Problem 15 - Chemistry The Central Science

Question

Write balanced chemical equations corresponding to each of the following descriptions: (a) Solid calcium carbide, \(\mathrm{CaC}_{2}\) , reacts with water to form an aqueous solution of calcium hydroxide and acetylene gas, \(\mathrm{C}_{2} \mathrm{H}_{2}\) . (b) When solid potassium chlorate is heated, it decomposes to form solid potassium chloride and oxygen gas. (c) Solid zinc metal reacts with sulfuric acid to form hydrogen gas and an aqueous solution of zinc sulfate. (d) When liquid phosphorus trichloride is added to water, it reacts to form aqueous phosphorous acid, \(\mathrm{H}_{3} \mathrm{PO}_{3}(a q)\), and aqueous hydrochloric acid. (e) When hydrogen sulfide gas is passed over solid hot iron(III) hydroxide, the resulting reaction produces solid iron(II) sulfide and gaseous water.

Step-by-Step Solution

Answer

(a) CaC2 (s) + 2 H2O → Ca(OH)2 (aq) + C2H2 (g) (b) 2 KClO3 (s) → 2 KCl (s) + 3 O2 (g) (c) Zn (s) + H2SO4 → H2 (g) + ZnSO4 (aq) (d) PCl3 (l) + 3 H2O → H3PO3 (aq) + 3 HCl (aq) (e) 3 H2S (g) + 2 Fe(OH)3 (s) → 2 FeS (s) + 6 H2O (g)

Step 1: Write the Reactants and Products

The reactants are solid calcium carbide (CaC2) and water (H2O). The products are an aqueous solution of calcium hydroxide (Ca(OH)2) and acetylene gas (C2H2).

Step 2: Write the Unbalanced Equation

CaC2 (s) + H2O → Ca(OH)2 (aq) + C2H2 (g)

Step 3: Balance the Equation

To balance the equation, we'll adjust the number of reacting molecules: CaC2 (s) + 2 H2O → Ca(OH)2 (aq) + C2H2 (g) (b) Solid potassium chlorate is heated, it decomposes to form solid potassium chloride and oxygen gas.

Step 4: Write the Reactants and Products

The reactant is solid potassium chlorate (KClO3). The products are solid potassium chloride (KCl) and oxygen gas (O2).

Step 5: Write the Unbalanced Equation

KClO3 (s) → KCl (s) + O2 (g)

Step 6: Balance the Equation

To balance the equation, we will adjust the number of reacting molecules: 2 KClO3 (s) → 2 KCl (s) + 3 O2 (g) (c) Solid zinc metal reacts with sulfuric acid to form hydrogen gas and an aqueous solution of zinc sulfate.

Step 7: Write the Reactants and Products

The reactants are solid zinc metal (Zn) and sulfuric acid (H2SO4). The products are hydrogen gas (H2) and an aqueous solution of zinc sulfate (ZnSO4).

Step 8: Write the Unbalanced Equation

Zn (s) + H2SO4 → H2 (g) + ZnSO4 (aq)

Step 9: Balance the Equation

In this case, the equation is already balanced, so no adjustments are needed. (d) When liquid phosphorus trichloride is added to water, it reacts to form aqueous phosphorous acid, \(\mathrm{H}_{3} \mathrm{PO}_{3}(a q)\), and aqueous hydrochloric acid.

Step 10: Write the Reactants and Products

The reactants are liquid phosphorus trichloride (PCl3) and water (H2O). The products are aqueous phosphorous acid (H3PO3) and aqueous hydrochloric acid (HCl).

Step 11: Write the Unbalanced Equation

PCl3 (l) + H2O → H3PO3 (aq) + HCl (aq)

Step 12: Balance the Equation

To balance the equation, we'll adjust the number of reacting molecules: PCl3 (l) + 3 H2O → H3PO3 (aq) + 3 HCl (aq) (e) When hydrogen sulfide gas is passed over solid hot iron(III) hydroxide, the resulting reaction produces solid iron(II) sulfide and gaseous water.

