Qualitative Anion Analysis in General Chemistry Course

[Yanick]

Hi guys and gals. I'm new to the site and have been reading and learning a lot from here. I can't believe I didn't know about this place before. Anyway I'm having a pretty difficult time with a certain reaction that I had to do in my General Chemistry lab. Any help would be appreciated!

We were asked to add H₂SO_4(aq) to a bunch of different salts, watch for the physical manifestations of the reactions (if there were any) and then write a balanced equation/net ionic equation.

There are actually 2 that are giving me problems and I've been sitting here staring at them for about an hour now. I just can't seem to get it right.

1. KI_(s) + H₂SO_4(aq)

2. K₂CrO_4(s) + H₂SO_4(aq)

I just can't get an equation that seems right to me.

1. according to the lab manual, just adding the sulfuric acid to the potassium iodide at room temp you should see effervescense and a darkening of the solid, then you should smel hydrogen sulfide and see violet fumes of Iodine.

So my take on it is as follows;
2KI_(s) + H₂SO_4(aq) -> I_2(g) + H₂S_(g) + 2K⁺_(aq) + 2O_2(g)

The balancing seems to work out for this one but I am unsure if I have the states correct and if that is really the appropriate equation (I did a google search and saw stuff like SO₂ as a possible product)

2. Our instructor provided us with a formula on the board as follows;
2CrO₄^2- + H₂O -> Cr₂O₇^2- + 2OH^-

so my thought was that because K₂CrO₄ is an ionic compound it should dissociate upon addition of solvent but I just can't make the reaction work out on paper. It seems from what the instructor said the chromate should become dichromate and there should be a hydroxide product but, for the life of me, I can't figure out what the acid is doing in the reaction. It seems like whatever available water in the acid solution is reacting with chromate but then what happens to the hydrogen and sulfate ions? Are they spectators? I really just can't wrap my head around this problem. I feel as though there is some kind of information, or key concept, that I have not been taught yet because I just can't come up with an answer that sits right with me.

Thank for any help you guys can offer!

 

[Borek]

First reaction - charges are not balanced, it is not only about balancing atoms. And there will be no free oxygen between products, more like water.

As for the second - try to add H⁺ on both sides of the reaction. After all you used acid.

 

[Yanick]

First reaction - charges are not balanced, it is not only about balancing atoms. And there will be no free oxygen between products, more like water.

I may be thick in the head but I have no clue how I can possibly balance the charges with what I'm given here. Something tells me that this is a reaction which I cannot workout with the level of Chemistry education I have been given thus far. The instructor did mention that we wouldn't be able to work out some of the reactions because they are redox, and I'm getting the inkling that this is one of those.

As for the second - try to add H⁺ on both sides of the reaction. After all you used acid.

Well the thing that bothers about this one is that we are starting with K₂CrO₄ as a solid. The reaction that he put on the board is okay if you have the salt in a solution whereby the potassium can just be a spectator. So this is my shot in the dark upon, but I'm not hopeful

2K₂CrO₄(s) + 2H⁺(aq) -> Cr₂O₇^2-(aq) + H₂O(l) + 4K⁺(aq)

I left out the sulfate ion as a spectator, both sides have same charge (+2). It looks okay to me. Any opinions?