has the precise number of electrons required. Statement 2 is correct.

- Identify Metallic Hydrides

Metallic hydrides are formed by transition metals. Titanium hydride (TiH2) is a good example of a metallic hydride. Statement 3 is correct.

- Reactivity with Water

Metallic hydrides typically do not react with water to produce hydrogen. Most metallic hydrides are stable and do not decompose easily in water. Thus, statement 4 is incorrect.

Key concepts

electropositive elements

Electropositive elements are those that tend to lose electrons and form positive ions. These elements are typically found in Groups 1 and 2 of the periodic table, which include the alkali metals (like sodium and potassium) and alkaline earth metals (like calcium and magnesium).

Due to their high electropositive character, these elements can easily donate electrons, making them highly reactive. When these elements react with hydrogen, they form ionic hydrides, such as sodium hydride (NaH) and calcium hydride (CaH2). Ionic hydrides are characterized by the transfer of electrons from the electropositive metal to the hydrogen atom, creating a compound with strong ionic bonds.

electron-precise hydrides

Electron-precise hydrides are compounds in which the number of valence electrons around the central atom is exactly equal to the number required to form stable chemical bonds.

A classic example of an electron-precise hydride is phosphine (PH3). In phosphine, the phosphorus atom has five valence electrons, three of which form bonds with the hydrogen atoms, leaving a lone pair of electrons on the phosphorus. This precise arrangement allows the molecule to be stable and satisfy the valence shell electron requirements.

These hydrides are typically covalent in nature and exhibit specific geometric configurations to maintain stability.

transition metals

Transition metals are elements found in Groups 3–12 of the periodic table. They are characterized by their ability to form different oxidation states and their use of d orbitals in bonding.

A unique feature of transition metals is their ability to form metallic hydrides. These hydrides often have non-stoichiometric compositions, meaning the ratio of metal to hydrogen can vary. Titanium hydride (TiH2) is a typical example.

Metallic hydrides usually exhibit metallic properties and can have applications as catalysts, in hydrogen storage, and in various industrial processes. Due to the variability in their composition, they can be quite complex in their behavior and characteristics.

reactivity with water

The reactivity of hydrides with water depends on their type.

Ionic hydrides, formed by highly electropositive elements, react vigorously with water to produce hydrogen gas. For example, sodium hydride (NaH) reacts with water as follows:

2 NaH + 2 H2O → 2 NaOH + H2

However, metallic hydrides, generally formed by transition metals, do not react with water to produce hydrogen. These hydrides are typically stable and do not decompose easily when in contact with water. Therefore, saying that metallic hydrides react with water to produce hydrogen would be incorrect. The stability of metallic hydrides in water makes them useful in many practical applications where moisture resistance is required.