Problem 127 - Chemistry The Science in Context

Question

Polishing Silver Silver tarnish is the result of silver metal reacting with sulfur compounds, such as \(\mathrm{H}_{2} \mathrm{S}\), in the air. The tarnish on silverware \(\left(\mathrm{Ag}_{2} \mathrm{S}\right)\) can be removed by soaking in a solution of \(\mathrm{NaHCO}_{3}\) (baking soda) in a basin lined with aluminum foil. a. Write a balanced equation for the tarnishing of Ag to \(\mathrm{Ag}_{2} \mathrm{S},\) and assign oxidation numbers to the reactants and products. How many electrons are transferred per mole of silver? b. Write a balanced equation for the reaction of \(\mathrm{Ag}_{2} \mathrm{S}\) with Al metal, NaHCO \(_{3},\) and water to produce Al(OH) \(_{3}\) \(\mathrm{H}_{2} \mathrm{S}, \mathrm{H}_{2},\) and \(\mathrm{Ag}\) metal.

Step-by-Step Solution

Answer

Answer: The balanced equation for the reaction of tarnished silver (Ag2S) with aluminum (Al), sodium bicarbonate (NaHCO3), and water (H2O) is: 3 Ag2S + 6 Al + 6 NaHCO3 + 18 H2O → 6 Al(OH)3 + 6 H2S + 6 H2 + 6 Ag In this process, one mole of silver (Ag) loses one mole of electrons when it tarnishes to form silver sulfide (Ag2S).

Step 1: Write a balanced equation for silver tarnishing

To write a balanced equation, first, identify the reactants and products in the reaction. In this case, the reactants are silver (Ag) and hydrogen sulfide (H2S). The product is silver sulfide (Ag2S). Now balance the equation by adjusting the number of atoms of each element on both sides. The balanced equation is: 2 Ag + H2S → Ag2S + H2

Step 2: Assign oxidation numbers

Assign oxidation numbers to each atom in the reactants and products of the reaction: For Ag: 0 (in an elemental state) For S in H2S: -2 (combined with two H atoms) For H in H2S: +1 (combined with an S atom) In the product side (Ag2S): For Ag: +1 (combined with an S atom) For S in Ag2S: -2 (combined with two Ag atoms)

Step 3: Calculate the number of electrons transferred

According to the oxidation numbers, we can see that silver (Ag) has been oxidized from 0 to +1. So, each Ag atom loses one electron in the process. For one mole of silver (Ag), one mole of electrons will be transferred.

Step 4: Write a balanced equation for the reaction of Ag2S with Al, NaHCO3, and water

The reaction of silver sulfide (Ag2S), aluminum (Al), sodium bicarbonate (NaHCO3), and water (H2O) produces aluminum hydroxide (Al(OH)3), hydrogen sulfide (H2S), hydrogen gas (H2), and silver metal (Ag). Balance the equation by adjusting the number of atoms of each element on both sides. The balanced equation is: 3 Ag2S + 6 Al + 6 NaHCO3 + 18 H2O → 6 Al(OH)3 + 6 H2S + 6 H2 + 6 Ag

Key Concepts

Chemical Reactions

Understanding chemical reactions is key when exploring how silver tarnish is removed. A chemical reaction involves converting reactants into products through rearrangement of atoms. In the context of silver tarnishing, silver (Ag) reacts with hydrogen sulfide (\(\text{H}_2\text{S}\)), a compound found in air, to form silver sulfide (\(\text{Ag}_2\text{S}\)). This formation of tarnish is a chemical change.
To reverse the tarnish, silver sulfide (\(\text{Ag}_2\text{S}\)) is soaked with aluminum foil in a baking soda solution. The reaction of silver sulfide with aluminum (\(\text{Al}\)), water, and baking soda (\(\text{NaHCO}_3\)) can restore the silver's shiny appearance. Chemical reactions like these demonstrate how substances can undergo transformations to produce different substances with distinct properties.

Oxidation Numbers

To understand chemical reactions fully, assigning oxidation numbers is essential. Oxidation numbers help in determining how electrons are distributed among atoms in a molecule. In pure elements, like silver before tarnishing, the oxidation number is 0. In silver sulfide (\(\text{Ag}_2\text{S}\)), the silver atoms are bonded to sulfur, changing the oxidation number of silver to +1 and that of sulfur to -2.
Here's a quick guide:

• Free elements: oxidation number is 0.
• For \(\text{H}_2\text{S}\), sulfur has an oxidation number of -2 and hydrogen +1.
• In compounds, the sum of oxidation numbers equals the overall charge of the molecule.

Knowing these numbers helps identify which atoms lose or gain electrons during the reaction.

Electron Transfer

Electron transfer is at the heart of oxidation-reduction (redox) reactions, like the tarnishing of silver. When silver reacts with sulfur in hydrogen sulfide, it loses electrons and is oxidized. Specifically, each silver atom loses one electron, which shifts its oxidation state from 0 to +1.
When silver tarnish (\(\text{Ag}_2\text{S}\)) is removed, a redox reaction occurs again. In this process, aluminum acts as a reducing agent, transferring electrons back to silver, which gets reduced to its metallic form as sulfides in the compound undergo changes. Understanding how electrons are exchanged is crucial in these chemical transformations.

Balancing Equations

Balancing chemical equations ensures that the same number of each type of atom appears on both sides of the reaction, reflecting the conservation of mass. This process is an essential skill in chemistry.
For the tarnishing of silver:

• Reactants: 2 moles of \(\text{Ag}\) and 1 mole of \(\text{H}_2\text{S}\)
• Products: 1 mole of \(\text{Ag}_2\text{S}\) and 1 mole of \(\text{H}_2\)

Balancing involves adjusting coefficients to ensure each atom is accounted for.
Similarly, when reversing the tarnishing:

• Reactants: 3 moles of \(\text{Ag}_2\text{S}\), 6 moles of \(\text{Al}\), 6 moles of \(\text{NaHCO}_3\)
• Products: 6 moles of \(\text{Al(OH)}_3\), 6 moles of \(\text{H}_2\text{S}\), 6 moles of \(\text{H}_2\), and 6 moles of \(\text{Ag}\)

Balancing helps better understand the stoichiometry of reactions, crucial for working in labs or real-world scenarios.