Chemistry

hi there
when i ad KI to BR, why does the Br2 become 2Br- and the 2I- I2?
(sry for the bad english)

He is asking why the reaction occurs.

What you are describing is a redox (reduction-oxidation) reaction.  The Bromine is being reduced by the iodide; (you can see it goes from atomic to ionic, so it's charge is being reduced).  The two electrons it takes to reduce the two bromines have to come from somewhere...you guessed it, they come from the I-.  The iodide ions lose their electrons and revert to the atomic species.



Take a look at this.  These are the two half reactions at standard state (1M, standard temp, pressure).  The reaction potential in voltage indicates how favorable the reaction is (i.e., the greater change in free energy.  Higher voltage = more favorable)

(1) Br2 + 2e- ---> 2Br-     +1.07V
(2) I2 + 2e- ---> 2I-     +0.54V

We can reverse the second equation:

(1) Br2 + 2e- ---> 2Br-     +1.07V
(-2) 2I- ---> I2 + 2e-     -0.54V
----------------------------------------
2I- + Br2 --> 2Br- + I2  +0.53V

and add (1) and (-2) together to get your equation.  We also add the reaction potentials together as well.  Since this new reaction potential is positive, the forward reaction is favorable compared to the reverse.