Problem 69 - 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) Potassium metal is exposed to an atmosphere of chlorine gas. (b) Strontium oxide is added to water. (c) A fresh surface of lithium metal is exposed to oxygen gas. (d) Sodium metal reacts with molten sulfur.

Step-by-Step Solution

Answer

The balanced equations for the given reactions are: (a) \(2K + Cl_2 \rightarrow 2KCl\) (b) \(SrO + H_2O \rightarrow Sr(OH)_2\) (c) \(4Li + O_2 \rightarrow 2Li_2O\) (d) \(2Na + S \rightarrow Na_2S\)

Step 1: (a) Potassium metal reacts with chlorine gas

To write the balanced equation for the reaction between potassium metal (K) and chlorine gas (Cl₂), first identify the products. Potassium belongs to group I, hence it has +1 charge, and chlorine belongs to group VII, hence it has -1 charge. So, the product will be potassium chloride (KCl). Now, balance the equation: 2K + Cl₂ → 2KCl The balanced equation is: \(2K + Cl_2 \rightarrow 2KCl\)

Step 2: (b) Strontium oxide reacts with water

To write the balanced equation for the reaction between strontium oxide (SrO) and water (H₂O), first identify the products. SrO has a +2 charge on Sr and -2 charge on O; water has +1 charge on H and -2 charge on O. The product will be strontium hydroxide (Sr(OH)₂), as Sr has a +2 charge and OH⁻ has -1 charge. Now, balance the equation: SrO + H₂O → Sr(OH)₂ The balanced equation is: \(SrO + H_2O \rightarrow Sr(OH)_2\)

Step 3: (c) Lithium metal reacts with oxygen gas

To write the balanced equation for the reaction between lithium metal (Li) and oxygen gas (O₂), first identify the products. Lithium belongs to group I, hence it has +1 charge, and oxygen belongs to group VI, hence it has -2 charge. So, the product will be lithium oxide (Li₂O). Now, balance the equation: 4Li + O₂ → 2Li₂O The balanced equation is: \(4Li + O_2 \rightarrow 2Li_2O\)

Step 4: (d) Sodium metal reacts with molten sulfur

To write the balanced equation for the reaction between sodium metal (Na) and molten sulfur (S), first identify the products. Sodium belongs to group I, hence it has +1 charge, and sulfur belongs to group VI, hence it has -2 charge. So, the product will be sodium sulfide (Na₂S). Now, balance the equation: 2Na + S → Na₂S The balanced equation is: \(2Na + S \rightarrow Na_2S\)

Key Concepts

Redox Reactions

Redox reactions are a type of chemical reaction that involves a transfer of electrons between two species. This process is a combination of two related sub-processes: reduction, where an atom gains electrons, and oxidation, where an atom loses electrons. In the balanced equation for the reaction of potassium metal with chlorine gas, potassium (K) loses electrons to become K⁺ (oxidation), and chlorine (Cl₂) gains electrons to become Cl⁻ (reduction). Together, this forms potassium chloride (KCl), showing the clear redox process.

The balanced chemical equation demonstrates the changes in oxidation states. Potassium starts with an oxidation state of 0 and changes to +1. Chlorine starts with an oxidation state of 0, and each chlorine atom changes to -1. Redox reactions are key to understanding many chemical processes, as they show how elements interact and change at the atomic level, achieving stability by transferring electrons.

Group I Elements

Group I elements, known as alkali metals, include lithium (Li), sodium (Na), potassium (K), and others. These elements are characterized by their single valence electron, which they readily lose to achieve a noble gas configuration.

• Alkali metals are highly reactive, particularly with halogens, as seen in the reaction between potassium and chlorine.
• The reaction of lithium with oxygen to form lithium oxide ( 4Li + O₂ → 2Li₂O) shows how easily these metals form compounds.

This group is known for its vigorous reactions with water and non-metals due to the low ionization energy, indicating how easily electrons are lost.

Water and oxygen reactions are examples of reactions demonstrating the metals' ability to lose electrons and participate in the reduction-oxidation process. Group I elements are crucial in understanding basic reactivity patterns in chemistry, helping to predict the behavior of other elements in the same vertical column on the periodic table.

Stoichiometry

Stoichiometry involves calculating the relative quantities of reactants and products involved in chemical reactions. By balancing chemical equations, we ensure the conservation of mass and atoms during a reaction. Each of the balanced equations provided in the exercises exemplifies stoichiometry.

• The equation for lithium reacting with oxygen: 4Li + O₂ → 2Li₂O, shows a 4:1:2 relationship.
• For potassium and chlorine forming potassium chloride: 2K + Cl₂ → 2KCl, a 2:1:2 ratio is observed.

The coefficients in these equations indicate the molar relationships between substances, facilitating calculations of needed reactants or expected products.

Mastering stoichiometry is a fundamental skill in chemistry. It aids in lab preparation by predicting the amounts of reagents required and the yields of the products. Stoichiometry is not just about numbers; it's about understanding how matter transforms in a controlled, precise way.

Chemical Properties of Metals

Metals have unique chemical properties due to their atomic structure. They are typically good conductors of electricity and heat and demonstrate malleability and ductility. In chemical reactions, metals tend to lose electrons to form cations, a process evident in reactions like sodium with molten sulfur ( 2Na + S → Na₂S).

• Metals like sodium and potassium are highly reactive with nonmetals, forming ionic bonds.
• Metals demonstrate a variety of reactivity levels based on their position in the periodic table; for instance, transition metals react differently than alkali metals.

Understanding these properties aids in predicting how metals will behave in reactions and compounds.

The reactivity of metals is vital in many practical applications, such as the extraction of metals from ores and the formulation of materials with specific properties. Recognizing the tendencies of metals to participate in certain types of reactions enhances our prediction of chemical behavior, especially in industrial and natural processes.