Question

Which of the following statement is false about the alkali metals? (1) All alkali-metal salts impart a characteristic colour to the Bunsen flame. (2) Among \(\mathrm{LiOH}, \mathrm{CsOH}, \mathrm{KOH}\), and \(\mathrm{RbOH}\), CsOH has the highest solubility in water. (3) Among the alkali metals, cesium is the most reactive. (4) The reducing character of the alkali metal hydrides follow the order : \(\mathrm{LiH}>\mathrm{NaH}>\mathrm{KH}>\mathrm{RbH}>\mathrm{CsH} .\)

Short answer

Statement 4 is false. The correct reducing order is \(\text{CsH > RbH > KH > NaH > LiH}\).

Step-by-step solution

Understanding Statement 1

All alkali-metal salts do impart a characteristic color to the Bunsen flame. This statement is true. Examples include lithium producing a crimson flame, sodium producing a yellow flame, and potassium producing a lilac flame.

Analyzing Statement 2

Compare the solubility of \(\text{LiOH}, \text{CsOH}, \text{KOH}, \text{RbOH}\) in water. Cesium hydroxide (CsOH) is indeed highly soluble in water, more so than the other hydroxides mentioned.

Evaluating Statement 3

Assess the reactivity of cesium among the alkali metals. Cesium is the most reactive among the alkali metals, which makes the statement true. Reactivity increases down the group in the periodic table.

Investigating Statement 4

Consider the reducing character of alkali metal hydrides. Alkali metals' reducing strength typically increases with increasing atomic number, but hydrides like \(\text{LiH}\) are actually less reducing than \(\text{CsH}\). This makes the order \(\text{LiH > NaH > KH > RbH > CsH}\) incorrect. The correct order should be \(\text{CsH > RbH > KH > NaH > LiH}\). Thus, this statement is false.

Key concepts

Alkali Metals Reactivity

Alkali metals are highly reactive elements located in Group 1 of the periodic table. These metals include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr). Their reactivity increases as you move down the group.

The reactivity of alkali metals is due to their single valence electron, which they readily lose to achieve a stable electron configuration. This makes them highly reactive, especially with water and oxygen.

Some key points about their reactivity include:

• Reactivity increases down the group: Lithium is the least reactive, while francium is the most reactive.
• They react vigorously with water to form hydroxides and hydrogen gas.
• When exposed to air, they form oxides, peroxides, or superoxides depending on the metal.

For example, cesium is one of the most reactive alkali metals and can spontaneously ignite in air. In comparison, lithium reacts less violently but still requires careful handling.

Flame Test for Alkali Metals

The flame test is a simple yet effective method to identify alkali metals based on the color they emit when heated in a flame. This phenomenon occurs because the metal's electrons get excited when heated and release energy as light when they return to their ground state.

Each alkali metal imparts a characteristic color to the flame, making it easy to differentiate between them. Here are some typical flame colors for common alkali metals:

• Lithium: Crimson or red flame
• Sodium: Bright yellow flame
• Potassium: Lilac or light purple flame
• Rubidium: Red-violet flame
• Cesium: Blue or violet flame

The flame test is particularly useful in qualitative analysis because of its simplicity and the distinct colors produced by each metal. However, it's important to note that certain colors may be masked if multiple metals are present.

Solubility of Hydroxides

The solubility of alkali metal hydroxides in water varies depending on the metal. Generally, the solubility increases as you move down the group in the periodic table. This trend can be attributed to the lattice and hydration energies of the hydroxides.

Some important points about the solubility of alkali metal hydroxides include:

• Lithium hydroxide (LiOH): Moderately soluble
• Sodium hydroxide (NaOH): Highly soluble
• Potassium hydroxide (KOH): Very highly soluble
• Rubidium hydroxide (RbOH): More soluble than KOH
• Cesium hydroxide (CsOH): Extremely soluble

Among the alkali metal hydroxides mentioned in the exercise, cesium hydroxide (CsOH) is the most soluble in water. This increasing solubility can be attributed to the decrease in lattice energy and increasing ease of dissociation as you move down the group. High solubility of these hydroxides means they can form strong alkaline solutions, which are highly caustic.