QUESTION 140 (7 marks)
A nickel coating is required on 40.0 cm² of a metal object. Its uniform thickness must be 50.0 μm. Assume only Ni²⁺ reduction occurs at the object and current efficiency is 100%. Use density(Ni) = 8.90 g cm⁻³, M(Ni) = 58.69 g mol⁻¹ and F = 96485 C mol⁻¹. The current is 2.00 A. A pure nickel anode is used.
(a) Calculate the mass of nickel required. Use 1 μm = 10⁻⁴ cm. [2 marks]
(b) Calculate the plating time in minutes. [3 marks]
(c) Predict the ideal change in Ni²⁺ concentration and explain using the anode and cathode reactions. [2 marks]
Practice marking scheme
Answer
Required mass = 1.78 g; plating time = 48.8 min. Ni²⁺ concentration remains approximately constant as nickel dissolves at the anode at the same molar rate as it deposits at the cathode.
Working
Thickness is cm, so the coating volume is cm³ and mass is g. Nickel amount is mol, requiring coulombs. Time is s, or 48.8 min. The half-equations and replenish and consume the same ion amount.
Marking criteria
- Converts thickness to 0.00500 cm and calculates coating volume 0.200 cm³. [1 mark]
- Calculates nickel mass 1.78 g. [1 mark]
- Calculates nickel amount from mass and molar mass. [1 mark]
- Uses two electrons per Ni and q = n(e⁻)F. [1 mark]
- Calculates time ≈ 48.8 min using the current. [1 mark]
- Writes correct nickel oxidation and reduction half-equations. [1 mark]
- Explains approximately constant Ni²⁺ concentration by equal production and consumption. [1 mark]
Practice question aligned to the current QCAA syllabus; review the worked solution and marking criteria.
View the QCAA syllabusCompare your working with the guide above.