[ygao85]

When we are given a bunch of reaction how do we know which one is a redox reaction? Is there any trick that can tell you right away that a reaction is a redox rxn.

Also why is this eqn not a redox rxn:
Cr₂O₇^2-_(aq)+2OH^-_(aq)-->2CrO₄^2-_(aq)+H₂O_(l).

Thank you.

[xiankai]

one quick way to determine is see if there is any change in valency /oxidation number in anyone of the reactants.

your eqn is not redox becos Cr2O7 2- has chromium in 6+ oxidation state, but CrO4 2- also has chromium in 6+ oxidation state.

same goes for hydroxide. oxygen and hydrogen have no changes in valency/oxidation state.