2018 O-Level Pure Chemistry MCQ Paper 1 Answers (Subject code: 6092)

1.	B
2.	B
3.	C
4.	D
5.	A
6.	A
7.	B
8.	C
9.	D
10.	B
11.	C
12.	C
13.	B
14.	B
15.	A
16.	D
17.	C
18.	D
19.	B
20.	A
21.	B
22.	A
23.	A
24.	A
25.	D
26.	B
27.	D
28.	D
29.	A
30.	D
31.	C
32.	D
33.	A
34.	D
35.	B
36.	B
37.	D
38.	C
39.	C
40.	C

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O-Level Chemistry: Selective Discharge of Ions in Electrolytic Cells – Different Sodium Chloride Electrolytes

In this blog post, we take a look at the difference in the reactions that occur and the products obtained when different types of electrolytes are used. We will use inert platinum electrodes and 3 different sodium chloride electrolytes for the discussion.
The electrolytes are:

Molten Sodium Chloride
Dilute Sodium Chloride Solution
Concentrated Sodium Chloride Solution

The 3 factors involved in the selective discharge of ions with inert electrodes:

Selective Discharge of Cations
Selective Discharge of Anions
Effect of Concentration on the Selective Discharge of Anions

We have gone through the basic concepts involved previously in another blog post. If you have missed out on it, you should go and watch it first.

Note that the use of reactive electrodes will make the reactions more complicated. You can take a look at my other video to learn them also.

All the video tutorials are based on what we share with our Secondary 4 GCE O-Level Pure Chemistry and IP Chemistry students in our weekly Sec 4 Pure Chemistry Tuition Classes.

YouTube Video Tutorial on Selective Discharge of Ions in Electrolytic Cells – Different Sodium Chloride Electrolytes

In the YouTube Video below, you will learn in a step-by-step manner on how different reactions will occur at the electrodes when different sodium chloride electrolytes are being electrolysed in an electrolytic cell.

Length of Video: 9.28 minutes

Direct Video Link: https://youtu.be/1QdDTb-CGuk

I hope you find the content easy for your understanding and if you have anything valuable to add, leave me a comment below.

Feel free to share this blog post with your friends.

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PS: Under related articles below, there are several blog post discussions and questions related to Electrolysis concepts.?You can also do a keyword search using the search box at the top right hand corner. Enjoy learning!

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O-Level Chemistry: Selective Discharge of Ions in Electrolytic Cells | Part 2: Nature of Electrodes

Selective Discharge of Ions in Electrolytic Cells - Nature of Electrodes — Photo Credit: Andrew Adams (Creative Common)

In the previous blog post on Selective Discharge of Ions and Reactivity Series – Part 1, we have looked at the 3 factors on how ions in the electrolyte are preferentially being discharged based on (1)Selective Discharge of Cations & Anions and (2)Concentration Effect of Anions. Today, we will look at the last factor known as Nature of Electrodes when it comes to selective discharge of ions in electrolysis.

We will compare the electrolysis of copper (II) sulfate solution using two different types of electrodes: Inert Electrodes and Reactive Electrodes.

Nature of Electrodes: (A) INERT versus (B) REACTIVE

(A) Using Inert Electrodes e.g. Platinum or Graphite electrodes

At the anode:

Hydroxide and sulfate ions are attracted to the positive anode
Hydroxide ions are preferentially discharged to liberate oxygen gas
Sulfate ions are not discharged and remains in the electrolyte solution

At the cathode:

Hydrogen and copper (II) ions are attracted to the negative cathode
Copper is lower than hydrogen in the reactivity series
Thus, copper (II) ions are preferentially discharged
Reddish-brown copper metal is deposited at the cathode

In the electrolyte solution:

Blue colour of copper (II) sulfate solution gradually becomes lighter
Remaining hydrogen and sulfate ions combine to form sulfuric acid
Resulting electrolyte becomes increasingly more acidic

(B) Using Reactive Electrodes e.g. Copper electrodes

At the anode:

