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Fehling's Reagent (2 Viewers)
- Thread starter phenol
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phenol
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Lime water reacts with atmospheric CO2
Fehling reagent is copper 2+ ions in sodium potassium tartarate and sodium hydroxide.
Maybe CuCO3 formation/ppte? Anyway I cannot find the answer online
do any of you out there know?
Fehling's reagent (mainly light blue Cu(OH)2) reacts with reducing sugars under heating, to from brick red Cu2O precipitates. If heated using hot waterbath, Cu2O ppt would be large and brick red, if heated directly using bunsen, Cu2O would be smaller and yellow.
R-CHO + 2Cu 2+ + 5OH - -> R-CO2 + Cu2O + 3H2O
For this reagent , the two ingredient solutions should be made, stoppered and stored (NaOH and CuSO4) - they are then mixed to form Fehling's reagent when needed.
Why must the reagent be freshly made? The presence of the tartarate ion is to bond to Cu2+, preventing Cu(OH)2 ppt when two solutions are mixed. But why don't we just use CuSO4 solution then, since it contains free Cu2+? The redox reaction needs a basic medium, thus the NaOH with the sodium potassium tartarate. So the whole point of the reagent is to have free Cu2+ and a basic medium. Now if we mix the two solution together yonks before we actually use the resulting reagent, what would happen? Hmm, it can't be reaction with air, since CuSO4(aq) and the NaOH + tartarate solution would also have the same problem, since they all contain the same Cu2+ and OH- ions. So my guess is either:
1. Cu2O somehow precipitates, and making the reagent useless, or
2. something happens to the tartarate ion if you leave it with Cu2+ for too long, and Cu(OH)2 precipitates. And then when you actually use the reagent, Cu(OH)2, after heating would just decompose to CuO, making the reagent useless anyway
Just a side note, another reason you might see CuO at the end of the test is due to the oxidation of Cu2O to CuO, since Cu2O is only more stable at high temperature, so if you heat slowly, it's no good. Also, Cu(OH)2 decompose faster when heating slowly in water bath, so heating directly is probably better, although, you'd probably end up seeing small and yellow Cu2O ppt.
R-CHO + 2Cu 2+ + 5OH - -> R-CO2 + Cu2O + 3H2O
For this reagent , the two ingredient solutions should be made, stoppered and stored (NaOH and CuSO4) - they are then mixed to form Fehling's reagent when needed.
Why must the reagent be freshly made? The presence of the tartarate ion is to bond to Cu2+, preventing Cu(OH)2 ppt when two solutions are mixed. But why don't we just use CuSO4 solution then, since it contains free Cu2+? The redox reaction needs a basic medium, thus the NaOH with the sodium potassium tartarate. So the whole point of the reagent is to have free Cu2+ and a basic medium. Now if we mix the two solution together yonks before we actually use the resulting reagent, what would happen? Hmm, it can't be reaction with air, since CuSO4(aq) and the NaOH + tartarate solution would also have the same problem, since they all contain the same Cu2+ and OH- ions. So my guess is either:
1. Cu2O somehow precipitates, and making the reagent useless, or
2. something happens to the tartarate ion if you leave it with Cu2+ for too long, and Cu(OH)2 precipitates. And then when you actually use the reagent, Cu(OH)2, after heating would just decompose to CuO, making the reagent useless anyway
Just a side note, another reason you might see CuO at the end of the test is due to the oxidation of Cu2O to CuO, since Cu2O is only more stable at high temperature, so if you heat slowly, it's no good. Also, Cu(OH)2 decompose faster when heating slowly in water bath, so heating directly is probably better, although, you'd probably end up seeing small and yellow Cu2O ppt.
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abdooooo!!!
Banned
umm... what is this... omg what is wrong you people... who the hell is this Fehling person?
Fehling is the German dude who came up with Fehling's reagent (very similar to Benedict's Solution) used for reducing aldehydes. This reagent is frequently used for medicinal purposes. For example, when you go test for diabetes, you get a sample of urine, and use of Fehling's reagent would tell you whether it's got sugar content (reducing sugars like glucose), and that'd be important info for diagnosing diabetes. Of course it's not the only method
abdooooo!!!
