A chemical engineer designs a pilot plant to determine the conditions that will give the best results for copper plating different objects - VCE - SSCE Chemistry - Question 8 - 2011 - Paper 1

The presence of excess potassium cyanide, KCN, helps maintain a low concentration of Cu+(aq) ions in solution by shifting the equilibrium of the reaction:

ightleftharpoons Cu(CN)_2(aq)$$ Excess CN− ions push the position of equilibrium to the right, thereby decreasing the concentrations of free Cu+( ions. This is due to Le Chatelier's principle, which predicts that adding more reactants will shift the equilibrium to favor product formation.

The gas most likely produced at the cathode when the current is too high is hydrogen gas from the reduction of water. The balanced equation for this reaction is:

$2H_2O(l) + 2e^- → H_2(g) + 2OH^−(aq)$

First, calculate the mass of copper deposited:

Mass of copper deposited = Mass after plating - Mass before plating = 36.4 g - 25.2 g = 11.2 g

Now convert grams to moles using the molar mass of copper (Cu = 63.55 g/mol):

Number of moles, n(Cu) = (\frac{11.2 \text{ g}}{63.55 \text{ g/mol}} = 0.176 \text{ mol})

Using Faraday's laws of electrolysis, the charge required (Q) to deposit this mass can be calculated:

Q = n(F), where F = Faraday's constant (96500 C/mol)

Q = 0.176 mol × 96500 C/mol = 16940 C

Next, using the formula for time (t):

$t = \frac{Q}{I}$

t = (\frac{16940 C}{0.900 A} = 18822.22) seconds

Finally, convert seconds to minutes:

t in minutes = (\frac{18822.22}{60} ≈ 313.7) minutes.