27 March 2025

Summary

In this lecture, Sharizah Mohamed Amin delves into the fundamentals of redux reactions, which involve simultaneous reduction and oxidation processes. The session begins with a review of criteria to identify these reactions, including changes in oxygen, hydrogen, electrons, and oxidation states. Key concepts such as reducing and oxidizing agents are introduced, emphasizing how they affect the reaction's outcome. Practical tests for identifying redux reactions are outlined, highlighting the use of specific chemicals to detect oxidizing or reducing agents. The lecture concludes with examples and non-examples of redux reactions, reiterating the importance of understanding oxidation states and agent roles in chemical reactions.

Highlights

• Understanding redux reactions involves the simultaneous occurrence of reduction and oxidation! 🔄
• Criteria to identify redux reactions include changes in oxygen, hydrogen, electrons, and oxidation states! 🧪
• Magnesium's reaction with copper oxide serves as a classic example of oxidation and reduction in action! ⚙️
• Classifying agents: Reducing agents get oxidized, while oxidizing agents get reduced! 🔁
• The role of oxidation state is crucial in determining the nature of redux reactions! 📊
• Using chemicals like potassium iodide and acidified potassium permanganate helps test redux reactions! ⚗️

Key Takeaways

• Redux reactions involve both reduction and oxidation occurring simultaneously. This magical combo makes chemical reactions extra fascinating! 🌟
• Identifying redux reactions can be done by observing changes in oxygen, hydrogen, electron transfer, and oxidation states. Science is like detective work! 🔍
• Reducing agents get oxidized, and oxidizing agents get reduced. It's a bit of a role swap party in the chemical world! 🎉

Overview

Sharizah Mohamed Amin takes us on a riveting exploration of redux reactions, where reduction and oxidation processes happen simultaneously. The lecture starts by revisiting the critical criteria for identifying these intricate reactions. Whether it's the change in oxygen, hydrogen, or electrons, or understanding oxidation states, these elements are vital in spotting a redux reaction.

The session then introduces us to the essential roles of reducing and oxidizing agents. Magnesium and copper oxide provide a vivid glimpse into how substances undergo oxidation and reduction. It's all about who gives and who takes electrons! Throughout, the importance of accurately presenting oxidation states is highlighted, as it forms the backbone of understanding these reactions.

To wrap up, different methods to test for redux reactions are discussed, shedding light on the practical use of chemicals like potassium iodide and acidified potassium permanganate. From recognizing color changes to identifying product formations, this lecture sets the stage for dissecting chemical reactions with precision and clarity.

