We have discussed on the strategies to predict the products of electrolysis of molten compounds as electrolytes in the earlier post.
Today, we are going to examine the Strategies to Predict the Products of Electrolysis for Aqueous Solutions.
For aqueous solutions, there is more than 1 type of cations and anions, since water is present.
Water will contribute to H+ and OH- ions in addition to the ionic compound that is present.
When more than one type of cations or anions are present in a solution, only 1 cation & 1 anion is preferentially selected for discharge.
The selectivity discharge of ions depends on 3 factors:
1. Position of metal in the reactivity series
2. Concentration of the ion in the electrolyte
3. Nature of the electrode used
Let’s check out on the 1st Criterion to consider – Position of Metal in Reactivity Series
Ease of Discharge of Cations Increases down the list:
Potassium Ion, K+
Sodium Ion, Na+
Calcium Ion, Ca2+
Magnesium Ion, Mg2+
Aluminium Ion, Al3+
Zinc Ion, Zn2+
Iron Ion, Fe2+
Lead Ion, Pb2+
Hydrogen Ion, H+
Copper Ion, Cu2+
Silver Ion, Ag+
Ease of Discharge of Anions Increases down the list:
Sulphate Ion, SO42- (Do not discharge)
Nitrate Ion, NO3- (Do not discharge)
Chloride Ion, Cl-
Bromide Ion, Br-
Iodide Ion, I-
Hydroxide Ion, OH-
As such, Cu2+ is preferentially discharged @ the -ve cathode, if we have Cu2+, H+ and Fe2+ in aqueous solutions.
And for anions in aqueous solutions, we will have OH- ions preferentially discharged over NO3- and Cl- if we have all of them in the solutions.
So far so good? Let’s check one of question:
Question 1:
If we have a soluton of dilute aqueous NaCl solution and inert platinum electrode is being used,
(i) What ions are present in the electrolyte?
(ii) What are the products @ the cathode and the anode?
PS: Leave us your suggested answers directly below this post at “Leave Your Reply” section.
I hope you have been learning something so far. =))
Up next, we will visit the 2nd Criterion for predicting the products of electrolysis:
“Concentration of Aqueous Solutions”
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- O Level Chemistry – Secrets of Redox & Electrolysis Series
- O Level Chemistry – Definitions of Electrolysis
- O Level Chemistry – Products of Electrolysis of Molten Compound
Hello,Sean.
I got a simple question for you….
In an electrochemical cell,which electrode is positive??..which is negative??..which ion is positive??.. which is negative??.
(i)The species in the dilute solution of NaCl(aq)are Na+ Cl- H+ OH- and H2O.
(ii)predict the product at each electrode
the oxidation reaction can occur at Anode
4OH-(aq) –>2H2O(l)+O2 (g)+4e- (1) Eo = -0.40 V
2Cl-(aq) –>Cl2 (aq)+ 2e- (2) Eo = -1.36 V
2H2O (l) –>4H+(aq)+O2(g) +4e- (3) Eo = -1.23 V
according to the potential values we predict that reaction(1)will be occur first but [OH-]too low in neutral soution thus the reaction (3) is the best answer.
the reduction reaction can occur at Cathode
Na+(aq) +e- –> Na (s) (4) Eo= -2.71 V
2H+(aq) +2e- –>H2 (g) (5) Eo = 0.00 V
2H2O(l) +2e- –>H2 (g)+2OH-(aq) (6) E0 = -0.83 V
like the anode the reaction (3)is the best answer because of low concentration of H+ ion.
thanks for reply me
ions: H+,cl-,Na+,OH-
cathode: cl2
anode: h2
It really depends if you’re talking about molten copper chloride or in aqueous copper chloride, but since you didn’t mention, I’d assume it’s molten copper chloride.
The ions involved are only Cu2+ and Cl2-.
Metals are formed at the negative electrode (cathode),
therefore Cu2+ should be formed at the cathode, w
whereas Cl2- should be formed at the anode.
The ion present in the electrolyte is Na+,Cl2-,H+ and OH-.2) the product at the cathode nd anode is H+ at the cathode nd OH- at the anode
U hav cleared my basic concepts, CUD U PLEASE TELME THE PRODUCTS OF ELECTROLYSIS OF Cucl2 WITH PT ELECTRODE in detail
one more thing can any one please suggest me a way for quick revision
and how to make isomers
hey there is a best and easy way to learn reactivity series….it is like this
please =potassium
send =sodium
cats =calcium
monkeys =magnesium
and =aluminium
zebras =zinc
in =iron
large =lead
huge =hydrogen
cages =cooper
make =mercury
sure =silver
padlocked =platinum
if aqueous potassium bromide was electrolysed which ion would move to the anode?? is it OH- or bromide? if bromide does it mean that bromide is concentrated since OH is lower in the series and it should be the one discharged.
