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VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Chemistry => Topic started by: crayolé on June 04, 2010, 08:08:39 pm

Title: How do you work this out? :/
Post by: crayolé on June 04, 2010, 08:08:39 pm

Selenium dioxide (Se02) is an important reagent in organic syntheses, as it is both an oxidant and
weakly acidic. In a certain reaction, 0.142 g of selenium dioxide reacted with exactly 25.52 mL of
0.100 M chromium(II) sulfate, CrS04. In the reaction, the Cr2+(aq) ions were oxidised to Cr3+(aq).
To what oxidation state was the Se4+ converted in this reaction?

Title: Re: How do you work this out? :/
Post by: stonecold on June 04, 2010, 08:13:44 pm

I posted this somewhere but can't remember where now.

Basically, work out the mol for each.  It ends up being Cr:Se is 2:1

Then write up equation:

2Cr²⁺ + Se⁴⁺ ---> 2Cr³⁺ + Se^x

x must equal + 2 for the charges to balance.

Title: Re: How do you work this out? :/
Post by: kate26 on June 04, 2010, 08:28:10 pm

was this in a neap exam ? only cos i remember reading this and thinking wtf?

Title: Re: How do you work this out? :/
Post by: stonecold on June 04, 2010, 08:28:37 pm

yeah, i got it in like the final 3 minutes haha...

Title: Re: How do you work this out? :/
Post by: Martoman on June 04, 2010, 08:29:03 pm

NEAP 09 first short answer

Title: Re: How do you work this out? :/
Post by: cameron_15 on June 04, 2010, 08:29:41 pm

Relatively hard exam that was.

Title: Re: How do you work this out? :/
Post by: kate26 on June 04, 2010, 08:30:31 pm

haha stupid neap

Title: Re: How do you work this out? :/
Post by: cindyy on June 04, 2010, 09:10:48 pm

Quote from: stonecold on June 04, 2010, 08:13:44 pm

I posted this somewhere but can't remember where now.

Basically, work out the mol for each.  It ends up being Cr:Se is 2:1

Then write up equation:

2Cr²⁺ + Se⁴⁺ ---> 2Cr³⁺ + Se^x

x must equal + 2 for the charges to balance.

i dont get how you did that :S

Title: Re: How do you work this out? :/
Post by: stonecold on June 04, 2010, 09:15:01 pm

Did you work out the mol?

I don't exactly remember, but it was something like n(Cr)=0.0028 and n(SeO2)=0.0014.

So from this, you know the mol ratio.  You also know Cr goes from +2 to +3, and the Se is initially +4.

Then do my little equation thingy, remembering to take into account the mol ratio, and you will see the Se has to be reduced to the +2 oxidation state for the charges to balance.

Title: Re: How do you work this out? :/
Post by: azn_dj on June 04, 2010, 09:19:34 pm

Basically because you are not tweaking the balancing by adding electrons/hydrogens to either side
the left side overall charge must equal the right hand side charge
so the left is 2*+2 plus +4 which is +8 Overall
Right hand side - 2*+3 (Chromium) is +6. The difference is 2. So it ends up being 2+ for the Selenium.

Oxidation number in Stonecold's example - Very funny Stonecold

Title: Re: How do you work this out? :/
Post by: cindyy on June 04, 2010, 09:20:04 pm

OHHHHHHH! stupid me :D