Chapters

• 00:00 - 01:30: Introduction to Redux Reactions The chapter titled 'Introduction to Redux Reactions' begins with a brief review of the previous lesson, focusing on redux reactions. These are reactions that incorporate both reduction and oxidation processes simultaneously. The chapter also touches on the methodology for identifying such reactions.
• 01:30 - 03:00: Criteria for Identifying Redux Reactions The chapter discusses the criteria used to identify redux reactions. It outlines four main criteria: the role of oxygen, the presence and role of hydrogen, changes in electrons, and changes in oxidation state. The term 'oxidation state' is explained as being related to the charge of an element, such as magnesium.
• 03:00 - 05:00: Example of Oxygen in Redux Reactions The chapter discusses the oxidation state of magnesium, noting that while magnesium ions have a charge of 2+, the oxidation state is represented as +2. Emphasis is placed on the correct and accurate presentation of oxidation states.
• 05:00 - 07:30: Reducing and Oxidizing Agents The chapter discusses the process of identifying oxidation and reduction in chemical reactions through the sign and number criteria, emphasizing the importance of understanding these concepts.
• 07:30 - 09:00: Criteria: Hydrogen in Redux Reactions In the chapter titled 'Criteria: Hydrogen in Redux Reactions', the discussion begins with a focus on the behavior of oxygen in redux reactions. Using the example of a reaction between magnesium and copper oxide, it is highlighted that magnesium transforms into magnesium oxide, implying that magnesium gains oxygen. Conversely, copper oxide becomes copper, indicating that copper oxide loses oxygen. This transformation is key to understanding the principles of redux reactions involving hydrogen.
• 09:00 - 11:00: Criteria: Electrons and Ionic Equations The chapter discusses the concept of oxidation and reduction using the example of magnesium and copper 2 oxide. It explains that magnesium is oxidized as it gains oxygen and copper 2 oxide is reduced as it loses oxygen. There is an emphasis on correctly identifying the substances involved in oxidation and reduction when asked such questions.
• 11:00 - 15:00: Criteria: Oxidation State The chapter titled 'Criteria: Oxidation State' emphasizes the importance of discussing reactions with respect to reactants. It clarifies a common misconception by stating that magnesium oxide should not be described as being oxidized, as it is actually the product of the oxidation process. The chapter highlights the necessity of correctly identifying the substances undergoing oxidation or reduction in a chemical reaction.
• 15:00 - 23:00: Testing for Redux Reactions In this chapter, the concepts of reducing agents and oxidizing agents are introduced and discussed. It is explained that magnesium, when oxidized to form magnesium oxide, acts as a reducing agent. The nature of a reducing agent is such that it itself is oxidized while causing the reduction of another substance.
• 23:00 - 31:30: Examples of Redux and Non-Redux Reactions The transcript discusses the role of copper oxide as an oxidizing agent and explains the concept of oxidizing and reducing agents. In a reaction, the reducing agent is the substance that gets oxidized, whereas the oxidizing agent is the one that gets reduced. The summary emphasizes understanding the final state of the agents in the reaction.
• 31:30 - 38:30: Summary and Conclusion The chapter focuses on understanding oxidizing and reducing agents by examining the products of chemical reactions. It provides a specific example using magnesium oxide, which is oxidized, making magnesium the reducing agent. The chapter also introduces a second criterion focused on hydrogen, highlighting scenarios where hydrogen is involved instead of oxygen in reactions.