Hydroxide and sulfate ions are attracted to the positive anode
We would expect hydroxide ions to be preferentially discharged over sulfate ions based on selective discharge of anions
However, copper is a reactive electrode and itself will be preferentially discharged (oxidised) to form copper (II) ions
Cu(s) => Cu²⁺(aq) + 2e^–
Anode will decrease in size and mass

At the cathode:

Hydrogen and copper (II) ions are attracted to the negative cathode
Copper is lower than hydrogen in the reactivity series
Thus, copper (II) ions are preferentially discharged
Reddish-brown copper metal is deposited at the cathode

In the electrolyte solution:

There is a net transfer of copper (II) ions from the anode to the cathode
There is no net loss of copper (II) ions from the copper (II) sulfate solution
Blue colour and concentration of copper (II) sulfate solution remain unchanged

The table below further summarises the difference in products obtained from the electrolysis of copper (II) sulfate solution using inert (platinum or graphite) electrodes and reactive (copper) electrodes.

This is an important concept in Electrolysis. We always ensure that O-Level and IP Chemistry students in our recommended weekly Pure Chemistry Tuition Classes know it well. If you are still unsure about it, you can take a look at the video below.YouTube Video Tutorial on Selective Discharge of Ions in Electrolytic Cells – Part 2: Nature of Electrodes

In the YouTube Video below, you will learn in a step-by-step manner on how Nature of Electrodes will determine the selective discharge of ions in Electrolysis.

Length of Video: 11.59 minutes

Direct Video Link:https://youtu.be/njX27H_tZc8

I hope you find the content easy for your understanding and if you have anything valuable to add, leave me a comment below.Feel free to share this blog post with your friends.In the next blog post, i will be sharing a video on Why Different Sodium Chloride Electrolytes will give you Different Products when Inert Electrodes are used in Electrolytic Cells. Do stay tuned to it!

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PS: Under related articles below, there are several blog post discussions and questions related to Electrolysis concepts. You can also do a keyword search using the search box at the top right hand corner. Enjoy learning!

Electrode	Cathode	Anode	Reaction at Cathode	Reaction at Anode
Copper (II) sulfate solution	Platinum	Platinum	Reddish-brown copper metal is deposited. Cathode increase in size and mass.	Oxygen gas is liberated. Effervescence observed.
Copper (II) sulfate solution	Copper	Copper	Reddish-brown copper metal is deposited. Cathode increase in size and mass.	Copper anode dissolves. Anode increase in size and mass.

O-Level Chemistry: Selective Discharge of Ions in Electrolytic Cells | Part 1: Position of Reactivity Series & Concentration Effect

Selective Discharge of Ions in Electrolytic Cells - Position of Reactivity Series and Concentration Effect — Photo Credit: John Kreogh (Creative Commons)

From my observations in my weekly Sec 4 O-Level Pure Chemistry Tuition Classes, i noticed that many of the secondary 4 Chemistry students are very weak when it comes to application questions involving Selective Discharge of Ions in Electrolytic Cells. This is covered in the topic of Electrolysis in their school.

In an aqueous solution, more than one type of cation and anion are present in the electrolyte. However, only one cation and one anion are preferentially or selectively being discharged during the process of electrolysis.

If the electrodes used during electrolysis are inert, the ions being preferentially discharged will depend on three factors:

1) Selective Discharge of Cations
2) Selective Discharge of Anions
3) Effect of Concentration on the Selective Discharge of Anions

3 Factors Affecting Selective Discharge of Ions (using Inert Electrodes)

1. Selective Discharge of Cations

This is linked to the topic of Metals & Reactivity Series.

The ease of discharge of cations will depend on the position of the metals in the reactivity series.

You learnt that the more reactive a metal, the greater its tendency to react and form ions.

This simply means that in electrolysis, the more reactive the metal, the least tendency for the metal ions to be preferentially discharged at the cathode.

Ions of reactive metals like potassium and sodium will remain in the electrolyte solution and will not be discharged.

Hydrogen ion and ions of less reactive metals like copper and silver will be preferentially discharged.

