Problem 11 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Read the following statement and explanation and answer as per the options given below : Statement : The alkali metals can form ionic hydrides which contain the hydride ion \(\mathrm{H}^{-}\) Explanation : The alkali metals have low electronegativity; their hydrides conduct electricity when fused and liberate hydrogen at the anode. [ (a) Both \(\mathrm{S}\) and \(\mathrm{E}\) are true and \(\mathrm{E}\) is the correct explanation of \(\mathrm{S}\). (b) Both \(\mathrm{S}\) and \(\mathrm{E}\) are true but \(\mathrm{E}\) is not the correct explanation of \(\mathrm{S}\). (c) \(\mathrm{S}\) is true but \(\mathrm{E}\) is false. (d) \(\mathrm{S}\) is false but \(\mathrm{E}\) is true

Step-by-Step Solution

Answer

(a) Both S and E are true and E is the correct explanation of S.

Step 1: Understand Statement (S)

Statement (S) claims that alkali metals can form ionic hydrides which include the hydride ion \( \mathrm{H}^{-} \). Alkali metals like lithium, sodium, and potassium do form halides with hydrogen, resulting in compounds like \( \text{LiH} \) which contain \( \mathrm{H}^{-} \) ions.

Step 2: Analyze Explanation (E)

Explanation (E) suggests that alkali metals have low electronegativity and their hydrides conduct electricity when molten, releasing hydrogen at the anode. This is a correct observation because ionic compounds conduct electricity when molten. The hydrides dissociate into ions, allowing the conduction of electricity, and hydrogen gas is released at the anode.

Step 3: Determine Relationship Between S and E

Both S and E are true regarding the properties of alkali metal hydrides. Explanation (E) provides a logical explanation of Statement (S) by clarifying why these hydrides behave as they do.

Step 4: Select the Correct Option

Since both S and E are true and E is the correct explanation of S, the correct option to select is (a).

Key Concepts

Alkali Metals

Alkali metals are a group of elements located in the first column of the periodic table. These include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr). These metals are known for their softness and high reactivity.

As members of Group 1, alkali metals have one electron in their outermost shell. This single electron can be easily lost to form positively charged ions known as cations. The loss of this electron results in the formation of ionic compounds, including ionic hydrides when combined with hydrogen.

The ionic hydrides of alkali metals, such as lithium hydride (LiH) or sodium hydride (NaH), feature the hydride ion (\( ext{H}^- \)), where hydrogen gains an electron from the metal. This unique bond forms because of the large difference in electronegativity between alkali metals and hydrogen.

Electronegativity

Electronegativity refers to the tendency of an atom to attract a bonding pair of electrons. In the context of alkali metals, they have low electronegativity, meaning they do not strongly attract electrons.

This low electronegativity is a key factor that influences the formation of ionic hydrides. With hydrogen having higher electronegativity in comparison, an electron is transferred from the alkali metal to hydrogen, resulting in a negatively charged hydride ion (\( ext{H}^- \)).

The difference in electronegativity between alkali metals and hydrogen allows the formation of ionic bonds rather than covalent ones.

Notable points about electronegativity:

• It depends on the atomic number and the distance of the valence electrons from the nucleus.
• It determines the nature of bonds; higher differences usually lead to ionic bonds.
• Increases across a period and decreases down a group in the periodic table.

Conduction of Electricity

The conduction of electricity involves the movement of charged particles, such as electrons or ions, through a medium. In the case of ionic hydrides, these compounds conduct electricity when they are in a molten state or dissolved in water.

When an ionic hydride is heated and melts, its ions are free to move. This movement allows the hydride ions (\( ext{H}^- \)) and metallic cations to transport the electric current through the substance.

At the anode, the negative hydride ions release electrons to form hydrogen gas, (\( 2 ext{H}^- \rightarrow ext{H}_2 + 2e^- \)), illustrating how ionic compounds can decompose to release gas.

Key points about electrical conduction in ionic compounds:

• Occurs due to ion movement, not electron flow.
• Requires the ions to be free, typically in liquid form or aqueous solution.
• Results in chemical changes at the electrodes, such as gas evolution.