Why the Copper Deposit Is Too Light
A student electroplates a brass spoon with copper using a copper anode and a copper(II) sulphate solution. The spoon is cleaned, weighed (24.80 g), and made the cathode. A current of 0.50 A is passed for 25.0 minutes. The spoon is removed, rinsed with distilled water, dried with propanone, and reweighed; its mass is 25.02 g. The student expects a deposit of about 0.25 g from the calculation and is surprised that the mass gain is lower. The solution is still blue at the end. The student used a direct current supply set to 0.50 A and assumed that all the charge went into depositing copper. (1 faraday = 96 500 C mol⁻¹; Cu = 63.5.)
1. Calculate the theoretical mass of copper that should be deposited and compare it with the actual mass gain.
Read the worked answer
Charge passed:
Q = I × t = 0.50 A × (25.0 × 60 s) = 0.50 × 1500 = 750 C
Moles of electrons:
n(e⁻) = 750 C ÷ 96 500 C mol⁻¹ = 7.77 × 10⁻³ mol
Cathode reaction:
Cu²⁺(aq) + 2e⁻ → Cu(s)
n(Cu) = n(e⁻) ÷ 2 = 3.89 × 10⁻³ mol
mass(Cu) = 3.89 × 10⁻³ mol × 63.5 g mol⁻¹ = 0.247 g
Actual mass gain:
25.02 g − 24.80 g = 0.22 g
The theoretical mass is about 0.247 g, so the actual deposit is about 89% of the expected value. The calculation confirms the student's expectation of about 0.25 g; the mass gain is genuinely lower.
2. Give two reasons why the actual mass gain is lower than the theoretical value.
Read the worked answer
The current efficiency is less than 100%. Some of the electrical charge may be used in a side reaction at the cathode, such as reduction of H⁺ to hydrogen gas, especially if the copper(II) concentration near the cathode falls. That charge does not deposit copper.
Some copper may be lost during handling. The deposit may be powdery or loosely attached if the current density is too high; some can fall off or be washed away during rinsing. Incomplete plating of the back of the spoon, or a poor connection, would also reduce the mass gain.
A further possibility is that not all the spoon surface was immersed or electrically connected, so the effective cathode area was smaller than assumed.
3. Explain why a copper anode is used rather than an inert carbon anode, and state what would happen to the blue colour and the copper(II) ion concentration if a carbon anode were used.
Read the worked answer
With a copper anode, the anode reaction is:
Cu(s) → Cu²⁺(aq) + 2e⁻
Copper dissolves from the anode at the same rate as copper is deposited at the cathode. The copper(II) ion concentration in the solution therefore stays roughly constant, and the blue colour remains steady. The anode also replaces the copper used, so the process can continue.
With an inert carbon anode, copper cannot dissolve. The anode reaction becomes the oxidation of water:
2H₂O(l) → O₂(g) + 4H⁺(aq) + 4e⁻
Copper(II) ions are removed from the solution at the cathode but not replaced. The blue colour would fade, the copper(II) ion concentration would fall, and the solution would become more acidic.