Precipitation Chemistry

[Rex.]

This is from Igcse ques:


Solution A= Iron(III) chloride(aq)

Test: Add dilute nitric acid and silver nitrate into solution A.

What do you observe from it?

 

[Shadow Fury]

A white precipitate (which is silver chloride). I got A* in IGCSE Chemistry

 

[Rex.]

A white precipitate (which is silver chloride). I got A* in IGCSE Chemistry

Congrats dude! BTW, how do you get the answer? What way?

 

[Shadow Fury]

Congrats dude! BTW, how do you get the answer? What way?

It is stuff you memorise. This is the test for chloride ion Cl^-. Iron (III) displaced silver from its nitrate, becoming soluble iron (III) nitrate thus disappearing. Silver was soluble when it was silver nitrate but when iron displaced it, soluble silver nitrate became insoluble silver chloride, falling down as a white precipitate.

 

[Rex.]

Damn my silly question! I should have know this. Thanks Fury!

Edit: Does it mean we have to ignore the nitric acid?

 

[Shadow Fury]

Damn my silly question! I should have know this. Thanks Fury!

Edit: Does it mean we have to ignore the nitric acid?

Good that I rechecked the thread to see your Edit...

No, we must not ignore the nitric acid. It is required for this reaction to occur. You must acidify the solution.
It is always wise to use a nitrate because all nitrates are soluble. However, this doesn't mean that in the exam you'll always get a nitrate. You must memorise the solubilites of all these negative ions (SO₄^2-, NO₃^-, Cl^- etc...) with metals.

 

[Rex.]

Good that I rechecked the thread to see your Edit...

No, we must not ignore the nitric acid. It is required for this reaction to occur. You must acidify the solution.
It is always wise to use nitric acid because all nitrates are soluble. However, this doesn't mean that in the exam you'll always get it. You may find hydrochloric acid to test for a metal X sulphate where the metal X chloride would dissolve so you must memorise the solubilites of all these negative ions (SO₄^2-, NO₃^-, Cl^- etc...) with metals.

So the acid acts as the trigger for the reaction?

Edit: And also won't the substances react with the acid?

 

[Shadow Fury]

So the acid acts as the trigger for the reaction?

Edit: And also won't the substances react with the acid?

Yeah, the acid is needed to trigger the reaction. If you put solid silver nitrate and solid iron (III) chloride nothing will happen. Also, you are supposed to add *drops* of nitric acid just to see the precipitate.

FeCl₃ (soluble) + 3AgNO₃ (soluble) ---> Fe(NO₃)₃ (soluble) + 3AgCl (insoluble white precipitate)

As you see, nitric acid is not involved. It simply triggers the reaction.

 

[Rex.]

Ok, nitrate acid is straight forward and what about this?

Solution A= Iron(III) chloride(aq)

Test: Add dilute HCl acid and silver nitrate into solution A.

How do we dealt with this then?

 

[Shadow Fury]

Ok, nitrate acid is straight forward and what about this?

Solution A= Iron(III) chloride(aq)

Test: Add dilute HCl acid and silver nitrate into solution A.

How do we dealt with this then?

Won't change, still white ppt. The reaction is still the same. Oh right... I forgot... the silver should always be nitrate, not the acid always nitric
You can use any acid you want for this (sulphuric, hydrochloric, phosphoric...). It's just that during my first days of solving past papers I saw hydrochloric acid while in my book it was written nitric only so I got confused.

 

[Rex.]

Won't change, still white ppt. The reaction is still the same. Oh right... I forgot... the silver should always be nitrate, not the acid always nitric
You can use any acid you want for this (sulphuric, hydrochloric, phosphoric...). It's just that during my first days of solving past papers I saw hydrochloric acid while in my book it was written nitric only so I got confused.

So what about replacing the silver nitrate with silver bromide?

 

[Shadow Fury]

So what about replacing the silver nitrate with silver bromide?

It will not work very well and you're making it way too complicated. Silver bromide is insoluble and it will appear as a pale-yellow solid on the base. It will be displaced anyway but you'll get a coloured liquid (aqueous iron bromide) and a white precipitate of silver chloride. Good luck at seeing a white thing amongst a coloured liquid! I'm not even sure if that would be the result!

 

[Rex.]

Ok, one more. I just encounter one question now.

Liquid A= ethanoic acid

What is smell and the colour?

Is it in alkenes or something?

 

[Shadow Fury]

Ok, one more. I just encounter one question now.

Liquid A= ethanoic acid

What is smell and the colour?

Is it in alkenes or something?

Uhm, ethanoic acid is vinegar so its smell is like vinegar (that awful smell...) and it is obviously colourless. It can't be an alkene since it contains oxygen. It is an organic acid.

 

[Rex.]

What sub subject which we learn about the colour,smell, taste of the substance including acid and bases?

 

[Shadow Fury]

What sub subject which we learn about the colour,smell, taste?

You should have it in your Chemistry notes. If not, you got some crappy notes. It is included in the course so you must know them well.

 

[Rex.]

Uhm, ethanoic acid is vinegar so its smell is like vinegar (that awful smell...) and it is obviously colourless. It can't be an alkene since it contains oxygen. It is an organic acid.

That's what we call Pungent :|

 

[Zombie]