Banned
they are all Germans... scientist that is.
me just realised something... i've done this before. the reducing and non-reducing sugars business, and there is this other guy Tollen, he's got a reagent too lol.
biochemistry seems cool.
me just realised something... i've done this before. the reducing and non-reducing sugars business, and there is this other guy Tollen, he's got a reagent too lol.
biochemistry seems cool.
yeah! hehe, u know what's going on here
haha, benedict's solution is used more often than fehling's, coz it's safer to handle. As for Tollen... hehe, yeah, his reagent, silver mirror test... but like fehling's, this reagent must be made right before it's used, if you leave it for too long, it may be a lil dangerous -> explosive
haha, benedict's solution is used more often than fehling's, coz it's safer to handle. As for Tollen... hehe, yeah, his reagent, silver mirror test... but like fehling's, this reagent must be made right before it's used, if you leave it for too long, it may be a lil dangerous
-> explosive
abdooooo!!!
Banned
explosive? hehehe... i wonder why though? i suspect it would be due to the extreme oxidation properties of the reagents since they both explode... it could be that they oxidise too fast with air so it heats up over time or something and thus causing the eventual explosion... resulting in the death of many valuable chemists
chemistry is the best... all this dangerous and poisonous chemicals involved... is there any data on the age of the average chemist when they die compared to normal people? cause i think chemistry can cause premature death.
chemistry is the best... all this dangerous and poisonous chemicals involved... is there any data on the age of the average chemist when they die compared to normal people? cause i think chemistry can cause premature death.
abdooooo!!!
Banned
me thinks its the same reason as why TNT is explosive as an oxidising agent, since an explosion is a redox reaction that occurs too fast to allow heat to escape.
TNT = VERY very strong oxidising agent = BOMB = EXPLOSION!!!
ammonical silver = slightly weaker oxidising agent = less powerful bomb = tiny explosion!
TNT = VERY very strong oxidising agent = BOMB = EXPLOSION!!!
ammonical silver = slightly weaker oxidising agent = less powerful bomb = tiny explosion!
phenol
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err? how is TNT a strong oxidising agent?
Correct me if I am wrong but I think this rxn is entropy favoured and happens fast partly due to the emission of gas ...
and by your analogy, i am the possessor of ~ 100 grams of potassium dichromate, why haven't they exploded? sorry if it seems that i am having a go at you, this is not my intention
abdooooo!!!
Banned
im not sure about the exact reasons for TNT's explosive properties but i "think" it has to do with its oxidising properties or unstableness, and the presence both N and C because the products that can be formed such as N2, CO, CO2 are relatively stable and have strong bonds. the reason i think potassium dichromate is not explosive because they do not contain N or C. ammonical silver on the other hand contains N and both the other two reagents contain C, i think. so thats why i compared the relative oxidising strength with those two with the idea that they both contained N... whatever man... im just bullcrapping... im totally ignorant so don't feel sorry for having a go at me
hopefully mercury will have the answer
edit: what do you mean by the reaction happens fast partly due to the emission of gas? don't all explosive reaction produce gases? my understanding is that the speed of decomposition primarily depend on how unstable the chemical is, ie the amount of initiating force needed to start the reaction.
hopefully mercury will have the answer
edit: what do you mean by the reaction happens fast partly due to the emission of gas? don't all explosive reaction produce gases? my understanding is that the speed of decomposition primarily depend on how unstable the chemical is, ie the amount of initiating force needed to start the reaction.
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Explosiveness doesn't really have anything to do with redox
Tollen's reagent may cause explosions coz of the precipitation of fulminating silver, which is a black powder that could explode violently in the liquid in which it is formed. So when you see a dark ppt forming from ammoniacal silver solution, danger bell should ring in your head!! Fulminating silver is this complex (diff to silver fulminate, a fulminic acid salt that comes about when u react dissolved nitrate with ethanol) wif formula Ag2O.(NH3)2 that forms when silver oxide dissolves in ammonia. Basically this thing is really unstable and solution containing it could be set off to explode by minor things like sunbeam, friction and other punee things.