Step 13: Write the Reactants and Products

The reactants are hydrogen sulfide gas (H2S) and solid hot iron(III) hydroxide (Fe(OH)3). The products are solid iron(II) sulfide (FeS) and gaseous water (H2O).

Step 14: Write the Unbalanced Equation

H2S (g) + Fe(OH)3 (s) → FeS (s) + H2O (g)

Step 15: Balance the Equation

To balance the equation, we'll adjust the number of reacting molecules: 3 H2S (g) + 2 Fe(OH)3 (s) → 2 FeS (s) + 6 H2O (g)

Key Concepts

Chemical Reactions

Chemical reactions are processes where substances, known as reactants, interact to form one or more new substances, known as products. A chemical reaction involves the breaking of bonds in reactants and the formation of new bonds in products. These changes result in the conversion of substances into different chemical species with distinct properties. For example, when calcium carbide reacts with water, it results in the production of calcium hydroxide and acetylene gas.
Chemical reactions can be simple or complex, sometimes involving just one step or multiple stages. Understanding chemical reactions is crucial for explaining phenomena in chemistry, such as the reaction between zinc and sulfuric acid that produces hydrogen gas and zinc sulfate.

Equation Balancing

Equation balancing is the process of ensuring that a chemical equation follows the law of conservation of mass. This implies that the number of atoms for each element should be the same on both sides of the equation. To achieve this, coefficients are added in front of the chemical formulas. Balancing chemical equations is essential to accurately depict the chemical changes that occur during a reaction.
For instance, in the decomposition of potassium chlorate into potassium chloride and oxygen, the equation must be balanced as: \[ 2 \, \text{KClO}_3 \rightarrow 2 \, \text{KCl} + 3 \, \text{O}_2 \] This ensures that the number of oxygen and potassium atoms is consistent on both sides of the equation. Balancing equations is fundamental in predicting the amounts of products formed or reactants needed.

Reactants and Products

In a chemical reaction, reactants are the starting substances that undergo a transformation, while products are the new substances formed as a result of the reaction. Understanding the role of reactants and products helps in comprehending the overall direction and outcome of a reaction.
Consider the reaction of zinc metal with sulfuric acid. Zinc metal and sulfuric acid serve as the reactants, and through the reaction, they produce hydrogen gas and zinc sulfate as products.

• Reactants: Chemical substances initially present in a reaction.
• Products: Substances formed as a result of a chemical reaction.

Identifying reactants and products is the first step in writing and balancing chemical equations, fundamental to analyzing chemical processes.

Decomposition Reaction

A decomposition reaction occurs when a single compound breaks down into two or more simpler substances. This type of reaction is usually initiated by an external source of energy such as heat, light, or electricity.
An example of a decomposition reaction is the breakdown of potassium chlorate into potassium chloride and oxygen gas: \[ 2 \, \text{KClO}_3 \rightarrow 2 \, \text{KCl} + 3 \, \text{O}_2 \]
During decomposition, the complex molecule of potassium chlorate dissipates into simpler molecules of potassium chloride and oxygen, showcasing the versatility of chemical changes and the way energy affects chemical compounds. Decomposition reactions are fundamental in various industrial and laboratory processes.

Synthesis Reaction

A synthesis reaction, also known as an addition or composition reaction, involves two or more reactants combining to form a single product. This type of reaction is a fundamental process in chemistry, allowing the construction of complex molecules from simpler ones.
An example of a synthesis reaction is the combination of hydrogen sulfide gas with iron(III) hydroxide to produce iron(II) sulfide and water: \[ 3 \text{H}_2 \text{S} + 2 \text{Fe(OH)}_3 \rightarrow 2 \text{FeS} + 6 \text{H}_2\text{O} \]
Synthesis reactions play a critical role in various chemical industries, including the manufacturing of chemical compounds and pharmaceuticals. Understanding these reactions helps in comprehending how elements and simple compounds interact and bond to form complex substances.