A manufacturer of brass musical-instrument valves uses electroplating to give parts a hard nickel surface. Fig. 1 shows one valve connected in a plating tan...
Question 1 Report
A manufacturer of brass musical-instrument valves uses electroplating to give parts a hard nickel surface. Fig. 1 shows one valve connected in a plating tank. The electrolyte is nickel sulfate solution. The operator records the mass of a valve before and after plating at different currents for the same time. The coating must be even, so each valve is cleaned with detergent and rinsed with water before it enters the tank.
| Current / A | Plating time / min | Mass before plating / g | Mass after plating / g |
|---|---|---|---|
| 0.40 | 15 | 18.62 | 18.74 |
| 0.80 | 15 | 18.59 | 18.83 |
| 1.20 | 15 | 18.65 | 18.98 |
(a) Give the name of the electrode that the brass valve is connected to. [1]
(b) Give the name of the electrode made from nickel metal. [1]
(c) Describe the movement of nickel ions and electrons that produces a nickel coating on the valve. [3]
(d) Use Table 1 to calculate the mass of nickel deposited on the valve at a current of 0.80 A. [2]
(e) Use Table 1 to describe the relationship between current and mass deposited. [2]
(f) Describe why the valve is cleaned before electroplating. [2]
(g) Use a half-equation to show the reaction at the nickel electrode. [2]
(h) Give two changes that would increase the mass of nickel deposited without changing the metal compound in the solution. [3]
Answer Details
(a) The brass valve is the cathode, or negative electrode [1]. Nickel ions are reduced onto it.
(b) The nickel-metal electrode is the anode, or positive electrode [1].
(c) Nickel ions move towards the negative valve [1]. They gain electrons [1], forming nickel atoms that coat the valve [1].
(d) \[18.83\text{ g}-18.59\text{ g}=0.24\text{ g}\] The deposited nickel mass is 0.24 g [2].
(e) Mass deposited increases as current increases [1]. In 15 minutes, the gains are 0.12 g, 0.24 g and 0.33 g, showing approximate direct proportionality [1].
(f) Cleaning removes grease, dirt or oxide [1]. This gives good electrical contact and allows nickel to adhere evenly [1].
(g) At the nickel anode:
\[\mathrm{Ni(s)\rightarrow Ni^{2+}(aq)+2e^-}\] [2]
(h) Two valid changes are to increase the current and increase the plating time [maximum 3]. Increasing nickel sulfate concentration or using a suitable higher controlled temperature are also acceptable.
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