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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We have learned that cations are attracted towards the -ve cathode and being discharged, and the anions are attracted towards the +ve anode and being discharged. Questions with Molten Electrolyte will then be easy to solve since there is only one type of cations and one type of anions.

Try this questions out to see how much you understand.

Question 1:

(i) Name the ions present in molten copper (II) bromide and the products at the electrodes.

(ii) Write equations, including states symbols, to represent the reaction at the cathode and the anode.

(iii) Finally, write the overall reaction that occurs when molten copper(II) bromide is electrolysed.

PS: Leave your suggested answers in the “Comments Section” below. We will discuss on it very soon.

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ELECTROLYSIS – This must be one of the most hatred topic on basic Chemistry for many secondary school (local) and high school (regional and western countries) students.

Many find this topic very confusing, especially when application questions are asked in exams. This topic is closely related to REDOX chapter, and yes, if you are confused about REDOX, this is going to be a tought chapter for you. As such, before you try to learn about Electrolysis, do have a good grasp of REDOX first. You can visit our previous blog-posts on Redox by visitng those links shown all the way below.

Let’s start by defining Electrolysis.

Electrolysis à process of using electricity to break down(or decompose) a compound

Takes place in an Electrolytic Cell that has 3 components:
1. Battery
2. Electrodes
3. Electrolyte

Process of Electrolysis involves 3 things:
1. External Circuit
2. Reactions within the Electrolyte
3. Reactions at the surface of the electrodes

When electricity is passed through an electrolyte, chemical reactions take place at the electrodes and electrolyte is decomposed.

Electrolytes can conduct electricity because they contain charged particles (i.e. ions) that are mobile.

Process of gaining or losing electrons at the electrodes is called Discharge. When ions are discharged at the electrodes, they form atoms or molecules.

Redox reactions take place at the electrodes during electrolysis.
• @ Cathode, reduction takes place because cations gains electrons
• @ Anode, oxidation takes place because anions loses electrons

Let’s check out one exam-based questions to see how much you have learned:

Quick Check 1:
What is the equation for the reaction that takes place at the negative electrode (cathode) during the electrolysis of molten calcium chloride?
A. Ca2+ + 2e- –> Ca B. Ca –> Ca2+ + 2e-
C. Ca2+ + e- –> Ca+ D. 2Cl- –> Cl2 + 2e-

PS:Do try out the question & show your suggested answers in ”Comments Section” below

In the chapter on Redox Reactions, the most common errors that are made by students are on Oxidising & Reducing Agents.

Let’s take a look at these agents.

Do note again that Oxidising Agents are substances that Oxidise another substance & Reducing Agents are substances that Reduces another substance.

If you are still not sure about the above, always remember this analogy:

Agents always help another party

i.e. Property Agents – help another party to get property

i.e. Insurance Agents – help another party to get insurance coverage

i.e. Oxidising Agents – help another substance to get oxidised

i.e. Reducing Agents – help another substance to get reduced

and in the process of helping another substance, they themselves are changed in the opposite way.

i.e. Oxidising Agents – are reduced @ end of redox reactions

i.e. Reducing Agents – are oxidised @ end of redox reactions