How to write equation given compounds?

[Sammy1024]

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I was doing the following problem:

31. Which of the following is the ratio of Na2Cr2O7(aq) to Fe3+(aq) in the titration of Flask 2, where Cr2O72-(aq) is reduced into Cr2O3(s)?

And I wasn't sure on how to exactly go about writing the equation.

The answer says: To balance the reaction, first, you should write out the unbalanced equation:
Fe2+ + Na2Cr2O7 <--> Fe3+ +Cr2O3

From here, the first step is to balance the electrons in the two half-reactions. The ferrous (+2) cation loses one electron, while the dichromate gains three electrons per chromium (and thus six electrons per compound). To balance the electrons, the ferrous and ferric (+3) cations must both be multiplied by a factor of six.

6Fe2+ +1Na2Cr2O7 -> 6Fe3+ +1Cr2O3

But looking at the answer the unbalanced equation makes sense, but I don't understand the balanced portion. At the time of the question, I wasn't sure about if they separate, or how I would balance the charges, which the answer says is by adding H+'s to each side. Any suggestions?

 

[chenzt]

But looking at the answer the unbalanced equation makes sense, but I don't understand the balanced portion. At the time of the question, I wasn't sure about if they separate, or how I would balance the charges, which the answer says is by adding H+'s to each side. Any suggestions?

You add H+ to the left side and H2O to the right side to balance the hydrogen and oxygen.

8H+ 6Fe2+ +1Na2Cr2O7 -> 6Fe3+ +1Cr2O3 + 4H2O + 2Na+

Now it is balanced, the book omitted this portion because it is irrelevant to the question.

 

[Sammy1024]

How do you know to add those things? I know there's unbalanced number of Os that can't be balanced without adding something, but how do you know to add H and H2O (specifically)?

 

[chenzt]

First, count the number of oxygen. I noticed there's four missing on the right hand side. So I added four water. That means 8 hydrogen is missing on the left hand side. Then you tally up everything, notice that two sodium is missing on the right, and you're done.

 

[Sammy1024]

Oh I get it! So in a way H and H2O are the "neutral" species that won't alter the experiment, just balance everything out.

 

[zacharyw]

They aren't neutral. Count the charge of the equation before adding the H+ and H2O and then count it again after. Notice how H+ is providing the charge balance to the equation as well?

 

[Sammy1024]

Oh I understand that. I meant neutral like you can add them to balance the charges, add elements (like O) but they're not interfering with the original equation. They're just kinda there. Like how if you add HCl it messes with the reaction, but something like H or H2O won't.

 

[TheJourney]

Can someone explain how they figured they have to multiply by a factor of 6 in order to balance the charges or equation?

 

[Sammy1024]

Because Fe3+ x 2 is equivalent to Fe2+ x3

3 x 2 = 6

 

[chenzt]

Can someone explain how they figured they have to multiply by a factor of 6 in order to balance the charges or equation?

Because Fe3+ x 2 is equivalent to Fe2+ x3

3 x 2 = 6

Not really.

Fe2+ + Na2Cr2O7 -> Fe3+ +Cr2O3

Chromium on the left side has a charge of +6 (+12 for both) and on the right has a charge of +3 (+6 for both). So basically chromium gained six electrons from the reaction. Something else needs to give up six electrons to make it balanced. Iron in this case will give up one electron per atom, so you need six total.

6Fe2+ +1Na2Cr2O7 -> 6Fe3+ +1Cr2O3