Try it on paper first. A hint, the key idea and the worked solution are here when you need them.
A cell is made of two zinc rods. One dips into zinc sulphate solution and the other into zinc sulphate solution. The solutions are joined by a salt bridge. (a) Which rod is the anode, and in which direction do the electrons flow in the wire? (b) What is the emf at ? (c) What happens to the two concentrations as the cell runs, and when does it stop?
A concentration cell has two identical electrodes in solutions of the same ion at concentrations . and at . The dilute side is the anode. A saturated solution of a salt gives : for , , and for , . , with .
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Given. The two concentrations are and , and zinc ions have .
Step 1. The anode. The dilute side makes more ions: . The rod is the anode. Electrons go through the wire from that rod to the rod.
Step 2. The ratio. , and .
Step 3. The emf. .
Step 4. Running down. The dilute solution gains and the strong one loses it. The ratio falls towards 1. The cell stops when both solutions have the same concentration, and then .
Answer: the dilute rod is the anode and the emf is .
Check. The emf is a few hundredths of a volt, as it should be for a ratio of 60 and .
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