[funkyonion]

Hi, I'm having trouble finding equation for redox reaction. Question is in finnish so I'll try to translate.

Q: How many grams of sodium bisulfite ( NaHSO₃ ) you need to add to 250,0 ml of 0,500 mol/l potassium chromate ( K₂CrO₄ ) solution, so that all the chrome in chromate reduces to Cr³⁺ ions (solution is basic)?

I don't know how am I supposed to get the half reactions from that question. Provided solution gives the half reactions as:

Reduction:
$$ CrO_4^{2-} (aq) \rightarrow Cr^{3+} (aq)$$

Oxidation:
$$ HSO_3^- (aq) \rightarrow SO_4^{2-} (aq) $$

I know how these are balanced, but is there a way to figure out why HSO₃ is oxidized to SO₄^2- and not, for example, to SO₃^2-?

EDIT: Trying to make text readable...

[Borek]

Why do you think converting HSO₃^- to SO₃^2- is an oxidation?

[funkyonion]

Why do you think converting HSO₃^- to SO₃^2- is an oxidation?

Well... I don't know, it isn't? Those reactions I wrote are from the provided solutions so I didn't make them up. The problem I'm having is that how can I figure out those half-reactions. The half-reaction for reduction I could've figured out but I don't know how to figure out the latter reaction.

[Borek]

Why do you think converting HSO₃^- to SO₃^2- is an oxidation?

Well... I don't know, it isn't?

Try to assign oxidation numbers to all elements present on both sides of the reaction. Actually there is only one element that can change its ON.

Or, from different angle - how do you call this reaction:

HSO₃^- <-> H⁺ + SO₃^2-

Is it redox, or something else?

[funkyonion]

Why do you think converting HSO₃^- to SO₃^2- is an oxidation?

Well... I don't know, it isn't?

Try to assign oxidation numbers to all elements present on both sides of the reaction. Actually there is only one element that can change its ON.

Or, from different angle - how do you call this reaction:

HSO₃^- <-> H⁺ + SO₃^2-

Is it redox, or something else?

Ah, sorry I misread your previous answer, I thought it said SO₄. I'm not entirely sure what you meant with "Actually there is only one element that can change its ON." Might have something to do with that I'm not a native english speaker.

I have some difficulty with these redox assignments since it seems I'm unable to predict what kind of reaction happens. As in the question I wrote in the opening post, most of these don't tell nothing but two solutions are mixed together and then some metal is reduced and the resulting liquid is acidic/basic, and I have no idea how to get from there to forming equations for half-reactions.

In this question my initial guess was that the oxidation half reaction would be HSO₃^-  SO₃^-2 but it was HSO₃^-  SO₄^-2. Where does that fourth oxygen atom come from?

Btw, thank you for taking the time to answer!

[Borek]

ON means just the oxidation number (if that's what you have problem with).

In this question my initial guess was that the oxidation half reaction would be HSO₃^-  SO₃^-2

Please reread earlier posts, I was trying to guide you to the moment you will realize why it is NOT redox reaction.

Where does that fourth oxygen atom come from?

From water - plenty of water around.

In redox reactions taking place in water we don't care about the source of oxygen/hydrogen, as there is plenty of both around - so it is not difficult to balance reaction. For example if you need an oxygen atom on the right, you can put a water molecule on the left and two H⁺ between products, or two OH^- on the left and water molecule on the right - and atoms are balanced.

[funkyonion]

ON means just the oxidation number (if that's what you have problem with).

That explains it!

In this question my initial guess was that the oxidation half reaction would be HSO₃^-  SO₃^-2

Please reread earlier posts, I was trying to guide you to the moment you will realize why it is NOT redox reaction.

Where does that fourth oxygen atom come from?

From water - plenty of water around.

In redox reactions taking place in water we don't care about the source of oxygen/hydrogen, as there is plenty of both around - so it is not difficult to balance reaction. For example if you need an oxygen atom on the right, you can put a water molecule on the left and two H⁺ between products, or two OH^- on the left and water molecule on the right - and atoms are balanced.
[/quote]

Reaction I described would be more like dissolving? So as Cr reduces, something has to oxidize, and in this case it would be sulphur? Indeed adding fourth oxygen makes its oxidation number get more positive.

Someday I'll understand these. There was another problem where mixture of copper and silver were dissolved in nitric acid and I have no clue where the nitric oxide came from but maybe that's something I don't need to worry about today. Thank you for answering!

[Borek]

Reaction I described would be more like dissolving?

Better, although still not correct. It was just a simple dissociation.