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Topic: metal ions ag+ cu2+ zn+ being separated experiment  (Read 609 times)

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Offline Helly

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metal ions ag+ cu2+ zn+ being separated experiment
« on: January 18, 2020, 10:32:00 AM »
metal ions ag+ cu2+ zn+ being separated
1. adding HCl, what color the precipitates? and what is the precipitates?
the answer is black , agcl. why the one precipitates is ag+? while ag+ has higher potential reduction but the one gets oxidized? and how do i know exactly the color of precipitates?
2. the supernatant obtained from experiment 1., (a) was bubbled in to form (b) precipitates. the answer a = h2s, b = black, but how do i know what was bubbled and the color of the precipitates?
3. the supernatant obtained from 2, base was bubbled in to form white precipitates, the answer is h2s.

Offline AWK

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Re: metal ions ag+ cu2+ zn+ being separated experiment
« Reply #1 on: January 18, 2020, 11:04:45 AM »
You need to read about the qualitative analysis of cations.

In addition, AgCl is a white solid, under the influence of sunlight or UV radiation, it turns gray-blue due to photochemical decay to metallic silver. Only specially prepared fine-grained AgCl can give black color after decomposition (see the black and white photographic film).

https://www.chemix-chemistry-software.com/images/solubility/precipitate-colors.gif
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Offline Helly

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Re: metal ions ag+ cu2+ zn+ being separated experiment
« Reply #2 on: January 18, 2020, 12:40:01 PM »
From bubble in h2s in 2. , we go to 3. It says added base, but why also bubble in h2s again and form white precipitates?

Offline AWK

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Re: metal ions ag+ cu2+ zn+ being separated experiment
« Reply #3 on: January 18, 2020, 12:45:03 PM »
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You need to read about the qualitative analysis of cations.
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Offline Helly

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Offline AWK

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Re: metal ions ag+ cu2+ zn+ being separated experiment
« Reply #5 on: January 18, 2020, 01:12:49 PM »
This is the standard procedure for separating cations into groups. The supernatant after group II is alkalized with ammonia and then H2S is added (bubbled) - this forms (NH4)2S. Alkalization of NaOH is inadvisable because Al3+ may not precipitate in group III.
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Offline Helly

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Re: metal ions ag+ cu2+ zn+ being separated experiment
« Reply #6 on: January 18, 2020, 10:59:15 PM »
Ok. But there is no al3+ in the question?
What about steps 3? We want to precipitate Fe, why bubble in H2S ?

Offline AWK

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Re: metal ions ag+ cu2+ zn+ being separated experiment
« Reply #7 on: January 19, 2020, 01:46:33 AM »
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Ok. But there is no al3+ in the question?

Students may receive a mixture containing Al3+ and may make a mistake according to this scheme (only in the case of Al3+).

Quote
What about steps 3? We want to precipitate Fe, why bubble in H2S ?
Now iron has appeared? Earlier it was zinc.

Can you answer the question - what should be the concentration of S2- ions in the solution to precipitate Cu2+ and Zn2+ cations from the solution with concentrations, e.g. 0.1 M.? What must be the pH of the solution in order to obtain such a concentration of sulfide ions if we introduce hydrogen sulfide gas into the solution? Assume the maximum concentration of hydrogen sulfide in the acidic medium as 0.05 M.
If you can (and you have to calculate it) then you will understand the idea behind this scheme (for cations of groups II and III.
Otherwise, you have to memorize that.
Note that this scheme was created over 150 years ago and its creators did not understand the rules associated with it then. It was simply an experience of many chemists.
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Offline Helly

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Re: metal ions ag+ cu2+ zn+ being separated experiment
« Reply #8 on: January 19, 2020, 09:30:57 AM »
We need 0.2 M H2S ? For 0.1 M Cu2+ and 0.1 M Zn2+.

So in step 3 we add h2s again to make sure if theres still cation group 2?
Btw, what is the white percipitate from step 3 when we add h2s? There is left zn only in step 3 right? Since ag and cu been precipitate?

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