Polish Chemistry Comp - Help me !

[Guest Bebop]

Hello!

I live in Poland. I attend here the last class of junior high (a secondary school - I'm 16 years old). I've got many interests and Chemistry is one of them.

That's why I took part in a Polish Chemistry competition organised for people of my age. I got through to the second round aswell as my school mates (11 students), mostly because of the support of the greatest teacher ever (no one teaches Chemistry like she does).

The second round was organised on the beggining of this year (10th January). Before it everyone had been sure of getting through to the final, we were studying whole days & nights.

After the comp we were curious of one excercises. Most of us had the same solution, so we thought it was OK.

Few days later, our teacher told us that none of us worked this excercise well...

The comitee suggested really strange solution, no one got the idea and most of the people writting the second round didn't get through (99% :evil: ). Saying that differently - in the final there will probably be 10 students out of aprox. 500...

 

Now what I want to ask you to do... We want to write to the comitee to cancel this controversial task, because of the very strange solution suggested and no good solutions from the students.

In order to do this we need good reasons. That's why I ask YOU to sit and try to solve this task. Please, it will be very helpful for me and my friends.

 

The suggested solution will be provided in the nearest future .

 

Task:

'The admixture of natrium's hydroxide (NaOH) in natrium's carbonate (Na2CO3) can be detected in the following way : dissolve the sample of the carbonate (Na2CO3) in water (H2O), then add dilution (I'm not sure if it's understandable in English- otherwise spirit is OK?) of BaCl2 in excess and few drops of indicator showing if the dilution is basic (colourising the dilution in kind of pink - I don't what's the name of this indicator in English so I'm trying to describe it, in Polish it's 'fenoloftaleina'). The appearence of the pink color proclaim that there's admixture of NaOH in Na2CO3. Explain with reaction's equations and explaination in words this way of detection NaOH's admixture in Na2CO3.'

 

Inform me if anything is not understandable, cos it's the first time I use the chemical vocabulary .

[YT2095]

Phenolphthalein is the English and it`s usualy in Ethanol (C2H5OH).

it`s clear before it reacts, then goes a deep pink color on PH`s from 8.2 to 10.0

 

here also the word Natrium (I understand it) is called Sodium, it`s like K kalium (Potassium) it`s Latin words, but be sure I/we understand

 

also barium chloride and natrium hydroxide would react, makeing natrium chloride and barium hydroxide as they are the most stable chems left after the displacement reaction.

the natrium carbonate however seems to be a red herring?

 

such a strange experiment!

 

mozet toze, ty govoresh po-russki ?

 

[Guest Bebop]

Thank you very much for your reply.

I think I'll post the suggested solution on Friday.

 

Oh, and no, I don't speak Russian .

[Guest Bebop]

OK, this is how it was described by the judging comitee:

- dissolving Na2CO3 + 2 H2O -->2 NaOH + H2O + CO2 (1 point)

- BaCl2 + H2O + CO2 -->BaCO3(solid)+ 2 HCl (1 point)

- neutralizing base 2 NaOH + 2 HCl -->2 NaCl + 2 H2O (1 point)

- reason why the solution colorises with phenophtalein: excess of sodium hydroxide (NaOH)

in the sample of Na2CO3 that wasn't neutralized by created acid causes the appearance

of the pink color in the solution. (2 points)

TOTAL: 5 points

 

Please tell me what you think about it .

Again big thanks for your interest .

[Guest Simon]

excuse me!i am sorry if i interfere you.

I am a Malaysian.I would propbably help you if you can clarify your question better.

I mean to understand your question in a simple question.

By the way, i am Simon Chiew and i am also 16 years old.Hi! nice to meet you.

[Guest Simon]

Thanks.

In my opinion,all the oH- ion in the conical flaskwill neutralized the H+ ion.At this particular level,there is no H+ or OH- excessing more.Thus,the colour of the fenolftalein change to colourless.

I don't whether i'm rite.furthermore,i have bad english.

[pulkit]

Analyzing step by step ..........

 

Firstly the equation for dissoultion of [MATH]Na_2CO_3[/MATH] is certainly correct as it will produce [MATH]NaOH[/MATH] and [MATH]CO_2[/MATH]. Look at this reaction as the hydrolysis of [MATH]Na_2CO_3[/MATH], i.e.,

[MATH]Na_2CO_3 + H_2O \longrightarrow NaOH + H_2CO_3[/MATH]

 

When you now add [MATH]BaCl_2[/MATH] , being a highly reactive reagent it will most definately precipitate out [MATH]BaCO_3[/MATH] . Now, look at it like this [MATH]CO_2[/MATH] has in essence come from the solution of [MATH]H_2CO_3[/MATH] present already. The removal of [MATH]CO_2[/MATH] will lead to free [MATH]H^{+}[/MATH] attacking the [MATH]Cl^{-}[/MATH] ions, henece HCl will form.

 

The excess of alkali present as [MATH]NaOH[/MATH] will impart the pink colour because of phenolphthaleine.

 

It is a very nicely thought of and an absolutely correct qualitative analysis. The two critical points are initial dissolution of bicarbonate and addition of barium chloride only later, and also the presence of excess strong alkali in form of [MATH]NaOH[/MATH]