Hey guys!
For galvanic cells, does the mass loss of one electrode (anode) equal the mass gain of the other electrode (cathode)?
I assume no, but could someone explain why?
You are correct, it does not equal
UNLESS the species that are oxidised and reduced are the same thing. For example, take a copper electrolytic cell.
It has a copper anode and the cathode is the object to be plated and the electrolyte is a copper solution.
In this example let's say that 0.5 mole of copper is oxidised at the anode, it's safe to say that 0.5 mole of copper will be reduced at the cathode. Given that the molar masses are the same, the mass will be the same.
When the species differ, there are a number of things that makes the mass loss at the anode differ to the mass gained at the electrode.
One is the mole of electrons lost or gained by the species as stated by their respective half equations. Take for example a silver and copper battery.
Let's say that at the copper anode, 1 mole of copper is lost and two moles of electrons are 'formed.' Then at the silver half cell,
two moles of silver is reduced as a result. From this example, the mass loss at the anode is not the same as the mass gained at the cathode which we can deduce solely from the mole ratios.
Another reason why is the molar mass of the species. In a zinc and copper battery, 1 mole of zinc is oxidised and 1 mole of copper is reduced. To find the respective masses, you would have to times them by their molar mass which is 65.4 g/mol and 63.5 g/mol respectively. So of course the mass lost at the respective electrodes would differ.
Hope this helps!