Problem 70 - Chemistry: The Central Science in SI Units, Global Edition

Question

Write a balanced equation for the reaction that occurs in each of the following cases: (a) Lithium is added to water. (b) Calcium is added to water. (c) Potassium reacts with chlorine gas. (d) Rubidium reacts with oxygen.

Step-by-Step Solution

Answer

(a) 2 Li (s) + 2 H₂O (l) -> 2 LiOH (aq) + H₂ (g) (b) Ca (s) + 2 H₂O (l) -> Ca(OH)₂ (aq) + H₂ (g) (c) 2 K (s) + Cl₂ (g) -> 2 KCl (s) (d) 4 Rb (s) + O₂ (g) -> 2 Rb₂O (s)

Step 1: (a) Lithium is added to water.

Lithium (Li) is an alkali metal, and it reacts with water (H₂O) to form lithium hydroxide (LiOH) and hydrogen gas (H₂). The balanced equation for this reaction can be obtained by ensuring that the number of H and O atoms is equal on both sides. Thus, the balanced equation is: \( \) 2 Li (s) + 2 H₂O (l) -> 2 LiOH (aq) + H₂ (g) \( \)

Step 2: (b) Calcium is added to water.

Calcium (Ca) is an alkaline earth metal, and it reacts with water (H₂O) to form calcium hydroxide (Ca(OH)₂) and hydrogen gas (H₂). To balance this equation, ensure that the number of H and O atoms is equal on both sides. The balanced equation is: \( \) Ca (s) + 2 H₂O (l) -> Ca(OH)₂ (aq) + H₂ (g) \( \)

Step 3: (c) Potassium reacts with chlorine gas.

Potassium (K) is an alkali metal, and it reacts with chlorine gas (Cl₂) to form potassium chloride (KCl). To balance this equation, ensure that the number of K and Cl atoms is equal on both sides. The balanced equation is: \( \) 2 K (s) + Cl₂ (g) -> 2 KCl (s) \( \)

Step 4: (d) Rubidium reacts with oxygen.

Rubidium (Rb) is an alkali metal, and it reacts with oxygen (O₂) to form rubidium oxide (Rb₂O). To balance this equation, ensure that the number of Rb and O atoms is equal on both sides. The balanced equation is: \( \) 4 Rb (s) + O₂ (g) -> 2 Rb₂O (s) \( \)

Key Concepts

Lithium reaction with water

When lithium, a soft, silvery alkali metal, is introduced to water, an interesting chemical reaction occurs. As part of the alkali metal group, lithium is highly reactive. When it interacts with water, the reaction forms lithium hydroxide and releases hydrogen gas, which can be observed as bubbling on the surface. This type of reaction is exothermic, meaning it releases heat.
The balanced chemical equation for lithium reacting with water is: \[ 2 \text{Li (s)} + 2 \text{H}_2\text{O (l)} \rightarrow 2 \text{LiOH (aq)} + \text{H}_2\text{(g)} \]
This equation shows us:

• Two lithium atoms react with two water molecules.
• The products are two molecules of lithium hydroxide and one molecule of hydrogen gas.

Understanding the balanced equation helps ensure the conversation of mass, which is important in all chemical reactions.

Alkali metals reactions

Alkali metals, including lithium, sodium, potassium, rubidium, and cesium, share characteristics of high reactivity and low density. These metals react vigorously, often explosively, with water. Each of these reactions forms a metal hydroxide and hydrogen gas.
A few properties about alkali metals:

• They are located in Group 1 of the periodic table.
• They have a single electron in their outermost shell, which they eagerly give up to achieve a stable, noble gas configuration.
• Their reactivity increases down the group from lithium to cesium.

When balancing these reactions, ensure the number of metal atoms and water molecules are the same on both sides of the equation. These reactions highlight the characteristics of alkali metals as strong reducing agents.

Rubidium oxide formation

Rubidium, another member of the alkali metal family, displays typical reactivity traits when exposed to oxygen. When rubidium comes into contact with oxygen, it forms rubidium oxide. This reaction needs to be controlled because it is highly exothermic, hinting that even a small amount of rubidium can produce a significant release of energy.
The balanced chemical equation is: \[ 4 \text{Rb (s)} + \text{O}_2\text{(g)} \rightarrow 2 \text{Rb}_2\text{O (s)} \]
Key points about the reaction:

• Four rubidium atoms react with one oxygen molecule.
• Two units of rubidium oxide are formed.
• The equation must remain balanced by having the same number of each type of atom on both sides.

This reaction further illustrates rubidium's aggressive reactivity as an alkali metal.

Potassium reaction with chlorine

Potassium, a well-known alkali metal, reacts with chlorine gas to produce potassium chloride, a compound extensively used as a salt substitute. This reaction is vigorous and exothermic, releasing energy in the form of heat and light.
The balanced equation for this reaction states: \[ 2 \text{K (s)} + \text{Cl}_2\text{(g)} \rightarrow 2 \text{KCl (s)} \]
Here are some important notes:

• Two potassium atoms react with one chlorine molecule.
• Produces two units of solid potassium chloride.
• The reaction is balanced to maintain equal numbers of each type of atom.

Understanding these equations helps reveal the simplicity in how alkali metals form compounds with nonmetals, maintaining charge balance as well.