Chemistry How does this example of electroplating tungsten onto base metal work?

[zenterix]

Homework Statement

A jeweler is investigating a novel method for electroplating tungsten onto base metal.

The jeweler passes a 30A current through a solution for 1 hour and 100g of tungsten is deposited on the ring.

Relevant Equations

What is the oxidation number of tungsten in the solution.

From the molar mass of tungsten of 183.84 g/mol, we know that $100/183.84\approx 0.55$ mol was deposited.

We also know how many mols of electrons passed through the solution in an hour.

This comes out to about 1.12 mol of electrons.

So, at a very superficial level of understanding, I concluded that you need about two electrons per atom of tungsten.

The answer to the problem is that the oxidation number is two.

I'd like to understand this problem a bit better.

To electroplate tungsten onto a base metal, it seems we could have an electrochemical cell where the cathode is a base metal plus tungsten ions in solution.

This doesn't seem to be what is happening in the problem however.

What does it mean that the jeweler passes current through a solution in this example?

 

[QuarkyMeson]

The jeweler is simply passing a current from an anode (probably something rather inert) to the cathode (the ring) through some solution (something novel containing tungsten ions) and tungsten ions in solution are being reduced at the cathode (being driven by the electrical field to the cathode).

Here is a real life patent from the 50's for tungsten plating: old patent

 

[Borek]

This is just Faradays law of electrolysis at its simplest, don't overdo.

And yes, there is a cell present, not sure why you think otherwise. Before the electrodeposition starts there is no W on the surface and you can argue initially there is no typical Me/Meⁿ⁺ cell, but simple models don't work at such border cases. The only thing that is not simple to determine using basic cell model is the initial electrodeposition potential, reduction as the process of adding electrons works perfectly OK.

 

[zenterix]

I've never heard of Faraday's law of electrolysis.

The jeweler is simply passing a current from an anode (probably something rather inert) to the cathode (the ring) through some solution (something novel containing tungsten ions) and tungsten ions in solution are being reduced at the cathode (being driven by the electrical field to the cathode).

Here is a real life patent from the 50's for tungsten plating: old patent

So, where does the electric field generating the current come from?

In other words, where does the emf come from?

In a simple electrochemical cell, we have a closed circuit formed by a wire connecting the electrodes and a salt bridge connnecting the solutions. The emf is generated by chemical reactions.

Please realize that though this may be familiar to you it is not to me.

You say a current is passed from an anode to a cathode through a solution. Is there a connecting wire in this setup you speak of?

 

[QuarkyMeson]

The emf comes from either an external current or voltage source, almost always a voltage source, i.e. a battery. There are wires connecting the external power source, with the positive terminal connected to the anode and the return connected to the cathode via wire. The solution itself then completes the circuit.

Here.

 

[Borek]

Google for the difference between electrolytic and galvanic cells.