[samuraix]

 Our chemistry labouratory instructor did experiment which is ahead of our syllabus. So , some of the experiment i did not quite understand . Hope you can help me with it.
Post-labouratory question:
1. Ni-Cad batteries are rechargeable and are commonly used in cordless appliances . Although such batteries actually function under basic condition , imagine an electrochemical cell using the setup below:
                      The left side of the beaker containing solution of 1M Cd2+ ion  and Cd electrode. The right side of beaker containing soluion of 1M Ni2+ ion and Ni electrode. Both beaker is connected with a salt bridge. Both electrode is connected with an external wire with voltmeter.
                       The equation and Eo:
                       Cd2+ (aq) + 2e become Cd(s)     Eo=-0.40 V
                        Ni2+ (aq) + 2e become Ni (s)    Eo=-0.25V
 
a) What is Eo for the cell?
b)Which metal is the cathode?
c)Describe the flow of electrons and NO3- ions ( from the salt bridge) in the cell.
d) If 0.050 A are drawn from the battery , how long (in hour) can it last if you begin with 100 mL of each of the solutions and each was initially 0.10M in dissolved species ? The electrodes each weigh 50.0 g in the beginning.
 
2. You are given a sample of an unknown metal that can be oxidized by one electron. You were told to determine the element. What could be a possible means of determining the element you have?
 
 
The part i most didn't understand is question 1b) . As both of the ion receive 2 electrons , how can i determine which metal is cathode?
Really thank you if you can solve this problem.I very appreciate.

[Borek]

The part i most didn't understand is question 1b) . As both of the ion receive 2 electrons , how can i determine which metal is cathode?

Just because standard potential is given for a reduction reaction doesn't mean both metals will be reduced. One will be oxidized, one will be reduced - you have to reverse one of the reactions.

[Sev]

Borek is right, oxidation/reduction is always coupled.
To determine which metal is reduced, look at the reduction potentials.  Ni has a higher red. potential so it will be reduced.

The emf is the difference in the standard red. potentials: E(red.) - E(ox.).  A spontaneous process would have emf>0, so E(red.) > E(ox.)

Reduction ocurrs at the cathode, oxidation at the anode.