Question

Write a balanced equation for each of the following reactions. (You may have to guess at one or more of the reaction products, but you should be able to make a reasonable guess, based on your study of this chapter.) (a) Hydrogen selenide can be prepared by reaction of an aqueous acid solution on aluminum selenide. (b) Sodium thiosulfate is used to remove excess \(\mathrm{Cl}_{2}\) from chlorine-bleached fabrics. The thiosulfate ion forms \(\mathrm{SO}_{4}{ }^{2-}\) and elemental sulfur, while \(\mathrm{Cl}_{2}\) is reduced to \(\mathrm{Cl}^{-}\). Nitrogen and the Other Group 5A Elements (Sections 22.7 and 22.8)

Short answer

Reaction (a): Formation of hydrogen selenide: Balanced equation: \(2 Al_2 Se_3 + 6 H^+ \rightarrow 6 H_2 Se + 4 Al\) Reaction (b): Sodium thiosulfate reacting with chlorine gas: Balanced equation: \(4 Na_2 S_2 O_3 + 8 Cl_2 \rightarrow 8 NaCl + 4 Na_2 SO_4 + 8 S\)

Step-by-step solution

Reaction (a): Formation of hydrogen selenide

The first reaction involves the reaction of an aqueous acid solution (likely with H+) ions on aluminum selenide (Al2Se3) to produce hydrogen selenide (H2Se). First, we will write an unbalanced equation and then balance it. Unbalanced equation: Al2Se3 + H+ → H2Se + Al Now we will balance the equation: Balanced equation: 2 Al2Se3 + 6 H+ → 6 H2Se + 4 Al

Reaction (b): Sodium thiosulfate reacting with chlorine gas

In this reaction, sodium thiosulfate (Na2S2O3) is used to remove excess chlorine gas (Cl2) from fabrics. The thiosulfate ion (S2O3^2-) forms sulfate ion (SO4^2-) and elemental sulfur (S), while Cl2 is reduced to chloride ion (Cl-). We start by writing an unbalanced equation and then balance it. Unbalanced equation: Na2S2O3 + Cl2 → Na2SO4 + S + 2 NaCl Now we will balance the equation: Balanced equation: 4 Na2S2O3 + 8 Cl2 → 8 NaCl + 4 Na2SO4 + 8 S

Key concepts

Chemical Reactions

Chemical reactions are processes where substances, known as reactants, transform into different substances called products. They are characterized by changes in the properties of substances involved, such as color, temperature, solubility, and the formation of precipitates or gas. Each reaction follows the law of conservation of mass, which states that matter can neither be created nor destroyed within a chemical reaction, meaning the mass of the products must equal the mass of the reactants.

For instance, in the reaction where hydrogen selenide is formed by reacting an aqueous acid solution with aluminum selenide, the reactants undergo a chemical change to produce hydrogen selenide and aluminum. Understanding the types of reactants and the expected products helps in predicting the outcome of the reaction, which is crucial in writing a balanced chemical equation.

Reaction Stoichiometry

Reaction stoichiometry involves the quantitative relationships between reactants and products in a chemical reaction. It explains the ratio in which the reactants combine and the ratio in which the products form. When writing equations, stoichiometry provides the coefficients that balance the reaction, ensuring that the number of atoms for each element is the same on both sides of the equation.

In practice, the reaction between sodium thiosulfate and chlorine to produce sulfate ion, elemental sulfur, and chloride ion, for example, requires a precise stoichiometric ratio to ensure all chlorine is reacted and the correct amounts of products form as anticipated. Enforcing the concept of moles helps students better understand reaction stoichiometry, as it adds a tangible way to measure the amount of substances involved in chemical reactions.

Balancing Chemical Equations

Balancing chemical equations is a methodical process of adjusting coefficients to obey the law of conservation of mass. During this process, students often find it helpful to first balance elements that appear in the fewest compounds and leave hydrogen and oxygen for last, as they appear in multiple compounds.

For example, balancing the aluminum selenide reaction required ensuring that there are equal numbers of each type of atom on both sides of the equation. This involved multiplying the number of moles of some reactants and products to achieve balance. By practicing the skill of balancing equations, students increase their understanding of chemical reactions and the fixed proportions in which chemical compounds react.

Oxidation-Reduction Reactions

Oxidation-reduction reactions, or redox reactions, are a type of chemical reaction that involves the transfer of electrons between substances. Oxidation refers to the loss of electrons, whereas reduction refers to the gain of electrons. The substance that loses electrons is said to be oxidized, while the substance that gains electrons is reduced.

In the context of the chlorine reaction with sodium thiosulfate, chlorine gas is reduced to chloride ions because it gains electrons from the thiosulfate, which is oxidized as it loses electrons. One method of identifying redox reactions is by assigning oxidation numbers to each atom before and after the reaction to track the loss or gain of electrons. Recognizing redox reactions is essential for correctly balancing complex reactions, as both the mass and charge must balance overall.