[MPQC]

Well. I'm doing some equilibrium homework, and it wants me to write some equations:

a) The reaction between H⁺ & Cr₂O₄^2-, which produces Cr₂O₇^2-.

b) The reaction between OH^- and Cr₂O₇^2-, which produces CrO₄^2- & H₂O

c) The summation equation showing both H⁺ and OH^- in the equilibrium reaction between CrO₄^2- & Cr₂O₇^2-

Alright. Starting on a, I get this:

a) 2H⁺ + 2CrO₄^2-  H₂O + Cr₂O₇^2-

b) 2OH^- + Cr₂O₇^2-    2CrO₄^2- + H₂O

c) This is where I need some help. I'm really not sure what it means by summation equation. I tried adding the separate rows together, but all they do is all cancel out. I can't find anything in my texts that refers to summation equations. So what exactly do I do?

[MOTOBALL]

You essentially have 2 simultaneous eqns. from which you need to eliminate H2O.

You do this via    (a) - (b) not (a) + (b)

Also, note that your top eqn "(a) H+ & Cr2O4(2-) produces Cr2O7(2-)" is not correct, although you had it right in the bottom (a). 

Motoball

[MPQC]

Whoops, missed adding H₂O in the question. Nice catch. But how exactly do I subtract them properly? (I don't ever recall subtracting equations.)

If I "logically" subtract, I get this:

2H⁺ + 2CrO₄^2- - 2OH^- - Cr₂O₇^2- H₂O + Cr₂O₇^2- - 2CrO₄^2- - H₂O

Which the H₂O's cancel out, so the end equation is like this...

2H⁺ + 2CrO₄^2- - 2OH^- - Cr₂O₇^2- ::equil::Cr₂O₇^2- - 2CrO₄^2-

But now I've got some unbalanced elements, which are the H⁺. Do I just remove them?

[MOTOBALL]

Your last eqn. now needs to be slightly re-arranged

        2 H(+) + 2 CrO4(2-) - 2 OH(-) - Cr2O7(2-)  Cr2O7(2-) - 2 CrO4(2-)

becomes

       2 H(+) + 2 CrO4(2-) + 2 CrO4(2-)    Cr2O7(2-) + Cr2O7(2-) +  2 OH(-)

becomes

               H(+) +  2 CrO4(2-)     Cr2O7(2-) + OH(-)

Motoball

[MPQC]

Two more questions..

So while you're reorganizing, you're simply just switching their sides so that they're positive?

And.. How do I know which element I'm trying to get rid of?

[MOTOBALL]

(1) Yes/No.  You rearrange the eqns. to separate the two chemical entities that are in a state of eqlbm. In general terms, all of the A on the LHS and all of the B on The RHS.

(2) From eqn. (c) of your original post, by implication H₂O is not expected to be in the summation eqn.  You are not interested in the eqlbm. of H⁺ and OH^- with H₂O

Motoball