Ashendale
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Exercises
Today I recieved a exercise page from my chemistry teacher to do at home.
Anyway, I'm not quite sure I have the right answers. Would you check them and tell where I am wrong (If I am)?
1. How to make Cu soluble compound without using acid?
2AgNO3 + Cu -> Cu(NO3)2 + 2Ag
CuO + SO3 -> CuSO4
2. 6 test tubes contain KOH, CaO, ZnO, CaCl2, SiO2 and P4O10. By experiementing, make sure which test tube contains which chemical. You can use water,
dilute H2SO4 and universal indicator.
P4O10 + 6H2O -> 2H3PO4
H3PO4 + 3ZnO -> Zn3(PO4)2 + 3H2O
CaO + H2O -> Ca(OH)2 (Add universal indicator)
ZnO + H2SO4 -> ZnSO4 + H2O | ZnSO4 + KOH -> Zn(OH)2 + K2SO4 | Zn(OH)2 (temp)-> ZnO + H2O
CaCl2 + H2SO4 -> HCl + CaSO4 | CaSO4 (temp)-> CaO + SO3 | CaO + H2O -> Ca(OH)2
I wasn't really sure about how to prove SiO2...I was thinking about adding water and showing that it won't react..But then again, ZnO
doesn't react either. It won't dissolve too..Any ideas?
About KOH, I was thinking about adding water and showing it gets really hot. (If I remember correctly, making hydroxide solution is quite exothermic?)
3. You are given 4 solid chemicals: MgCl2, K2CO3, ZnS and CuSO4. You are also given universal indicator, filter paper, concentrated acids, alkalis
and water. How to identify each chemical using as few reagants as possible?
ZnS + 2HNO3 -> Zn(NO3)2 + H2S (I was thinking you can identify sulfide from H2S gas?)
CuSO4 + 2NaOH -> Na2SO4 + Cu(OH)2 | Cu(OH)2 (temp)->CuO + H2O (yay, black CuO)
K2CO3 (temp)-> K2O + CO2 K2O + H2O -> 2KOH (Aah, the magical universal indicator shows us it's a base)
For MgCl2 I have got no ideas. At first I was thinking about reacting with silver(I)nitrate, but then again, it would only prove it's a
chloride..
4. You got 4 tes tubes with Na2SiO3, Na2S, Na2SO3 and Na2CO3. Using one reagant, make sure which test tube has which chemical in it.
Again, I've got no ideas. At first I thought about reacting with Lead nitrate to form insoluble salts..But teacher told that the person
identifying those chemicals should know the colour of the salt. Sitting there, I thought maybe hyrdochloric acid would do it. And it would, for Na2S
and Na2CO3. They both form gas. But the other's dont. So now I don't have ANY ideas. Sulphuric acid wouldn't work neither, as Na2SO4 is
soluble in water  And neither would nitric acid. I can't react it with a base,
because I can't think of any chemical that would be soluble when reacting with sodium salt and then form insoluble compound
[Edited on 27-9-2005 by Ashendale]
[Edited on 27-9-2005 by Ashendale]
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Ashendale
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Hmm, seems the allmighty Edit button is lost..At least I can't find it anywhere o_O
Anyway, spotted a mistake
"K2CO3 (temp)-> K2O + H2O" doesn't clearly do that, so ignore that.
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Darkblade48
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| Quote: | Originally posted by Ashendale
Anyway, spotted a mistake
"K2CO3 (temp)-> K2O + H2O" doesn't clearly do that, so ignore that. |
From an MSDS
| Quote: |
Hazardous Decomposition Products:
Contact with acids and involvement in a fire can cause formation of carbon dioxide. Thermal decomposition may also form potassium oxide.
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So, K2O can form with heat, but the other decomposition product would be CO2 and not water
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12AX7
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I've never heard of alkali carbonate melts decomposing too quickly so it probably isn't a viable route. Burning calcium carbonate then
slaking the CaO to Ca(OH)2 then precipitating carbonate with the K2CO3 would yield a solution of KOH, but that's not K2O.
If you want to get extravagant, you can always reduce it with carbon at white heat (K2CO3 + 2C + heat = 2K + 3CO; beware of (KCO)6 (potassium salt of
hexahydroxybenzene) which is claimed to be explosive), distill off potassium metal and burn that in a limited amount of oxygen to produce K2O. 
Tim
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woelen
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| Quote: | What teacher do you have? The level of this is quite high and this is not trivial for high school chemistry.
| Quote: | Originally posted by Ashendale
Today I recieved a exercise page from my chemistry teacher to do at home.
Anyway, I'm not quite sure I have the right answers. Would you check them and tell where I am wrong (If I am)?
