The Student Room Group

chem group 2 alevel question

The bottle containing Compound 2 has a 'TOXIC' hazard symbol. Use the information in the table to identify Compound 2. Explain both observations in the reaction with H2SO4(aq).
http://www.chemhume.co.uk/AS%20AQA%20CHEM/AS%20Revision/Exam%20PMS/1CMS.PDF table is on page 5
i don't get how it's BaCO3 (i understand the carbonate part of it) since it says the solid remains insoluble when added to NaOH however, if it was Ba then Ba would dissolve according to the solubility rules of hydroxides becoming increasingly soluble down the group. can someone explain? am i missing something
(edited 3 weeks ago)
Reply 1
Original post by ricecakes1
The bottle containing Compound 2 has a 'TOXIC' hazard symbol. Use the information in the table to identify Compound 2. Explain both observations in the reaction with H2SO4(aq).
http://www.chemhume.co.uk/AS%20AQA%20CHEM/AS%20Revision/Exam%20PMS/1CMS.PDF table is on page 5
i don't get how it's BaCO3 (i understand the carbonate part of it) since it says the solid remains insoluble when added to NaOH however, if it was Ba then Ba would dissolve according to the solubility rules of hydroxides becoming increasingly soluble down the group. can someone explain? am i missing something

BaCO3 is insoluble so it won't dissolve and you won't form Ba(OH)2.

Personally my issue with the question is how you can determine which of compound 2 and 4 is the Ba compound and which is the Sr compound...
Reply 2
Original post by Methene
BaCO3 is insoluble so it won't dissolve and you won't form Ba(OH)2.
Personally my issue with the question is how you can determine which of compound 2 and 4 is the Ba compound and which is the Sr compound...

how do u know that BaCO3 is insoluble is that just ur prior knowledge? cant be Sr as it says there's a white precipitate formed when reacted with h2so4, if it was Sr it'd say smth like slight white precipitate
Reply 3
Original post by ricecakes1
how do u know that BaCO3 is insoluble is that just ur prior knowledge? cant be Sr as it says there's a white precipitate formed when reacted with h2so4, if it was Sr it'd say smth like slight white precipitate

Carbonates are insoluble except K/Na/NH3 compounds - we had to learn this for gcse

And I'm pretty sure SrSO4 is just a white ppt, like BaSO4. Sr(OH)2 is the supposed identity of compound 4, which the table shows gives a white ppt. with H2SO4 rather than "slightly white"
Original post by Methene
BaCO3 is insoluble so it won't dissolve and you won't form Ba(OH)2.
Personally my issue with the question is how you can determine which of compound 2 and 4 is the Ba compound and which is the Sr compound...

The toxicity label is the biggest giveaway - you need to know that barium compounds are toxic (except BaSO4 - and strontium compounds usually are much less so).

@ricecakes1 there are also some solubility rules worth learning:

-All carbonates (except Na2CO3, K2CO3 and (NH4)2CO3) are insoluble.

-The sulphates of group 2 become less soluble down the group, though they are practically insoluble by the time you get to Ca.

-The hydroxides of group 2 become more soluble down the group.

You should note that although Ba(OH)2 is soluble, it won’t form if you toss BaCO3 or BaSO4 into a solution of hydroxide ions and so the solid will remain unchanged.
Reply 5
Original post by UtterlyUseless69
The toxicity label is the biggest giveaway - you need to know that barium compounds are toxic (except BaSO4 - and strontium compounds usually are much less so).
@ricecakes1 there are also some solubility rules worth learning:
-All carbonates (except Na2CO3, K2CO3 and (NH4)2CO3) are insoluble.
-The sulphates of group 2 become less soluble down the group, though they are practically insoluble by the time you get to Ca.
-The hydroxides of group 2 become more soluble down the group.
You should note that although Ba(OH)2 is soluble, it won’t form if you toss BaCO3 or BaSO4 into a solution of hydroxide ions and so the solid will remain unchanged.

Ohh, never even considered toxicity, that's good to know! Thanks :smile:
Reply 6
Original post by UtterlyUseless69
The toxicity label is the biggest giveaway - you need to know that barium compounds are toxic (except BaSO4 - and strontium compounds usually are much less so).
@ricecakes1 there are also some solubility rules worth learning:
-All carbonates (except Na2CO3, K2CO3 and (NH4)2CO3) are insoluble.
-The sulphates of group 2 become less soluble down the group, though they are practically insoluble by the time you get to Ca.
-The hydroxides of group 2 become more soluble down the group.
You should note that although Ba(OH)2 is soluble, it won’t form if you toss BaCO3 or BaSO4 into a solution of hydroxide ions and so the solid will remain unchanged.

thanks

Quick Reply

Latest