PDA

View Full Version : Chemistry Help!

xyz823
10-08-2010, 06:13 PM
Trying to work out the following question.

Need balanced half equations, fully balanced redox equations and what would be observed.

A freshly prepared solution of sodium hydrogen sulfite is added to a solution on iron (III) chloride.

TIA

pctek
10-08-2010, 06:35 PM
A freshly prepared solution of sodium hydrogen sulfite is added to a solution on iron (III) chloride.

That's a question? Looks like statement to me.....

nofam
10-08-2010, 06:37 PM
It might help if you put any workings you have so far - otherwise you're likely to get whacked with the DYOFH stick!

:thumbs:

Renmoo
10-08-2010, 06:39 PM
Fe^3+ + e^- ---> Fe^2+

HSO3^- + H2O ---> SO4^2- + 3H+ + 2e^-

I'm sure you know what to do from here.

Renmoo
10-08-2010, 06:40 PM

2Fe3+ + HSO3- + H2O ----> 2Fe2+ + SO42- + 3H+ + 2e-

user
10-08-2010, 06:43 PM
H2O + HSO3 (-1) -2e = SO4 (-2) + 3H(+1)
Fe (+3) + e = Fe(+2)

H20 + HSO3 + 2 Fe (+3) = SO4 (-2) + 2Fe(+2) + 3H(+1)

Colour change from yellow/brown to pale green

My chemistry is a little rusty though...

xyz823
10-08-2010, 06:48 PM
Cheers guys, just wasn't sure what was actually being reduced and what was being oxidised! Got myself all confused :/

user
10-08-2010, 06:49 PM

2Fe3+ + HSO3- + H2O ----> 2Fe2+ + SO42- + 3H+ + 2e-

You don't have any free electrons in a balanced redox equation!

Terry Porritt
10-08-2010, 06:52 PM
It's been 55 years since I did any chemistry, but I'm wondering where did the Sodium go to ? NaHS03

user
10-08-2010, 07:01 PM
It's been 55 years since I did any chemistry, but I'm wondering where did the Sodium go to ? NaHS03

Sodium is a spectator ion and takes no part in the reaction. You can add them onto both sides of the equation if you like but it is not necessary in a redox equation. It is the same for the chloride ions.

Renmoo
10-08-2010, 07:03 PM
You don't have any free electrons in a balanced redox equation!
Sans the electrons lol.

Well spotted, thanks :)

xyz823
10-08-2010, 07:06 PM
H2O + HSO3 (-1) -2e = SO4 (-2) + 3H(+1)
Fe (+3) + e = Fe(+2)

H20 + HSO3 + 2 Fe (+3) = SO4 (-2) + 2Fe(+2) + 3H(+1)

Colour change from yellow/brown to pale green

My chemistry is a little rusty though...

Correct me if I'm wrong but,

Don't you add electrons on to the left hand side of one half equation and add electrons the the right hand side of the other one? Meaning you would have to leave the hydrogen out giving this...

SO3 (-1) + H2O = SO4 (-2) + 2H (+1) + e
Fe (+3) + e = Fe (+2)

SO3 (-1) + H2O + Fe (+3) = SO4 (-2) + 2H (+1) + Fe (+2)

If I'm wrong don't hesitate to correct me!

Renmoo
10-08-2010, 07:10 PM
SO3 (-1) + H2O = SO4 (-2) + 2H (+1) + e

It is HSO3(-1)

The hydrogen is part of the entity. Na+ is left out.

user
10-08-2010, 07:15 PM
Correct me if I'm wrong but,

Don't you add electrons on to the left hand side of one half equation and add electrons the the right hand side of the other one? Meaning you would have to leave the hydrogen out giving this...

SO3 (-1) + H2O = SO4 (-2) + 2H (+1) + e
Fe (+3) + e = Fe (+2)

SO3 (-1) + H2O + Fe (+3) = SO4 (-2) + 2H (+1) + Fe (+2)

If I'm wrong don't hesitate to correct me!

You have mistakenly written the bisulphite ion as SO3- instead of HSO3-
This accounts for the 3H+ on the RHS.
It does not matter whether you have electrons added on the left and right of the respective half reactions, it is the same overall if you subtract them from the half equation showing the oxidation. As long as everything balances in the end (like an algebra equation).

xyz823
10-08-2010, 07:15 PM
SO3 (-1) + H2O = SO4 (-2) + 2H (+1) + e

It is HSO3(-1)

The hydrogen is part of the entity. Na+ is left out.