Hi Sean,
I refer to your reply to Jasmine above. If OH- is at the bottom of the electrochemical series and we’re talking about a dilute sodium chloride solution, oxygen gas is produced at the anode as a result of the position of ions in the electrochemical series, but if it is concentrated sodium chloride, then chlorine gas is produced as a result of the concentration factor.
That aside, with reference to the electrochemical series for anions, whereby F- is at the top and OH- at the bottom, isn’t the anions at the bottom more reactive hence easier to discharge in comparison to the anions above? So wouldn’t OH- be more reactive than Cl-, hence the preference for OH- to be discharged (assuming dilute solution)? If something is not reactive, I assume that it the probability of it being discharged (transformed to something else) is smaller compared to one that is more reactive.
I am getting contradicting information that the anions further up the series are more reactive than the ones at the bottom. I agree that F is reactive, that’s why it becomes F- which is more stable, so to say that F- is reactive is not accurate is it?
Look forward to your reply.
Thank you!
Hi, would like to ask the products formed at both cathode and anode for (i) lead nitrate solution and (ii) Nickel sulfate solution?
For lead nitrate solution,
Is it true that cathode : lead metal is formed instead of hydrogen gas being produced? i thought according to the position of metals in reactivity series, lead ions is above ‘hydrogen’?
As for nickel sulfate solution,
Is it true as well that cathode : nickel metal is formed instead of hydrogen gas being produced? i thought according to the position of metals in reactivity series, nickel ions is above ‘hydrogen’ as well?
Thanks for the encouragement KS!
All The Best!
btw miranda… add HCl also can… cuz the ppt formed from lead ion is insoluble…
aluminium chloride is soluble..
sorry again!! haha but i got another question.. sorry for posting three comments in a row… i am not spamming haha!
ok my question is, the electrolysis with inert electrode, electrolyte being potassium nitrate, then what is the overall or final equation of the electrolysis?
do i put it as :
4H+ + 4OH- ——> 2H2 + 2H2O + O2
or
4 H2O ——-> 2H2 + 2H2O + O2
which one is it? or is it both correct? or both wrong?
and thanks again!
ah.. sorry but i do not know how to edit my previous post.. but i got a question to add on..
my question is that if there is an electrolysis reaction with inert electrode and the electrolyte is concentrated aqueous potassium nitrate, what would be the ions that will be discharged at the cathode and the anode?
my focus is on the anode, since it is concentrated, by right the nitrate ion will be discharged, but i see the information above, you put that nitrate ion (Do Not Discharge), meaning even if the electrolyte is concentrated, hydroxide ion would be discharged instead?
please clear my doubts and thanks a lot!
hey! i was looking for the reactivity series of the electrolysis thing( i dun really know wat is it called, the one with ion).. and i found your website…
just posting to tell u that u had been a great help for us students.. keep it up!!
and i like ur website… really thanks!
Cheers
KS
Hi Miranda,
Yes! Of course i do cos it is covered under the chapter: Qualitative Analysis or commonly also known as Chemical Analysis.
Both Lead and Aluminum ions are Cations (+ve ions).
We can differentiate them using Sodium Hydroxide or Aqueous Ammonia.
Do you know the colour/state change when we add either Sodium Hydroxide or Aqueous Ammonia to Lead or Aluminum ions???
PS: Hear from you.
PPS: Anyone out there would like to contribute to this question?
Cheers,
Sean Chua
Master Trainer @ SimpleChemConcepts.com
Author of Up Your Chemistry Grades Now! book
Hi! I was just wondering if you knew how to differentiate between lead and aluminum ions?
Hi Jasmine,
You are spot on!
In the blogpost, i have share with you that the products of electrolysis are determined by:
1. Position of metal in the reactivity series
2. Concentration of the ion in the electrolyte
3. Nature of the electrode used
Concentration of Ions is an important factor to consider when it comes to predicting the products of electrolysis.
In this case, although Cl- is more reactive to be discharged at the +ve cathode, OH- ions is present in higher concentration (since DILUTE SOLUTIONS) & will be selectively discharged at the platinum inert cathode.
Hope this helps in getting your Chemistry Concepts on the right track before your major chemistry exams.
All The Best to Your Academic Success in Chemistry,
Sean
Master Trainer
when is Cl- discharged at the anode? Is it when the solution is concentrated instead of dilute whereby the ion with higher concentration is discharged at the anode?
Hi Jasmine,
Good try.
Part (i) is correct. For aqueous salt solutions, there are H+ and OH- bsides the Na+ and Cl- ions.
For Part (ii), do note that @ Anode, Oxygen gas is produced.
This is because OH- ions are preferentially discharged over Cl- ions.
Hope this helps.
PS: I am launching the ebook ?Up Your Chemistry Grades Nows!? before it is being published into hardcopy. There is a major section that have a comprehensive discussion of major / common mistakes made by students.
To your Academic Success in Chemistry,
Sean Chua
Master Trainer
http://www.SimpleChemConcepts.com
i)Na+ , H+ , Cl- , OH-
ii) Cathode : Hydrogen gas
Anode : Chlorine gas