Posted by max on Wednesday, May 16, 2012 at 12:17am.
First we note that we have 1 I on the left and 2 on the right. In order to compare apples with apples we need to have the same number; therefore, we stick a 2 on the left so we start with the same number of I atoms.
2I^- ==>I2
So we have changed from -2 (for two I atoms) to zero (for two I atoms). That is a loss of two electrons.
2I^- ==> I2 + 2e
That first step is almost never addressed in redox instructions and it is the difference between success and failure. It is common to do this with I^- ==> I2, Cr2O7^2- ==> Cr^3+, Cl2 ==> Cl^-, S2O4^2- ==> S4O6^2-, C2O4^2- ==> CO2, etc.
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