electrochemistry question?

Question

I am in ap chem and have the ap test on tuesday. In electrochem problems, standard cell potential is cathode-anode, right? So if they give you the problem with 2 half reactions both beng reduced, how do u know which one to flip and make the cathode?

and the problem on the 2003 ap test it gives the following info:
Br2 + 2e- --> 2Br- E=+1.065
BrO3 - + 6H+ ---> Br2 + 3H2O E=+1.52

so i thought E=cathode-anode=1.065-1.52, but the solution says 1.52-1.065. why is the second one the cathode?

Answer 1

These are two half reactions both being reduced

We need a redox reaction :

oxidizing agent : 2BrO3- + 12 H+ +10e- >> Br2+6H2O E = + 1.52 V ( highter number)

reducing agent 2Br- >> Br2 + 2e- E = - 1..065 (smaller number )

1.52 - 1.065 > 0 in a galvanic cell E >0

Answer 2

Turn round the one that is more negative, always.

Then its (more positive) - (more negative).