AP Chemistry 9.11: Faraday’s Law

Practice AP Chemistry questions on calculating charge flow, current, time, deposited mass, and electron transfer in electrochemical cells.

Exam points

Relate current, charge, and time using I = q/t in an electrochemical cell.
Use redox stoichiometry and Faraday’s constant to calculate electron flow from mass or ionic charge.
Calculate deposited electrode mass or electrolysis time from charge, current, and electron transfer.

[Maximum number: 2]

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 $\mathrm{Ag}_{2} \mathrm{~S}(s)$ when it reacts with hydrogen sulfide, as represented by the following equation.

2 \mathrm{Ag}(s)+\mathrm{H}_{2} \mathrm{~S}(g) \rightarrow \mathrm{Ag}_{2} \mathrm{~S}(s)+\mathrm{H}_{2}(g)

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.

For the correct calculated value of moles of electrons (may be implicit): 1 point

2.8 \mathrm{~g} \mathrm{Rh} \times \frac{1 \mathrm{~mol} \mathrm{Rh}}{102.9 \mathrm{~g} \mathrm{Rh}} \times \frac{3 \mathrm{~mol} e^{-}}{1 \mathrm{~mol} \mathrm{Rh}}=0.082 \mathrm{~mol} e^{-}

For the correct calculated value of time: 1 point

0.082 \mathrm{~mol} e^{-} \times \frac{96,485 \mathrm{C}}{1 \mathrm{~mol} e^{-}} \times \frac{1 \text { second }}{2.0 \mathrm{C}}=3900 \text { seconds }

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9.11.A—Calculate the amount of charge flow based on changes in the amounts of reactants and products in an… question 1