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Which is a redox reaction?

A2KBr + F2 ---> 2KF + Br2
B2HCL + Mg(OH)2 --> 2HOH + MgCl2
C2NaCl + H2SO4 --> Na2SO4 + 2HCl
DCa(OH)2 + Pb(NO3)2 --> Ca(NO3)2 + Pb(OH)2

I know the answer is A, but confused as to why that is. Can someone explain this for me? Thanks.

Verified Answer :

You have to look at the oxidation states of each atom in the reaction.  Since redox reactions involve the transfer of electrons from one element to another, 2 elements need to have oxidation state changes.  
option a is the only option that has a change in oxidation sates.  Fluorine goes from 0 to -1 indicating that it gained electrons (got reduced) while bromine goes from -1 to 0 indicating it loses electrons (got oxidized).
(2Br⁻→Br₂+2e⁻ and F₂+2e⁻→2F⁻).

I hope this helps.  Let me know if anything is unclear.

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