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Author Topic: What are displacement reactions?  (Read 12702 times)

Offline Seany

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What are displacement reactions?
« on: 05/05/2007 23:56:27 »
In experiments, where you put Iron into Copper Sulphate.. Then the Iron displaces the Copper.

How come, for vigorous experiments such as Magnesium and Copper Sulphate, there are many gases given off? If it is just a "displacement" shouldn't there be no bubbles or gases?
« Last Edit: 19/05/2007 11:53:17 by chris »


 

Offline daveshorts

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Re: What are displacement reactions?
« Reply #1 on: 06/05/2007 01:36:39 »
It all depends on what you are displacing. In water copper sulphate CuSO4 splits up into
Cu2+ and SO42-
the copper has given the sulphate 2 electrons

if you add iron which is more reactive than the copper so it will give it some electrons dissolving the iron and turning the copper back to metal.
Cu2+ + Fe -> Cu + Fe2+

If you add magnesium a similar reaction will take place
Mg2+ + Fe -> Cu + Mg2+

However there also water in your solution - H2O will sometimes naturally split into H+ and OH-

H2O reversible arrow H+ + OH-

Now iron isn't reactive enough to substitute for the H+ but magnesium is

Mg + 2H+ -> Mg2+ + H2

the hydrogen will be released as bubbles because it is a gas
 

Offline abdillah

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What are displacement reactions?
« Reply #2 on: 22/08/2007 03:36:23 »
Interesting, i don't see any other possibilities for the gas. However, the bubbles are seen to be forming quite readily which is surprising given that if you conduct an experiment with just magnesium and water, there are no bubbles observed.

Anyway, we can test the gas by collecting it and testing it. Has anyone tried?

So why is it that the rate of the evolution of this gas is quite fast?
 

Offline Cut Chemist

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What are displacement reactions?
« Reply #3 on: 22/08/2007 06:54:27 »
Could it be from a lowering of the pH??

When copper sulfate dissolves it normally drops the pH of the solution. 
Also some of those electrolysis experiments also include sulfuric acid in the solution.

Mg + 2H+ -> Mg2+ + H2

More H+ ions in solution would make this reaction more likely to occur.
 

Offline lightarrow

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What are displacement reactions?
« Reply #4 on: 22/08/2007 11:52:23 »
Could it be from a lowering of the pH??

When copper sulfate dissolves it normally drops the pH of the solution. 
Also some of those electrolysis experiments also include sulfuric acid in the solution.

Mg + 2H+ -> Mg2+ + H2

More H+ ions in solution would make this reaction more likely to occur.
Maybe. Which is the pH of a 1M CuSO4 solution? Also is possible that Cu++ or Cu or both acts as catalyzer for the reaction you have written (Cu is often used as catalyzer in reductions with metals).
 

Offline Cut Chemist

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What are displacement reactions?
« Reply #5 on: 22/08/2007 19:57:16 »
I have a 0.100 M solution and the pH is 4.2

I think Cu++ will also form Cu(OH)2 in aqueous solution.
This would explain why CuSO4 decreases the pH. 
(At high pH's Cu(OH)2 precipitates out of solution.)


Could Cu(OH)2 act as a catalyst?
« Last Edit: 22/08/2007 20:09:24 by Cut Chemist »
 

Offline abdillah

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What are displacement reactions?
« Reply #6 on: 24/08/2007 03:32:32 »
hmmm... that is plausible. Lowering the pH does make it more possible for the magnesium to react. I guess in other displacement reactions with more reactive metals like zinc, theres' the appearance of bubbles.

My other question is when I did an experiment involving copper (II) nitrate and magnesium, after taking out the magnesium metal, there are some blue deposits on the metal amongst the black/dark reddish brown copper metal displaced.

What could be this blue deposit? it must be a copper compound. but what? Cut chemist did u mean Cu(OH)2 precipitate out at low pH or high pH? becos pH 4.2 is low rite as u mentioned?

hmmm, second observation, i did this experiment with my class, when I added magnesium to copper(II)nitrate, one would expect the blue colour of copper (II) nitrate solution to fade and become colourless in the end. I did not do the reaction to completion, however it was observed that the blue colour became green. Is there any reason to it?
« Last Edit: 24/08/2007 10:02:28 by abdillah »
 

Offline Cut Chemist

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What are displacement reactions?
« Reply #7 on: 25/08/2007 05:03:23 »
Cu(OH)2 precipitates out of solution at a pH >7 (in alkaline solutions.)
Yes, pH 4.2 is a relatively low pH, but only slightly acidic.  There still might be some precipitation if the concentration is high enough.

Copper turns green, blue and/or black when it is oxidized. 

Cu(OH)2 is blue  (forms from Cu2+ ions)
CuO is black  (forms from Cu2+ ions)
Cu2O is green  (forms from Cu1+ ions)

There are many other copper hydrate compounds that are blue or green. 
I believe these normally contain carbonate or sulfate.

I don't know if copper forms compounds with nitrate.
« Last Edit: 25/08/2007 05:06:32 by Cut Chemist »
 

Offline lightarrow

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What are displacement reactions?
« Reply #8 on: 25/08/2007 14:26:10 »
Cu2O is green  (forms from Cu1+ ions)
No, it's brick-red.
http://en.wikipedia.org/wiki/Copper%28I%29_oxide
Quote
I don't know if copper forms compounds with nitrate.
You mean insoluble compounds? It's unprobable.
 

Offline Bored chemist

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What are displacement reactions?
« Reply #9 on: 26/08/2007 12:50:16 »
Copper nitrate is well documented as overtime pay for policemen.
(sorry; old joke)
The anhydrous form is unusual because it's a fairly volatile salt of a transition metal. The hydrated salt (which is rather more common) is a nice blue colour. It's very soluble in water.
I doubt that there's a copper (I) nitrate since Cu(I) is a srong reducing agent and nitrate is strongly oxidising.
 

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What are displacement reactions?
« Reply #9 on: 26/08/2007 12:50:16 »

 

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