27 March 2025 Transcription

• 00:00 - 00:30 Okay, good morning everyone. So, um I hope you have your notes with you. So, I'm just going to summarize u what we did last lesson. So, we look at redux. All right. So, we were looking at redux and what is redux reaction? They are basically reactions that involve reduction and oxidation um reactions at the same time. Now the next thing that we looked at was how do we identify these reactions? So what
• 00:30 - 01:00 criteria do we use? So there are actually four. So is there's oxygen. We'll look at what happened to oxygen. We look at what happened to hydrogen if there is in the reaction. We can also look at um changes in electrons. And lastly we can also look at changes in oxidation state. All right. So what is oxidation state actually? So oxidation state is related to the charge of an element. Okay. So for example, magnesium
• 01:00 - 01:30 being magnesium ion as you can see here has a charge of 2+. However, we said the oxidation state of magnesium is + two. Okay. Yeah. So for um I'm going to just hang around here first to to talk about oxidation state. So please remember to present oxidation state correctly and accurately. Okay. So for oxidation state
• 01:30 - 02:00 you need to state the sign first whether it's positive or negative followed by the number. Okay. So please remember that. Okay. Now um now that we know what's the criteria so how do we then um really look that um each of these criteria and to know whether it's oxidation or reduction. So let's jump into it. So let's look at
• 02:00 - 02:30 page two of your notes again. Okay. So let's look at the first criteria which is looking at oxygen. Okay. That's criteria number one. So using this example of magnesium plus copper oxide. So you'll notice that at the end of the reaction, magnesium has become magnesium oxide. Copper oxide has become copper. So it's quite obvious that magnesium has gained oxygen while copper oxide loses
• 02:30 - 03:00 its oxygen to become copper. So we say magnesium has been oxidized because it has gained oxygen while copper 2 oxide has been reduced because it loses hydrogen uh it loses oxygen. Okay. So with that I would like to emphasize that when asked okay which substance has been oxidized please write your
• 03:00 - 03:30 answer with respect to the reactants. Okay. All right. All answers must be with respect to reactants not product. So it is wrong to say magnesium oxide has been oxidized. Okay. Magnesium oxide is the outcome of the oxidation. Okay. All right. Then we also looked at
• 03:30 - 04:00 I'm going to introduce to you the the other two terms, okay, that we also briefly looked at the reducing agent and oxidizing agent. Now let's look at each of them. Magnesium has been oxidized to magnesium oxide. So that makes it that makes it the reducing agent. A reducing agent oxidized is being oxidized at the end. Okay? while it reduces others.
• 04:00 - 04:30 Okay. And copper oxide on the other hand is actually an oxidizing agent because it actually gets reduced at the end. Okay. So in summary, a reducing agent gets oxidized at the end of the reaction while the oxidizing agent gets reduced at the end of the reaction. So to identify which substance is the reducing
• 04:30 - 05:00 or oxidizing agent, you just have to look at the product. Okay? Like for example, magnesium oxide has been oxidized, which makes magnesium the reducing agent. Okay? All right. Let's look at criteria number two, which is actually hydrogen. Now in this case as you can see there's nothing else there's no oxygen but instead you have hydrogen. So
• 05:00 - 05:30 let's trace the movement of hydrogen. So as you can see here hydrogen sulfide has become sulfur which makes it um which means it has lose its uh its hydrogen which means it has been oxidized. Okay. And chlorine on the other hand has gained hydrogen which makes it reduced. Okay. Okay. Let's move on to electrons. Okay. Now electrons
• 05:30 - 06:00 um when you know that you are supposed to use electrons as one of the criteria then usually the reaction will be in terms of an ionic equation. So as you can see here this is an example of an ionic equation. So if you trace the movement of zinc in this case it has become zinc 2+ meaning it has lose electrons. So we say it has been
• 06:00 - 06:30 oxidized. So basically metals tend to be um metals tend to lose electrons right? So metals tend to get oxidized in the process. Now on the other hand for copper as you can see for copper it has been reduced because copper ion has now become copper. Um how do you get copper ion to become copper is actually through the gain of electrons.
• 06:30 - 07:00 Okay. Now lastly the last criteria and by far the most important criteria because this concept will be largely assessed okay in in your exams or in the if you take a look at your topical or your yearly TYS you will see that you are required to have an understanding a good understanding of oxidation state. Okay. So an oxidation state is related to the
• 07:00 - 07:30 um to the changes in electrons. Okay. All right. So this table over here um summarizes some rules. Okay. These are rules and trends um that you need to be familiar with in order for you to determine oxidation state of substance. Okay. Yeah. So I'll make sure you are very familiar with this table. Okay. Now
• 07:30 - 08:00 how do we use oxidation state to tell us whether oxidation or reduction has taken place. So please highlight to yourself keyword you use not lose or gain but instead use the word increase or decrease in oxidation state. So I repeat, you do not use the word lose, loss or gain is not to be used with oxidation
