Problem 90 - Chemistry An Atoms First Approach

Question

Which of the following statement(s) is(are) true? a. The alkali metals are found in the earth's crust in the form of pure elements. b. Gallium has one of the highest melting points known for metals. c. When calcium metal reacts with water, one of the products is \(\mathrm{H}_{2}(\mathrm{~g})\) d. When \(\mathrm{AlCl}_{3}\) is dissolved in water, it produces an acidic solution. e. Lithium reacts in the presence of excess oxygen gas to form lithium superoxide.

Step-by-Step Solution

Answer

The true statements are c and d. Statement c: When calcium metal reacts with water, one of the products is $\mathrm{H}_{2}(\mathrm{~g})$. Statement d: When $\mathrm{AlCl}_{3}$ is dissolved in water, it produces an acidic solution.

Step 1: Statement a: Alkali metals in earth's crust

Alkali metals are very reactive and are not found in nature in their pure elemental form. They are typically found as compounds or in mineral form. So, statement a is false.

Step 2: Statement b: Melting point of Gallium

Gallium is a soft metal with a very low melting point of 29.76°C (85.57°F). It is not one of the highest melting points known for metals. Statement b is false.

Step 3: Statement c: Reaction of calcium with water

Calcium reacts with water to form calcium hydroxide and hydrogen gas. The balanced chemical reaction is: \( \mathrm{Ca}(s)+ 2\mathrm{H}_{2}\mathrm{O}(l) \to \mathrm{Ca}(\mathrm{OH})_2(aq) + \mathrm{H}_{2}(g) \) So, statement c is true.

Step 4: Statement d: Behavior of \(\mathrm{AlCl}_{3}\) in water

When aluminum chloride (\(\mathrm{AlCl}_{3}\)) dissolves in water, it undergoes hydrolysis to form aluminum hydroxide and hydrochloric acid. The balanced chemical reaction is: \( \mathrm{AlCl}_{3}(s) + 3\mathrm{H}_{2}\mathrm{O}(l) \to \mathrm{Al}(\mathrm{OH})_{3}(s) + 3\mathrm{HCl}(aq) \) The solution contains hydrochloric acid, which is acidic. Therefore, statement d is true.

Step 5: Statement e: Lithium in the presence of excess oxygen

When lithium reacts with excess oxygen gas, it forms lithium peroxide, not lithium superoxide. The balanced chemical equation is: \( 2\mathrm{Li}(s) + \mathrm{O}_{2}(g) \to 2\mathrm{Li}_2\mathrm{O}_2(s) \) So, statement e is false. In conclusion, the true statements are c and d.

Key Concepts

Alkali Metals

Alkali metals are a fascinating group of elements found in group 1 of the periodic table. These metals include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr). They are known for several interesting characteristics:

• High reactivity: Alkali metals are highly reactive, especially with water, because they have just one electron in their outermost shell, which they readily lose to form positive ions.
• Never found pure: Due to their reactivity, these metals do not exist in nature in their pure elemental form. Instead, they are always found in combined forms, such as minerals or salts, like sodium chloride (NaCl) or potassium bromide (KBr).
• Softness: These metals are generally soft and can be cut with a knife.
• Low density: Some, like lithium, sodium, and potassium, are so light they can float on water.

It's important to handle these metals carefully in a laboratory setting, due to their tendency to react vigorously, sometimes explosively, with air or moisture.

Melting Point

Melting point refers to the temperature at which a solid becomes a liquid. It is a critical property that varies widely across different substances. Metals usually have high melting points because of the strong metallic bonds holding the atoms in a rigid lattice structure. However, some exceptions exist, such as with gallium:

• Gallium (Ga) has one of the lowest melting points among metals at about 29.76°C (85.57°F), allowing it to melt in the palm of your hand.
• Such a low melting point is not typical when compared to metals like tungsten, which melts at temperatures above 3000°C (5400°F).
• This property is part of what makes gallium useful in certain alloys and applications where low melting points are beneficial.

The melting point provides insight into the structure and strength of atomic bonding in a substance.

Hydrolysis

Hydrolysis is a chemical process where water is used to break down compounds. In the case of aluminum chloride (\( \mathrm{AlCl}_{3} \)), the process of hydrolysis involves a reaction with water, producing new substances:

• When \( \mathrm{AlCl}_{3} \) is dissolved in water, it forms aluminum hydroxide (\( \mathrm{Al(OH)}_3 \)) and hydrochloric acid (\( \mathrm{HCl} \)).
• The chemical equation for this hydrolysis reaction is: \( \mathrm{AlCl}_{3}(s) + 3 ext{H}_{2} ext{O}(l) o ext{Al}( ext{OH})_{3}(s) + 3 ext{HCl}(aq) \).
• This reaction results in the formation of an acidic solution due to hydrochloric acid being one of the products.

Hydrolysis is a key concept in chemistry with many applications, crucial for understanding how substances interact with water and change their chemical structure.

Acidic Solutions

An acidic solution is one that has a pH level less than 7. These solutions have a higher concentration of hydrogen ions (\( ext{H}^+ \)) compared to pure water. Acidic solutions are crucial in various chemical processes:

• They are typically the result of the dissolution of acidic substances, like hydrochloric acid (\( ext{HCl} \)), in water.
• For example, when \( ext{AlCl}_{3} \) undergoes hydrolysis, the resulting hydrochloric acid contributes to a lower pH, rendering the solution acidic.
• The property of being acidic affects both the reactivity and the applications of the solution in industries and laboratories.

Understanding the nature and behavior of acidic solutions helps in applications ranging from digestion in biology to manufacturing in industrial chemistry. They play a pivotal role in neutralization reactions, where an acid reacts with a base to form water and salt.