# Electrochemistry question

1. Feb 28, 2005

### josephcollins

Hi people, I have the following question. Fe2+, Fe3+, Cr2O7(2-) and Cr3+ are contained in a solution in standard states with dilute sulphuric acid. Deduce what happens. I am wondering here what effect the sulphuric acid has, is it simply a catalyst for the reaction in which the Cr2O72- is reduced to Cr3+ and the Fe2+ is oxidised to Fe3+? Could someone check this for me, thx. Joe

2. Feb 28, 2005

### The Bob

$$Fe^{2+} _{(aq)} + Fe^{3+} _{(aq)} + Cr_2 O_7 _{(aq)}^{2-} + Cr^{3+} _{(aq)} + H_2 SO_4 _{(aq)}$$

By the looks of it, I would say the sulphuric acid is a catalyst because all the other substances are ions and the sulphuric acid is not so is not really in a state to be used. If this is the case then I can see nothing wrong with your redox statements.

However if it was not used as a catalyst, I think the sulpate ion from the sulphuric acid would bond with the $$Fe^{2+}$$ I haven't got time to see if this will change the oxidation states because I have to be out of the house in -3 minutes.

Hope that might have helped. Gurus, enlighten us

3. Feb 28, 2005

### Staff: Mentor

H2SO4 in aqueous solution is dissociated as well. So you have made a mistake at the beginning of your reasoning...

Here goes the enlightenement:

Sulphuric acid server two purposes.

First, it is necessary in the solution containing Fe3+ to ensure pH is low enough to disallow precipitation of Fe(OH)3.

Second, in order for the Cr2O72- ions to work as oxidizer H+ ions are necessary (so they are not catalysing reaction, they are a substrate), as the half reaction is

$$Cr_2O_7^{2-} + 14H^+ +6e^- \rightarrow 2Cr^{3+} + 7H_2O$$

Fe2+ ions are oxidized to Fe3+.

4. Feb 28, 2005

### The Bob

I realised what I had implied when I turned my PC off but I didn't have time to turn it on, change it and get to college on time.

I do apologise.

5. Feb 28, 2005

### GCT

Simply balance the redox reaction equation, your gen chem text should teach you how to do this, it's very simple.

6. Feb 28, 2005

### GCT

Overall, it may be that the addition of the acid is necessary to increase the conductivity of water; this is a redox reaction after all. This is my guess, though I'm not certain.