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

Excess potassium cyanide (KCN) pushes the position of equilibrium to the right, which decreases the concentration of Cu^(aq) ions in solution. The reaction between Cu⁺ ions and CN^- ions forms the complex ion Cu(CN)₄^3- effectively removing Cu^(aq) and maintaining a lower concentration of free copper ions.

The balanced equation for the gas most likely produced at high current conditions at the cathode is:

$2H_2O(l) \rightarrow H_2(g) + 2OH^-(aq)$

To determine the time taken for copper plating, use the formula:

1. Calculate the mass of copper deposited: Mass deposited = Mass after - Mass before = 36.4 g - 25.2 g = 11.2 g

2. Convert the mass of copper to moles: Molar mass of Cu = 63.55 g/mol Moles of Cu = ( \frac{11.2 g}{63.55 g/mol} = 0.176 moles )

3. Use Faraday's law to find time:

   The total charge required = n × F, where n is the moles of electrons and F is Faraday’s constant (96485 C/mol). The reaction shows that 2 moles of electrons are required to deposit 1 mole of Cu, so: Moles of electrons = 2 × 0.176 = 0.352 moles Total charge required = 0.352 moles × 96485 C/mol = 33959.92 C

4. Use the current to find time: Time (in seconds) = Total charge / Current = 33959.92 C / 0.900 A = 37622.13 s

5. Convert seconds to minutes: Time (in minutes) = ( \frac{37622.13}{60} \approx 626.87 \text{ minutes} )

Thus, time taken to copper plate the medal is approximately 627 minutes.