• 08:00 - 08:30 state. Instead use the word decrease or increase. Okay. So let's take a look at this. A similarly they will use ionic equation um can be used to actually um highlight to you the how do you use oxidation state. Okay. So in this case if you look at magnesium um at the start it's magnesium element then it has become magnesium ion. So
• 08:30 - 09:00 magnesium in its elemental state has an oxidation state of zero. Then after the reaction it becomes magnesium ion. So you'll notice that the oxidation state has become plus2. So from 0 to +2 you know there has been an increase. All right. So therefore magnesium has been oxidized. Okay. So how do you actually write your answer or represent your
• 09:00 - 09:30 answer here? If when asked which substance has been oxidized. So you say magnesium has been oxidized as or because its oxidation state increase from please state the evidence to support the increase. So you need to be able to compute the oxidation state. Okay. Now in this case ion 3 has then
• 09:30 - 10:00 become iron 2 plus. So you'll notice that the oxidation state decreases from + 3 to +2 and therefore we say that iron 3 has been reduced. Okay. All right. So this one was the example that we actually did uh in class previously. So the rule of um determining oxidation state is to treat compounds as ionic.
• 10:00 - 10:30 Okay, now let's look at the last part. Okay, I think this is important. Okay, um how do we test if a reaction is a redux reaction? Okay, so we need to make use of chemicals. Okay, so we need to know tests. So in the previous chapter on qualitative analysis you know test um you know how to test for cation ion how to test for gases how to test for nion. So here we are going to introduce to you
• 10:30 - 11:00 test as well but in this case the purpose is to determine if um the reaction is a redux reaction. All right. So um how do we do that? Okay, we actually use this um we need to test for the presence of oxidizing and reducing agent. Okay, so now this part here is very important. So please get ready to annotate along. So the first one let's
• 11:00 - 11:30 say you are now going to find out uh okay determine um if the unknown solution contains oxidizing agent. Okay. So in order for you to determine if that solution contains oxidizing agent, what is the chemical to use? The chemical to use is potassium iodide. Okay. All right. So potassium iodide is a known reducing agent. So here I would like to
• 11:30 - 12:00 highlight to you to test for oxidizing agent you use reducing agent. Okay? Because they work in opposite manner. All right. And potassium iodide is a required reducing agent that you must remember. Okay. Now, potassium iodide is actually a colorless solution. So, you must remember this. But at the end of the reaction, okay, it will become brown. So, if it becomes uh if it
• 12:00 - 12:30 produces a brown solution, this confirms the presence of oxidizing agent. So this is what we call a positive result. Okay. Now why? What's the chemistry behind it? So potassium iodide actually contains iodide ions. Okay. This is the iodide ions. So the iodide ions then become iodine. Okay. So in that process of
• 12:30 - 13:00 becoming iodine, the colorless iod ion becomes brown. So please do not confuse this iodine in the equal state with the iodine in the solid state that you learned before as halogens being purplish black. So please do not confuse they are actually two is still talking about iodine but in different context. Okay let's move on. So here what we want to know is the next chemical that I want all of us to know
• 13:00 - 13:30 is acidified potassium magnanate 7. Okay. So this is actually a known oxidizing agent. So its job is to look out for reducing agent. Okay. So what is the color of acidified potassium magnanate? Is actually a deep dark purple. All right. And um this purplish um uh color will actually stain your fingers. Okay? So just remember if
• 13:30 - 14:00 you're handling potassium magnate 7, you have to handle with care so that you don't stain your clothes, you don't stain your hand. Okay? So it is purple. Um then at the end of it um at the end of the whole reaction then it will become colorless. Okay? Okay, the purple color will become colorless to indicate that there is presence of reducing agent because the magnanate
• 14:00 - 14:30 ion all right which has an oxidation state of plus 7 has been reduced um because the oxidation state decreases to plus two. Okay. So when there's a change of color from purple to colorless you can safely conclude that there is presence of a reducing agent. Okay. So in summary to test for oxidizing agent you
• 14:30 - 15:00 use reducing agent. All right. And what reducing agent do you need to know? Number one, you need to know potassium iodide and you need to know the color changes. Okay, because you may be asked to describe. Okay, number two. Okay, what are the tests? You can use your starch iodine, starch iodide, sorry. So the
• 15:00 - 15:30 iodine produced um because the iodide that will be um that will be actually um oxidized okay to iodine um will then interact with starch to give you that blue color which will tell you that um yes oxidizing agent is present. Okay. The next chemical is your iron 2 sulfate. Okay. uh followed by sodium hydroxide. So this sodium
• 15:30 - 16:00 hydroxide is added. Okay, because it's like your cat ion test, you want a precipitate to form. Okay, so how does this work? So to test for oxidizing agent, your iron 2 will become oxidized to iron 3. So and then when sodium hydroxide is added, you can see that it gets um you can see the formation of red brown precipitate. So that is a positive result.