1. How to make Cu soluble compound without using acid?
2AgNO3 + Cu -> Cu(NO3)2 + 2Ag
CuO + SO3 -> CuSO4 |
First one looks quite good. The second one is questionable. SO3 is the anhydride of a strong acid.
| Quote: | 2. 6 test tubes contain KOH, CaO, ZnO, CaCl2, SiO2 and P4O10. By experiementing, make sure which test tube contains which chemical. You can use water,
dilute H2SO4 and universal indicator.
P4O10 + 6H2O -> 2H3PO4
H3PO4 + 3ZnO -> Zn3(PO4)2 + 3H2O
CaO + H2O -> Ca(OH)2 (Add universal indicator)
ZnO + H2SO4 -> ZnSO4 + H2O | ZnSO4 + KOH -> Zn(OH)2 + K2SO4 | Zn(OH)2 (temp)-> ZnO + H2O
CaCl2 + H2SO4 -> HCl + CaSO4 | CaSO4 (temp)-> CaO + SO3 | CaO + H2O -> Ca(OH)2
I wasn't really sure about how to prove SiO2...I was thinking about adding water and showing that it won't react..But then again, ZnO
doesn't react either. It won't dissolve too..Any ideas?
About KOH, I was thinking about adding water and showing it gets really hot. (If I remember correctly, making hydroxide solution is quite exothermic?)
|
Add water to all of the test tubes. Some solids do not dissolve, some dissolve. P4O10 can be selected as first. Acidic with universal pH indicator.
Now, only 5 tubes are left.
Add drops of the solutions or suspension to dilute H2SO4. Three of them give a white precipitate as final result.
The one, which initially is clear and gives a white precipitate with dilute H2SO4 is CaCl2.
Of the other two, which give a white precipitate or suspension, one is SiO2, the other is CaO. You can distinguish these by testing pH. The alkaline
one is the CaO.
Now only two test tubes are left. The clear one is KOH, the suspension is ZnO.
| Quote: | 3. You are given 4 solid chemicals: MgCl2, K2CO3, ZnS and CuSO4. You are also given universal indicator, filter paper, concentrated acids, alkalis
and water. How to identify each chemical using as few reagants as possible?
ZnS + 2HNO3 -> Zn(NO3)2 + H2S (I was thinking you can identify sulfide from H2S gas?)
CuSO4 + 2NaOH -> Na2SO4 + Cu(OH)2 | Cu(OH)2 (temp)->CuO + H2O (yay, black CuO)
K2CO3 (temp)-> K2O + CO2 K2O + H2O -> 2KOH (Aah, the magical universal indicator shows us it's a base)
For MgCl2 I have got no ideas. At first I was thinking about reacting with silver(I)nitrate, but then again, it would only prove it's a
chloride.. |
Add water to a small part of each of the solids. The one which becomes blue is CuSO4.
Add dilute HCl to part of the other three solids. Two of them will bubble. The one, which smells badly is ZnS, the other one is K2CO3. The remaining
one is MgCl2.
| Quote: | 4. You got 4 tes tubes with Na2SiO3, Na2S, Na2SO3 and Na2CO3. Using one reagant, make sure which test tube has which chemical in it.
Again, I've got no ideas. At first I thought about reacting with Lead nitrate to form insoluble salts..But teacher told that the person
identifying those chemicals should know the colour of the salt. Sitting there, I thought maybe hyrdochloric acid would do it. And it would, for Na2S
and Na2CO3. They both form gas. But the other's dont. So now I don't have ANY ideas. Sulphuric acid wouldn't work neither, as Na2SO4 is
soluble in water And neither would nitric acid. I can't react it with a base,
because I can't think of any chemical that would be soluble when reacting with sodium salt and then form insoluble compound |
[Edited on 27-9-2005 by Ashendale]
[Edited on 27-9-2005 by Ashendale] |
Add dilute H2SO4 to all of them. The one, which bubbles and gives an odorless gas is Na2CO3, the other one which bubbles is Na2S (gives a rotten egg
smell). The compound, which gives a glass-like precipitate is Na2SiO3. The remaining one is Na2SO3. Although this does not bubble, it has a
distinctive pungent smell. With some heating, this also can be made to bubble, producing bubbles of SO2.
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Ashendale
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Thank you for your answers 
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Bot0nist
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This is a good teacher you have. I wish I would have been challenged to think critical in my high school chem class. These questions require knowledge
of chemistry and logical deduction to find the answers. Glean all you can from him/her. Good teachers are rare, IME.
Good luck on your Journey.
U.T.F.S.E. and learn the joys of autodidacticism!
Don't judge each day only by the harvest you reap, but also by the seeds you sow.
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woelen
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Please be aware that the original question was asked 6.5 years ago 
But of course, thinking about how to solve this kind of questions can be an interesting and valuable educational experience.
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