Ohk. Pretty sure I get it.

Paul.Cov
11-08-2010, 06:56 AM
Once again we need to salute the collective wisdom of those haunting these forums.

For me this has served as a reminder of what a nightmare I found chemistry, and of how much I've forgotten over the last 25 years. I was at a loss to post anything useful, and probably would still have struggled to answer this 25 years ago when the teaching was still fresh in my mind.

sarel
11-08-2010, 07:23 AM
B*gger. Saw this question and thought "hmmmm, should be easy meat" because one of my majors at Uni was Chemistry. Suffering succotash, I could not remember a frickin thing - 'tis probably true what they say, use it or lose it.

sarel

Renmoo
11-08-2010, 08:46 AM
www.studyit.org.nz

Great website that I use to frequent every half a day five years ago. The tutors over there > my teachers in high school.

Especially with Biology :stare:

Renmoo
11-08-2010, 08:47 AM
www.studyit.org.nz

Great website that I use to frequent every half a day five years ago. The tutors over there > my teachers in high school.

kahawai chaser
11-08-2010, 09:06 AM
Must admit I struggled working through the question, but been early 80's since I graduated in chemistry. I recall the charge transfer transition state? and/or ligand field theory that causes the colour change? I also thought that double arrows are used for equilibrium rather than the = sign?

Because Cl and Na are present what prevents (or could induce) them from precipitating/forming a salt solution?

user
11-08-2010, 09:34 AM
Double arrows are used for an equilibrium equation. I used an equals sign as the easier alternative to finding out how to type in an arrow. I also don't think it would be an equilibrium reaction since it would essentially go to completion.

The Na and Cl would not precipitate out for the same reason that they don't in the sea. NaCl is rather soluble and would stay in solution.

kahawai chaser
11-08-2010, 10:53 AM
Double arrows are used for an equilibrium equation. I used an equals sign as the easier alternative to finding out how to type in an arrow. I also don't think it would be an equilibrium reaction since it would essentially go to completion.

The Na and Cl would not precipitate out for the same reason that they don't in the sea. NaCl is rather soluble and would stay in solution.

I see. I think I was thinking of organic chemistry reactions which are more likely to establish equilibrium. I guess the green colour is from the chlorine irons.

Renmoo
11-08-2010, 11:51 AM
I guess the green colour is from the chlorine irons.
You are thinking of chlorine gas (Cl2) rather than chloride ions here.

Chloride ions = colourless. A glass of salt solution is colourless. :)

Renmoo
11-08-2010, 11:52 AM
Double arrows are used for an equilibrium equation. I used an equals sign as the easier alternative to finding out how to type in an arrow. I also don't think it would be an equilibrium reaction since it would essentially go to completion.
I don't mean to be too pedantic here, but technically all chemical reactions are equilibrium reactions.

The favoured direction depends on the condition and entropy.

Gobe1
11-08-2010, 11:52 AM
God im glad i dont go to school, i feel like a dummy

user
11-08-2010, 11:57 AM
I don't mean to be too pedantic here, but technically all chemical reactions are equilibrium reactions.

The favoured direction depends on the condition and entropy.

Technically yes. For practical considerations, not all. Consider the reaction of hydrogen and oxygen to form water. Once water is formed, the activation energy for the reverse reaction at room temp makes it essentially non-reversible. Also the formation of polymers is not an equilibrium reaction.

The green colour is owing to the hydrated ferrous ion in the solution.

Renmoo
11-08-2010, 12:01 PM
Because Cl and Na are present what prevents (or could induce) them from precipitating/forming a salt solution?
Since user has answered the first half of the question, I will attempt to explain the second half.

Common ion effect - The addition of either positive ions or negative ions can induce precipitation. The presence of NaCl salt in solution has the following equilibrium:

NaCl (s) < = > Na+(l) + Cl-(l)

Where s = solid, l = liquid.

When more of the positive ions or negative ions are added to the solution, it can push the equilibrium towards the left side (Le Chatelier's principle), whereby the ions are induced to form the solid form again.

Cheers :)

Renmoo
11-08-2010, 12:04 PM
The green colour is owing to the hydrated ferrous ion in the solution.
Does the complex contain two or three H2O molecules? It's been a while...

xyz823
11-08-2010, 12:13 PM
H2O + HSO3 (-1) = SO4 (-2) + 3H (+1) + 2e
Fe (+3) + e = Fe (+2)

H20 + HSO3 + 2Fe (+3) = SO4 (-2) + 2Fe(+2) + 3H(+1)

Talked to the chem teacher, had the electrons on the wrong side man.

