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 \(\mathrm{H}_{2}(g) .\) d. When 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.

Short answer

Statements c, d, and e are true.

Step-by-step solution

Statement a

: Alkali metals are highly reactive and do not exist in their pure elemental form in the earth's crust. They are found as components in various compounds such as halides and oxides. Therefore, statement a is false.

Statement b

: Gallium has a relatively low melting point of about 29.8\(^\circ\)C (85.6\(^\circ\)F). There are many metals with much higher melting points, such as tungsten, which has the highest melting point for metals at around 3,422\(^\circ\)C (6,192\(^\circ\)F). So, statement b is false.

Statement c

: When calcium metal reacts with water, the balanced chemical reaction is: Ca(s) + 2H2O(l) → Ca(OH)2(aq) + H2(g) As we can see from this reaction, one of the products is indeed hydrogen gas, H2(g). Therefore, statement c is true.

Statement d

: When aluminum chloride (AlCl3) is dissolved in water, it hydrolyzes to produce an acidic solution. The balanced chemical reaction is: AlCl3(s) + 3H2O(l) ↔ Al(OH)3(s) + 3HCl(aq) The HCl produced in this reaction generates an acidic solution. Hence, statement d is true.

Statement e

: When lithium reacts in the presence of excess oxygen gas, it forms lithium superoxide (LiO2). The balanced chemical reaction for this is: 2Li(s) + O2(g) → 2LiO2(s) Therefore, statement e is true. In conclusion, statements c, d, and e are true.

Key concepts

Alkali Metals

Alkali metals include fascinating elements like lithium, sodium, and potassium. They belong to Group 1 of the periodic table. These metals are known for their high reactivity. Because of this reactivity, they do not occur freely in nature in their pure form. Instead, they are found as part of compounds, such as oxides or halides. Their reactivity is so high that when exposed to air or water, they can react vigorously. This makes them very interesting and sometimes even dangerous to handle without the proper precautions.

For example, when sodium is added to water, it reacts to form sodium hydroxide and hydrogen gas. This reaction is highly exothermic and often produces a flame! Each alkali metal increases in reactivity as you move down the group in the periodic table. So, potassium is more reactive than sodium, and cesium is even more reactive than potassium.

Here are some key points about alkali metals:

• They are soft and can be cut with a knife.
• They have low densities and can often float on water.
• Each has a single electron in its outermost shell, making them highly reactive.
• They are shiny and have a silvery luster when freshly cut.

Acidic Solutions

Acidic solutions result from substances that release hydrogen ions, H extsuperscript{+}, when dissolved in water. One common example occurs when aluminum chloride (AlCl extsubscript{3}) is dissolved. It undergoes hydrolysis, which leads to an acidic solution. The chemical equation displaying this hydrolysis is:
AlCl extsubscript{3(s)} + 3H extsubscript{2}O extsubscript{(l)} \( \rightleftharpoons \) Al(OH) extsubscript{3(s)} + 3HCl extsubscript{(aq)}

As shown, aluminum chloride reacts with water to produce aluminum hydroxide and hydrochloric acid. The presence of hydrochloric acid increases the concentration of hydrogen ions, making the solution acidic. This is an important reaction to consider in various applications, especially in chemical industries and laboratories.

Some key points to remember about acidic solutions:

• They have a pH less than 7.
• They can conduct electricity because they ionize in water.
• They can turn litmus paper red.
• They often have a sour taste, though tasting chemicals is not recommended for safety reasons.
• Common acidic solutions include vinegar and lemon juice.

Lithium Superoxide

Lithium superoxide (LiO extsubscript{2}) is a unique compound formed when lithium reacts with an excess of oxygen. Normally, alkali metals form peroxides or simple oxides when reacting with oxygen. However, lithium, being smaller and having a distinct position in the group, forms a superoxide instead. This is illustrated in the following reaction:

2Li extsubscript{(s)} + O extsubscript{2(g)} \( \rightarrow \) 2LiO extsubscript{2(s)}

This compound is of interest because of its potential use in advanced energy storage and other technological applications. Lithium superoxide can act as an oxidizer because it contains a larger amount of available oxygen than regular oxides do. This property makes it useful in specific contexts, including lithium-air batteries.

Some important points about lithium superoxide:

• It forms under conditions where oxygen is abundant.
• It features lithium ions and superoxide ions (O extsubscript{2} extsuperscript{-}).
• It has potential applications in the field of energy, especially in battery technologies.
• Lithium superoxide serves as a model for understanding other superoxide compounds, making it important in scientific research.