In aqueous solution, hydrogen ions will be preferentially discharged over the ions of the metals above hydrogen in the reactivity series. Hydrogen gas will be liberated.

Likewise, if the cations come from a metal which is below hydrogen in the reactivity series, then the metal ions will be preferentially discharged. A metal will be produced.

2. Selective Discharge of Anions

Ease of discharge of anions increases down the list:

Sulfate ion, SO₄^2-
Nitrate ion, NO₃^–
Chloride ion, Cl^–
Bromide ion, Br^–
Iodide ion, I^–
Hydroxide ion, OH^–

Note that sulfate and nitrate ions will remain in the electrolyte solution. They are not electrolysed during electrolysis.

Hydroxide ions has the highest ease of discharge because it gives out electrons most readily (easy to be oxidised) during electrolysis. Oxygen gas will be produced.

Half equation for the discharge of hydroxide ions: 4OH^–(aq) => 2H₂O(l) + O₂(g) + 4e^–

3. Effect of Concentration on the Selective Discharge of Anions

It is important to note that an anion in higher concentration is always being preferentially discharged, regardless of the ease of discharge of anions (factor 2).

Let me give you a simple example to discuss the effect of concentration on the selective discharge of anions.

In the electrolysis of concentrated sodium chloride solution, the ions available in the electrolyte are:

Sodium ion, Na⁺
Chloride ion, Cl^–
Hydrogen ion, H⁺ (due to partial dissociation of water)
Hydroxide ion, OH^– (due to partial dissociation of water)

Both chloride and hydroxide ions are attracted to the positive anode.

Based on the ease of discharge of anions (factor 2), hydroxide ions should be preferentially discharged.

However, in concentrated solution, there are a lot more chloride ions than hydroxide ions.

Therefore, chloride ions are preferentially being discharged over the hydroxide ions at the anode.

YouTube Video Tutorial on Selective Discharge of Ions in Electrolytic Cells – Part 1: Position of Reactivity Series and Concentration Effect

In the YouTube Video below, you will learn in a step-by-step manner on how Position of Reactivity Series and Concentration Effect will determine the selective discharge of ions in Electrolysis.

Length of Video: 14.16 minutes

Direct Video Link:https://youtu.be/gQVimP_UtJg

I hope you find the content easy for your understanding and if you have anything valuable to add, leave me a comment below.Feel free to share this blog post with your friends and learn the key O-Level Pure Chemistry concepts together.

In the next blog post, i will be sharing a video on Selective Discharge of Ions in Electrolytic Cells based on the Nature of Electrodes. Do stay tuned to it!

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O-Level Chemistry: Basic Concepts of Electrolytic Cells in Electrolysis

Today, we shall discuss on the Basic Concepts of Electrolytic Cells in Electrolysis. This is a very important concept when it comes to Secondary 4 O-Level Pure Chemistry and IP Advanced Chemistry Examinations.

Basic Concepts of Electrolytic Cells in Electrolysis

Electrolysis is the decomposition of a compound using electricity.

During electrolysis, electrical energy is converted into chemical energy to bring about a chemical change.

The apparatus used for electrolysis is called an Electrolytic Cell and it consists of 3 main components:

Battery or Power Source
Electrodes
Electrolytes

It consists of two electrodes in an electrolyte.

An electrode is essentially a rod or plate where electricity enters or leaves an electrolyte during electrolysis.

An electrolyte could be an ionic compound in molten state or dissolved in aqueous solution, or an acid or alkali solution. Presence of mobile ions allows it to conducts an electric current and is decomposed in the electrolysis process.

A direct current power source or battery is connected to the electrodes.

An electric current will flow through the electrolyte, decomposing it into simpler substances.

Cathode is the negative electrode connected to the negative terminal of the battery or power source.

Anode is the positive electrode connected to the positive terminal of the battery or power source.

Cation, which is an ion having a positive charge, will be attracted to the negatively charged cathode and gets reduced.

Anion, which is an ion having a negative charge, will be attracted to the positively charged anode and gets oxidised.

Note that during electrolysis, reactions only occur at the electrodes and not inside the electrolyte.