Left hand side overall is -1. Right hand side is +1 (-2 + 3) so add 2 electrons to the right hand side to balance each side. And apparently if you have electrons on the same side in each equation you've done something wrong.

Renmoo
11-08-2010, 12:33 PM
H2O + HSO3 (-1) -2e = SO4 (-2) + 3H(+1)
Fe (+3) + e = Fe(+2)

Note the negative sign posted by user.

SoniKalien
11-08-2010, 12:42 PM
Cheers guys, just wasn't sure what was actually being reduced and what was being oxidised! Got myself all confused :/

LEO (the lion) goes GER!

Meaning LEO = Loss of Electrons = Oxidation
GER = Gain of Electrons = Reduction

hth :)

xyz823
11-08-2010, 12:42 PM
Note the negative sign posted by user.

Hmm we havnt been over that yet, only adding things. Whats the difference? Presume its just the opposite though?

Renmoo
11-08-2010, 02:08 PM
Hmm we havnt been over that yet, only adding things. Whats the difference? Presume its just the opposite though?
Yup!

It's like:

3 - 2 = 1
3 = 1 + 2

user
11-08-2010, 02:10 PM
Hmm we havnt been over that yet, only adding things. Whats the difference? Presume its just the opposite though?

That is simply the way I taught it many years ago. Logical since oxidation is the loss of electrons so it appears as a minus electrons on the LHS, reduction is the gain so it appears as a plus electrons on the RHS.

I think the ferrous ion probably has 6 water ligands surrounding it in solution.

The common ion effect would have no practical effect in causing NaCl to ppt in the redox reaction under discussion. It is far too soluble for any effect to show under the concentrations normally encountered.

As an aside, ferrous sulphate was known as green vitriol. Heating the salt over a flame produces sulphur trioxide, which can be dissolved in water to form oil of vitriol or sulphuric acid. I think this was how the acid was first produced, thus its old name. (If memory serves me correct - not to imply that I was around at the time of discovery though)

kahawai chaser
12-08-2010, 06:12 PM
Since user has answered the first half of the question, I will attempt to explain the second half.

Common ion effect - The addition of either positive ions or negative ions can induce precipitation. The presence of NaCl salt in solution has the following equilibrium:

NaCl (s) < = > Na+(l) + Cl-(l)

Where s = solid, l = liquid.

When more of the positive ions or negative ions are added to the solution, it can push the equilibrium towards the left side (Le Chatelier's principle), whereby the ions are induced to form the solid form again.

Cheers :)

OK - Thanks. Must reach it's saturation point, based on solubility product (I think) when crystals form.

xyz823
15-08-2010, 05:13 PM
Going through some more similar questions, came across this one here.

Hydrogen Peroxide is reacted with Iron(II) Sulfate. Write the reaction and the observations. Water is a product.

Any help would be much appreciated. I presume that the Iron (II) is reduced to Iron (III)? But not quite sure what is being oxidised etc.

Renmoo
15-08-2010, 05:24 PM
H2O2 + 2H+ + 2e- = H2O + H2O (i.e. two H2O molecules being produced)

Fe2+ = Fe3+ + e
2Fe2+ = 2Fe3+ + 2e

Now you can "add" both half equations

H2O2 + 2H+ + 2Fe2+ = 2H2O + 2Fe3+

Renmoo
15-08-2010, 05:27 PM
Regarding this query:

But not quite sure what is being oxidised etc.
In a redox reaction, when a compound is reduced, the other participating compound must have been reduced. Since you have made the guess that Ferrous Sulphate would be oxidised to Ferric Sulphate, therefore hydrogen peroxide must undergo a reduction reaction.

You know water is the product:

H2O2 = H2O

Balance the oxygen first:

H2O2 = H2O + H2O

Balance the hydrogen now by adding H+ to the left side:

H2O2 + 2H+ = H2O + H2O

Now balance the charges by adding the correct number of electrons:

H2O2 + 2H+ + 2e = 2H2O

:)

xyz823
15-08-2010, 05:31 PM
H2O2 + 2H+ + 2e- = H2O + H2O (i.e. two H2O molecules being produced)

Fe2+ = Fe3+ + e
2Fe2+ = 2Fe3+ + 2e

Now you can "add" both half equations

H2O2 + 2H+ + 2Fe2+ = 2H2O + 2Fe3+

Cheers. So the H2O2 is oxidised to two H2O molecules?

Renmoo
15-08-2010, 06:01 PM
Yes