[jagger04]

Cr2O7^(-2) + Se ---> H2SeO3 +Cr^(+3)


basically it says to balance both the oxidation and reduction, but for some reason i can't get it. please explain with step by step directions..

thanks in advance..

[BetaAmyloid]

Are you sure this is all of the reaction?...It appears that a reactant is missing - the hydrogen.

 

[jagger04]

this is all that was given, if that is what you are asking.

[gregdwulet]

Does the reaction occur in an acid solution?
If so, then the hydrogen would come from the H⁺ ions.

[renge ishyo]

I'll give you a start. The first step is to break the initial question down into two half reactions. Choose the two species that look most similar to each other on both sides of the reaction to give:

Cr₂O₇^2-  Cr⁺³

Se  H₂SeO₃

Now balance each half reaction one at a time. I will do the first half reaction for you. First, balance the Cr atoms in the first equation.

Cr₂O₇^2-  2Cr⁺³

Next, balance the oxygens. You do this by adding H₂0 to the side of the equation that lacks oxygen.

Cr₂O₇^2-  2Cr⁺³ + 7H₂0

Next, balance the hydrogens by adding H⁺ to the left side (yes, this is an implied acidic solution because of the dichromate ion).

14H⁺ + Cr₂O₇^2-  2Cr⁺³ + 7H₂0

Finally, you have to balance the total charge on each side of the equation. There is a total charge of +12 on the left side of the equation (+14+ (-2) = +12) and a total charge of +6 on the right side of the equation (2*(+3) = +6). So add +6e^- to the left side of the equation so that the total charge on both sides will now be +6.

14H⁺ + Cr₂O₇^2- + 6e^- 2Cr⁺³ + 7H₂0

Please do the same thing to the second half reaction, and after that add the two equations together.