The reaction which takes place during electrolysis is a redox reaction.

Oxidation occurs at the anode while reduction occurs at the cathode.

Strategy (acronym) to remember this concept easily is by using AN OX; RED CAT.

YouTube Video Tutorial on Electrolytic Cells

In the YouTube Video below, you will learn from Mr Sean Chua in a step-by-step manner on how electrolysis occurs in Electrolytic Cells. He is a highly recommended O-Level Pure Chemistry Tuition Teacher (Tutor Specialist) with 19 years of teaching experience and invited book author for the MOE approved O-Level Pure Chemistry Ten Years Series Books.

Length of Video: 11.09 minutes

Direct Video Link: https://youtu.be/7OaB0nyHPtA

I hope you find the content easy for your understanding and if you have any questions, leave me a comment below. Feel free to share this blog post with your friends and learn the key O-Level Pure Chemistry concepts together.

In the next blog post, i will be sharing a video on Selective Discharge of Ions in Electrolytic Cells based on the Position of Reactivity Series & Concentration Effect. Do stay tuned to it!

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O-Level Chemistry: Tests for Oxidising Agents and Reducing Agents

Test for Oxidising Agents and Reducing Agents - O Level Pure Chemistry Tuition — Photo Credit: Pete (Creative Commons)

Today, we shall take a look at the Tests for Oxidising Agents and Reducing Agents using common chemical reagents in our school laboratories.

Before we can continue, is important to first understand what is a Redox Reaction and how to identify one.

You will also need to know the definitions of Oxidising Agents and Reducing Agents.

A reducing agent is a substance that reduces another substance by:

giving hydrogen to the substance
removing oxygen from the substance
giving electrons to the substance

In the process of reducing another substance, the reducing agent itself becomes oxidised.

Common Reducing Agents are:

Potassium iodide, KI (colourless solution)
Reactive metals such as Mg, Zn
Hydrogen gas, H₂
Carbon, C
Carbon monoxide, CO
Hydrogen sulfide, H₂S
Sulfur dioxide, SO₂
etc

while

An oxidising agent is a substance that oxidises another substance by

giving oxygen to the substance
removing hydrogen from the substance
receiving electrons to the substance

In the process of oxidising another substance, the oxidising agent itself becomes reduced. I have blogged about these agents before using the analogy of a property and an insurance agents.

Common Oxidising Agents are:

Acidified potassium manganate (VII), KMnO₄ (purple solution)
Acidified potassium dichromate (VI), K₂Cr₂O₇ (orange solution)
Halogens e.g. Cl₂, Br₂
Concentrated sulfuric acid, H₂SO₄
Nitric acid, HNO₃
Oxygen, O₂
etc

In the video below, i will share with you on the step-by-step strategy to test for Oxidising and Reducing Agents using common laboratory reagents such as aqueous potassium iodide, KI(aq) and acidified potassium manganate (VII), KMnO₄(aq) solutions.

Test for Oxidising Agent:

Use of a Reducing Agent e.g. aqueous potassium iodide, KI(aq)

Observation: Colourless solution turns brown

Chemistry behind it: 2I^–(aq) => I₂(aq) + 2e^–

Test for Reducing Agent:

Use of an Oxidising Agent e.g. acidified potassium manganate (VII)

Observation: Purple solution decolourises

Chemistry behind it: MnO₄^–(aq) + 8H⁺(aq) + 5e^–=> Mn²⁺(aq) + 4H₂O(l)

YouTube Video Tutorial

Let’s take a look at the YouTube Video now….

Length of video: 16.11 minutes

Direct video link: https://youtu.be/1YQsHwUrT2c

I hope you find the content easy for your understanding and if you have any questions, leave me a comment below. Feel free to share this blog post with your friends and learn the key O-Level Pure Chemistry concepts together.

Subscribe to my blog to receive 2 updates per month sent to your email!

PS: Under related articles below, there are several blog post discussions and questions related to Redox Reaction concepts. You can also do a keyword search using the search box at the top right hand corner.