TNT is explosive for two main reaons:
1. it contains C, O, N, meaning when it burns, it produces highly stable sbustances with strong bonds, eg. CO, CO2 and N2, so realease a great deal of energy (wrong guess ab.... we wouldn't be here if oxidising reagents are all explosive) Like, most explosives contain lots of N and O. Essentially tho, it's the high velocity at which the energy is released that produces the blast pressure - like phenol said
2. TNT is chemically unstable, the nitro groups packed too closely so they suffer from a lot of strain and hindrance. So it doesn't take a lotta force to break some of these strained bonds and the molecule flies apart (note the high velocity here ) And mind you, TNT doesn't just spontaneously explode... it needs to be detonated by smth like lead azide, Pb(N3)2.
Tollen's reagent may cause explosions coz of the precipitation of fulminating silver, which is a black powder that could explode violently in the liquid in which it is formed. So when you see a dark ppt forming from ammoniacal silver solution, danger bell should ring in your head!!
Fulminating silver is this complex (diff to silver fulminate, a fulminic acid salt that comes about when u react dissolved nitrate with ethanol) wif formula Ag2O.(NH3)2 that forms when silver oxide dissolves in ammonia. Basically this thing is really unstable and solution containing it could be set off to explode by minor things like sunbeam, friction and other punee things. TNT is explosive for two main reaons:
1. it contains C, O, N, meaning when it burns, it produces highly stable sbustances with strong bonds, eg. CO, CO2 and N2, so realease a great deal of energy (wrong guess ab.... we wouldn't be here if oxidising reagents are all explosive
) Like, most explosives contain lots of N and O. Essentially tho, it's the high velocity at which the energy is released that produces the blast pressure - like phenol said2. TNT is chemically unstable, the nitro groups packed too closely so they suffer from a lot of strain and hindrance. So it doesn't take a lotta force to break some of these strained bonds and the molecule flies apart (note the high velocity here
) And mind you, TNT doesn't just spontaneously explode... it needs to be detonated by smth like lead azide, Pb(N3)2.
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phenol
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I think there's some problem with your formula of fulminating silver. Entropy wise, your thing "can" explode but not THAT preferred. Maybe since thermodynamically, Ag wants to go elemental and grabs e- from N3 giving N2 and water or maybe a lil H2? But a lotta bonds must be broken to do this so makes me wonder the activation energy esp for the NH3 bit. (i cannot source this info and correct me if my guesses are wrong)
I did some Googling(http://www.ab.ust.hk/sepo/chem_info/Shock and Explosive Chemical.htm) and found fulminating silver mainly comprised of something like AgN3.
Now this is much more convincing, the N3 - group is known to give off N2 gas in mild explosions (seat bags NaN3 etc) and since Ag is a stable metal, it has good tendency to go elemental in redox. I am guessing down the lines of
AgN3 --shock--> Ag + N2
And again, entropy and N2 bond energy causes the large increase in volume and exothermicity. Rate prolly due to low potential well of AgN3. Again, correct me if I am wrong merc
I did some Googling(http://www.ab.ust.hk/sepo/chem_info/Shock and Explosive Chemical.htm) and found fulminating silver mainly comprised of something like AgN3.
Now this is much more convincing, the N3 - group is known to give off N2 gas in mild explosions (seat bags NaN3 etc) and since Ag is a stable metal, it has good tendency to go elemental in redox. I am guessing down the lines of
AgN3 --shock--> Ag + N2
And again, entropy and N2 bond energy causes the large increase in volume and exothermicity. Rate prolly due to low potential well of AgN3. Again, correct me if I am wrong merc
abdooooo!!!
Banned
why do you need to know? i doubt that asking on a hsc chemistry forum would produce an answer... you should try to find an actual chemistry forum or something.
but i was taught that redox reactions which happens too fast due the unstable nature of the chemicals involved can produce large amounts of heated gase in a short time which expands so quickly that it causes an effect known as chemical explosion... is my understanding flawed?
it doesn't???Originally posted by mercury
Explosiveness doesn't really have anything to do with redox
but i was taught that redox reactions which happens too fast due the unstable nature of the chemicals involved can produce large amounts of heated gase in a short time which expands so quickly that it causes an effect known as chemical explosion... is my understanding flawed?
abdooooo!!!
Banned
umm... i kinda realised that... at least im not completely wrong.