Naked Science Forum
Non Life Sciences => Chemistry => Topic started by: Blendiman on 28/01/2004 18:24:14
-
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% ). 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 .
-
Ok, I think I managed to interpret what you are asking. Here's a detailed explanation of what is happening in that procedure.
This is a simple qualitative test where you are precipitating an un-needed ion so that you can test for another. This is the dissolution reaction for the test sample in water:
Na2CO3(aq) -> 2Na(+) + CO3(2-)
NaOH(aq) -> Na(+) + OH(-)
BaCl2 is added because the salt Barium Carbonate (BaCO3) is insoluble and will precipitate out of solution. This is done to eliminate all carbonate anions, as they can act as a base and change the pH of the solution.
BaCl2(aq) -> Ba(2+) + 2 Cl(-)
CO3(2-) + Ba(2+) -> BaCO3(solid)
Once the BaCO3 precipitates out, you will be left with only Na(+), Ba(2+), Cl(-), and OH(-) ions in solution. Hydroxide (OH) picks up acidic protons (H+) and thus reduces the proton concentration of the solution. You call the logarithm of the proton concentration the pH.
OH(-) + H(+) -> H20
If you have a neutral solution, your H(+) concentration is 10^-7 mol/L. The negative logarithm of this is 7, therefore neutral pH is 7. The presence of OH(-) decreases this acidic proton concentration to raise the pH. Your indicator (phenolphthalein in English) will turn pink at pH levels above approximately 8.
This should be how your judging committee explains it.
This message brought to you by The Council of People Who Are Sick of Seeing More People
-
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 .
Big thanks for your interest .
-
Ok, I see what you were asking you now. I thought you were saying the original NaCO3 has NaOH also present in the sample. I disagree with their approach of the chemistry, however.
Carbonates in water do NOT form hydroxides, they form bicarbonates. Bicarbonates can pick up any excess protons and form carbonic acid, which can then spit out a CO2 molecule, leaving behind water. Theoretically, this proton could come from water, but you've already formed some hydroxide when you created the bicarbonate, so the acidic hydrogen concentration is far too low to allow bicarbonate to get another proton.
Also, even if their chemistry in the first reaction was accurate, all of the CO2 that is formed would escape as gas. CO2 dissolved in water is Carbonic Acid (H2CO3) and its protons would be immediately eaten up by the hydroxide ions present.
Next, since there is no CO2 around to do any reaction, their BaCl2 reaction is pure fantasy. The Ba ions would immediately react with the carbonate ions, forming an insoluble precipitate. Also, you're not making HCl in the presence of hydroxide...no way.
Finally, if as they claim, HCl is produced and neutralizes the NaOH, why then would your indicator turn pink?
Your judging committee is grossly misinformed.
This message brought to you by The Council of People Who Are Sick of Seeing More People
-
Cannabinoid, if you believe in that, you should write to them so they can improve their competition.
Am I dead? Am I alive? I'm both!
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.stupid-boy.com%2Fsmilies%2Fkao%2Fotn%2Fcat.gif&hash=e4b91a72c020cc1c5d28487fff5428f1)
-
He didn't exactly provide me with contact information or the names of the people on the committee, or I would have.
This message brought to you by The Council of People Who Are Sick of Seeing More People
-
Ask him too, if he comes back to this site :-P
Am I dead? Am I alive? I'm both!
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.stupid-boy.com%2Fsmilies%2Fkao%2Fotn%2Fcat.gif&hash=e4b91a72c020cc1c5d28487fff5428f1)