Electrochemistry- to what extent we can consider a solution is concentrated?

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[Tay Guan Piang]

To all Chemistry teachers,
May i know when we study the second factor of the selectivity of electrode when dealing with concentrated solution, to what extent that we can consider when considering a concentrated solution? Got students asked me whether a solution with the concentration of 2.0 mol dm^-3 is considered concentrated?

How should i clarify this matter? Any sharing is very much appreciated. Thanks.

[Lim Tze Khai]

In chemistry, 2 mol dm-3 is consider as concentrated solution in general.

But to explain the concentration factor in electrolysis, we don't use concentrated solution but higher concentration.

As we did the experiment, Cl- ions were selected for discharge when the concentration is higher then OH-, the concentration were about 0.5 mol dm-3.

So explain to student without using concentrated solution but higher concentration of halide io ns.
Hope can help.

[Tay Guan Piang]

Oh, thanks a lot for the information, Mr. Lim. Your sharing is very much appreciated.

[Sim Thiam Huat]

Mr. Lim
    sorry the issue might be overdued. But what about 0.001 M and 1.0 M HCl in electrolysis. Will there be any difference in the products at the electrodes. I consider both solutions as dilute enough to give the same products

[Jong Kah Yin]

For electrolysis of aqueous solution, the factor of concentration of solution will affect the product at cathode and anode but for SPM syllabus, focus on the product at anode only. this factor is only applicable for circumstances involving Cl^-/Br^-/ I^-. For this factor of concentration, 0.001 mole dm^-3 is considered dilute and any concentration higher than this is considered concentrated. The best way is to try out the experiment in the lab.
For 1 mole dm^-3 HCl, one can easily get chlorine gas at anode.
Hope I answer your question.

[Tay Guan Piang]

For electrolysis of aqueous solution, the factor of concentration of solution will affect the product at cathode and anode but for SPM syllabus, focus on the product at anode only. this factor is only applicable for circumstances involving Cl^-/Br^-/ I^-. For this factor of concentration, 0.001 mole dm^-3 is considered dilute and any concentration higher than this is considered concentrated. The best way is to try out the experiment in the lab.
For 1 mole dm^-3 HCl, one can easily get chlorine gas at anode.
Hope I answer your question.

Mr. Jong, factor of concentration discussed so far in SPM level is more focus on anion, but what about the cation? I wonder if Mr. Jong can share more about the product formed at cathode when we study electrolysis of concentrated solution? Thanks.

[Liew Hui Lee]

Just for sharing, during my lesson, I usually tell my students to consider the concentration is the factor that affecting the selectively discharged of ions, the concentration of the solution must be 1.0 mol dm^-3 or more than 1.0 mol dm^-3.  For anions, we only consider the halides ions, while for cation, we only consider the lead(II) ion, Pb²⁺.

Examples :  1.  Electrolysis of 1.0 mol dm^-3 of lead(II)
                     nitrate solution using carbon electrodes
                     - Selectively-discharged cation : Pb²⁺ due to the 
                       higher concentration compared to H⁺
                     - Selectively-discharged anion : OH^- due to the 
                       lower position in the ECS compared to NO₃^-

                2.  Electrolysis of saturated sodium chloride solution using
                     carbon electrodes
                     - Selectively-discharged cation : H⁺ due to the 
                       lower position in the ECS compared to Na⁺
                     - Selectively-discharged anion : Cl^- due to the 
                       higher concentration compared to OH^-

[Jong Kah Yin]

As for Mr. Tay's question, concentration of ions plays a role in determining the products at cathode but for SPM, the concern is for anode only.
I did try out experimentally the effect of concentration of ions at cathode. Those metal ions near to H⁺ will be discharged if it is of higher concentration - the effect can be seen for ions as high as Fe²⁺ ions.One can see the greyish metal formed at cathode.

[Tay Guan Piang]

Oh, thanks a lot, Mr. Jong and Mdm. Liew. Your sharing is kindly appreciated. 

[Faridah Esa]

Regarding Cikgu Tay's question about the cations that will be discharged in the concentrated electrolyte used although usually it will not be asked in the SPM, one of my student did asked me the same question.I still could not get the answer from other chemistry teachers.I came across an answer during surfing the internet saying that all the cations above zinc in the electrochemical series will not be discharged if a  concentrated electrolyte is used but hydrogen gas will be produced.And thanks Mr. Jong for answering the question about to what extent an electrolyte is considered as concentrated, that means any concentration of electrolytes more than 0.001 M is considered as concentrated rite?

[Ahmad Maulana Bin Mohamed Jadit]

Hello tay, below 1.0 M usually considered as dilute, above 1.0 M considered concentrated already. For cations, factor of concentration does not affect product accept for Lead.

[Tay Guan Piang]

Thanks a lot, Cgu Maulana. Your sharing is kindly appreciated.

[Iskunadi Bin Liap]

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