In the previous blogpost, we have discussed on the 2nd criterion for predicting products of electrolysis for aqueous solutions under the Series on “Secrets of Redox and Electrolysis“.

Today we will end the series by introducing the 3rd criterion:

Consider: Nature of Electrodes Used in Electrolysis

Different products are discharged at the electrode when different type of electrodes are used.
When we say different types of electrodes used, it simply means:

Inert Electrode (such as Carbon and Platinum) vs Reactive Electrode (such as Cu or Silver)

We shall once again look at it in a step-by-step simplified approach.

Lets consider 2 electrolysis reactions using Copper(II) Sulphate as the electrolyte.

Set-Up A: Inert Electrodes such as Platinum

At the +ve Anode:

• 4OH-(aq) –> 2H2O(l) + O2(g) + 4e-
• SO42- ions remain in solution

At the –ve Cathode:

• Cu2+(aq) + 2e- –> Cu(l)
• H+ ions remain in solution

Summary:

• Overall reaction is: 2CuSO4(aq) + 2H2O(l) –> 2Cu(s) + O2(g) + 2H2SO4(aq)
• Acidic solution of H2SO4(aq) formed at end of reaction

Set-Up B: Reactive Electrodes such as Copper

At the +ve Anode:

• Cu(s) –> Cu2+(aq) + 2e-
• Anode dissolves to form Cu2+ ions in aqueous solution

At the –ve Cathode:

• Cu2+(aq) + 2e- –> Cu(l)
• H+ ions remain in solution
• Cathode becomes coated with a layer of reddish-brown Cu

Summary:

• Cu is transferred from the anode to the cathode
• Cu cathode slowly increases in mass
• Cu anode decreases in mass
• Colour & Concentration of CuSO4 remain unchanged
• No effective loss of Cu2+ ions from the CuSO4 solution

Let’s check out 1 question to see how much you understand.

Question:
Aqueous copper(II) sulphate is electrolyzed using copper electrodes. Which observations will be made?
Anode (+ve) Cathode (-ve) the electrolyte
A. anode dissolves           pink solid forms          blue colour fades
B. anode dissolves           pink solid forms           no change
C. colourless gas forms   colourless gas forms    no change
D. colourless gas forms   pink solid forms blue   colour fades

PS: Remember to leave down your answers in the “Leave A Reply” section below.

PPS: To learn everthing on Redox & Electrolysis, please visit the section right below called “Table of contents for Secrets of Redox & Electrolysis“

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”

PPS: I am launching a book called “Up Your Chemistry Grades Now!” in ebook version before i send it to publisher for the hardcopy book to be available in majoy bookstores.  As it takes sometime for the hardcopy book to be published, i want to ensure graduating students sitting for major examination soon, will have a chance to get hold of this wonderful book that i have compiled over 10 years of Chemistry Coaching. It includes all the Common Errors and Misconceptions that many students made batch after batch, and the Correct Essential concepts you need to know .  Also included are the Definitions that you need to know in O Level Chemistry Exams and also Tips on what the Examiners are Looking For. If you studying very hard for your Chemistry but yet to score the marks that you truly deserve, make sure you are the first to have your hand on it. You can sign-up for more information on the launch of this wonderful book here.

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

For the next 1 week or so, i will be writing and discussing on the basic essentials concepts for Redox & Electrolysis.

Starting with this blog-post i will kick start a Series knows as “Secrets of Redox & Electrolysis“, where we will again discussed on some exam-based questions to see how much we have learned.

For local students taking GCE ‘O’ Level Examinations, these is what you need to know academically:

• Pure Chemistry Students – Redox Chapter & Electrolysis Chapters
• Combine Science (Chemistry) Students – Redox Chapter only

Based on the SEAB syllabus requirement, you will need to know the following learning outcomes:

Redox:

• Define Oxidation & Reduction (Redox) in terms of oxygen/hydrogen gain/loss
• Define Redox in terms of electron transfer & changes in oxidation state
• Identify Redox reactions in terms of oxygen/hydrogen gain/loss, electron gain/loss & changes in oxidation state
• Aqueous potassium iodide & acidified potassium dichromate(VI) in testing for Oxidising & Reducing agents

Electrolysis:

• Define Electrolysis
• Apply the idea of Selective Discharge based on

(i) Cations: linked to the metal reactivity series
(ii) Anions: halides, hydroxides & sulphates
(iii) Concentration effects (electrolysis of conc. & dilute aqueous NaCl)

• Predict the likely products of the electrolysis of a molten binary compound such as NaCl
• Predict the likely products of the electrolysis of an aqueous electrolyte (dilute & conc. solutions)
• Predict the likely products of the electrolysis of water
• Construct ionic equations for the reactions occurring at the electrodes during the electrolysis
• Electrolysis of aqueous copper(II) sulphate with copper electrodes as a means of purifying copper
• Electroplating of metals, e.g. copper plating, and its uses
• Simple Cells & the production of electrical energy

And these are two chapters that have about 20 marks in your GCE ‘O’ Level Chemistry Examinations – which also means you need to know them! No way for you to skip them!

Coming soon in next blog-posts…………..Definition of Reduction, Oxidation, Redox Reactions & Exam-based questions

Stay Tuned!!!