Finding the mass of nickel and chromium?

Question

Electroplating is a widely used process. One of the most popular electroplating processes involves the plating of two metals, nickel and chromium, in separate vats. Nickel is plated first because it adheres better to iron, then the object is moved to a second vat where a layer of chromium is plated. Half-reactions occurring in each vat are shown below:

Plating of Nickel : Ni²⁺_(aq) + 2 e^- → Ni_(s)

Plating of Chromium: Cr₂O₇^2-_(aq) + 14 H⁺_(aq) + 12 e^- → 2 Cr_(s) + 7 H₂O_(l)

The power supply used in commercial electroplating provides each vat with approximately 1.50 x 10³ A of current for 30.0 minutes. In which vat is the greatest mass of metal deposited on a car bumper? Justify your answer by calculating the mass of each metal that would be deposited.

a) What mass of nickel would be deposited? Give your answer to the nearest gram.

 

b) What mass of chromium would be deposited? Give your answer to the nearest gram.

Answer 1

From the balanced redox equations, and using Faradays Law we can calculate the mass of metal deposited in a given amount of time.

1 amp = 1 coulomb(C) per sec

1.5x10³ amps = 1.5x10³ C/sec

1.5x10³ C/sec x 60 sec/min x 30 min = 2.7x10⁶ C of charge applied

For Ni:

96485 C/mole e- x 2 mol e- = 192,970 C = 1 mole Ni

192,970 C/mol = 2.7x10⁶ C/x mol and x = 13.99 moles Ni x 59 g/mol = 826 g Ni

For Cr:

96485 C/mole e- x 12 mol e- = 1.16x10⁶ C = 2 mol Cr

1.16x10⁶ C/2 mol = 2.7x10⁶ C/x mol and x = 4.7 moles Cr x 52 g/mol = 244 g Cr