3. Sterling silver is an alloy that is commonly used to make jewelry and consists of 92.5% silver an — Alloys, Atomic Structure, Stoichiometry, Electrochemistry Chemistry Question
Problem Context
3. Sterling silver is an alloy that is commonly used to make jewelry and consists of 92.5% silver and 7.5% other metals, such as copper, by mass. Over time, the alloy can form a tarnish of Ag2S(s) when it reacts with hydrogen sulfide, as represented by the following equation.
2 Ag(s) + H2S(g) → Ag2S(s) + H2(g)
(a) What are the oxidation numbers of silver in Ag(s) and Ag2S(s)?
Ag(s) ___________
Ag2S(s) ___________
Model Answer
Ag(s) 0 Ag2S(s) +1 [1].
The following table contains the atomic radii for silver and copper.
(i) Explain why sterling silver is better classified as a substitutional alloy than as an interstitial alloy.
Model Answer
Silver and copper have similar radii, so the alloy would be
substitutional versus interstitial [2].
(ii) Using principles of atomic structure and Coulomb’s law, explain why silver has a larger atomic radius than copper does.
Model Answer
Silver has more occupied electron shells (n = 5) than copper (n =
4); the electrons in the fifth shell experience weaker Coulombic attractions and
are farther away from the nucleus [2].
The Ag2S tarnish on sterling silver can be removed until only sterling silver remains. A student weighs a tarnished sterling silver sample both before and after removing the Ag2S(s) (molar mass 247.80 g / mol) and records the data in the following table.
(c) Assuming that only Ag2S(s) is removed, calculate the number of moles of silver atoms removed.
Model Answer
409.21 g - 398.94 g = 10.27 g, and 10.27 g × (1 mol Ag2S / 247.80 g
Ag2S) × (2 mol Ag / 1 mol Ag2S) = 0.08289 mol Ag [2, 3].
Rhodium plating is a process used to protect sterling silver from tarnishing. This involves electroplating (depositing) solid rhodium, Rh(s), onto the surface of the metal from an acidified solution of Rh2(SO4)3(aq). Oxygen gas is produced during this process.
(d) A table of half-reactions related to the overall reaction is provided.
(i) Write the balanced net ionic equation for plating Rh(s) from the acidified Rh2(SO4)3(aq) solution.
Model Answer
4 Rh3+(aq) + 6 H2O(l) → 4 Rh(s) + 3 O2(g) + 12 H+(aq) [3].
(ii) Calculate the value of E°cell for the reaction in part (d)(i).
Model Answer
E°cell = +0.80 V - 1.23 V = -0.43 V [3].
(iii) Based on your answer to part (d)(ii), explain why this process requires the use of an external power source.
Model Answer
E°cell is negative, which means the reaction is not
thermodynamically favorable [3].
(e) Calculate the length of time, in seconds, required to plate 2.8 g of Rh(s) onto a piece of sterling silver if 2.0 C / s of current is applied.
Model Answer
2.8 g Rh × (1 mol Rh / 102.9 g Rh) × (3 mol e- / 1 mol Rh) = 0.082 mol
e-, and 0.082 mol e- × (96,485 C / 1 mol e-) × (1 second / 2.0 C) = 3900 seconds
[4].
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