• 16:00 - 16:30 Okay. All right. Now let's take a look at reducing agent. As I said before, reducing agent use oxidizing agent to determine the presence of reducing agent. So they work opposite. So the first oxidizing agent you need remember is acidified potassium magnate solution. Okay. So um where the purple color will become colorless at the end. So that's the positive result that we
• 16:30 - 17:00 must look out for in order to tell us that reducing agent is present. Number two, um the chemical that you need to remember well enough is iron 3 sulfate which again um you need to add um sodium hydroxide so that it can produce a precipitate. So you observe what is the
• 17:00 - 17:30 color of the precipitate. So iron 3 um is then reduced to iron 2 which when added to sodium hydroxide will produce a dirty green precipitate. So this dirty green precipitate is actually the positive result to tell you that there's presence of reducing agent. Okay. So um please be familiar with that. Now the
• 17:30 - 18:00 last page is actually a very good summary page for you to actually identify what situations will actually give you um will actually give you redux and what situation um or what reactions are not considered redux. Okay, let's look at them one by one. So the first one is combination. As you can see from the screen, I've actually worked out the oxidation state of the elements and you can see that sodium has actually
• 18:00 - 18:30 increased its oxidation state from 0 to + one while chlorine actually the oxidation state decreases from 0 to minus1. So in this case there's both redux both reduction and oxidation. So I put a tick under redux. This is a redux. Now for combustion, I'm not using oxidation state because I can use um changes in um
• 18:30 - 19:00 oxygen. So for example, in this case um this is called methane. CH4 is methane. So you notice that CH4 um the hydrogen is lost. Instead is replaced by oxygen. So we say that methane has been oxidized. But on the other hand, your oxygen has now gained hydrogen. So we say that the oxygen has been reduced. So again, there's both
• 19:00 - 19:30 oxidation and reduction. So we say this is a redux. Now the next one which is neutralization which is a reaction between acid and alkali. Um please remember all neutralization is not a redux because there is no change in oxidation state of the elements. Okay. So with that we cannot say that's redux. So it is not a redux. Okay. Now next how
• 19:30 - 20:00 about metal displacement? So as you can see from the ionic equation zinc has become zinc 2+ which means the oxidation state actually increases from 0 to +2. So it has been oxidized while on the other hand lead 2+ has then become lead has been reduced. All right. So there's both oxidation reduction. So therefore this is redux. Okay. Next one. Let's take a look at halogen. So you have florine which
• 20:00 - 20:30 becomes fluoride. Um so with that you can see the oxidation state of florine decreases from zero to minus1. So we say that florine has been reduced. On the other hand the broomemide ion Br minus actually become Br2. You notice that the oxidation state actually increases. So from minus1 to zero. So which means oxidation has taken place. So by
• 20:30 - 21:00 definition if both reduction and oxygen oxidation occur at the same time this is redux. Now let's take a look at thermal decomposition of carbonates. Okay. This is another example of a non redux. Okay. Because once again um you notice that there's no change in the oxidation state of the elements. If you were to compute the oxidation state one by one. So thermal decomposition not
• 21:00 - 21:30 a redux. Next one reduction of metal oxides. Okay. So as you can see from for the three examples every one of them actually has both oxidation and reduction occurring at the same time. So these are redux. And lastly, precipitation. As you can see here,
• 21:30 - 22:00 you'll notice that this precipitation is not okay is not um an example of redux because if you look at it one by one, there is no change in the oxidation state of the elements involved in the reaction. Okay. So please remember there are three non-example. So number one neutralization is not a redux. Number two thermal decomposition is not a
• 22:00 - 22:30 redux. And thirdly precipitation is also not a redux. Okay. All right. So with that um we come to the end. So in summary. Okay. So redux reaction are described as oxidation and reduction reactions occurring at the same time. And there are four ways for us to determine whether a redux reaction has taken place. So we look at oxygen,
• 22:30 - 23:00 hydrogen, electrons and oxidation state. So my advice to you is this. Just remember one set. All right? So you can choose to remember oxidation or reduction accurately and then apply the opposite for the other substance. So um and we talk about oxidation state. So I want all of us to remember that oxidation state is actually the um is actually derived from the charge.
• 23:00 - 23:30 Okay. Now oxidation um is actually caused by oxidizing agent because oxidizing agent oxidize others but itself is reduced. Okay. Um and what are our important um you know important oxidizing agent number one acidified potassium magnanate 7 as well as iron 3 okay iron 3 ion. Then on the other hand for reducing agent okay you must know um
• 23:30 - 24:00 potassium iodide very well. All right. So please note that potassium iodide is a known reducing agent where the iodide will then um you know it takes time for the iodide then to become iodine colorless to brown. So once you see this change in color it's a positive um positive test result. Okay. to show you that there's presence of reducing agent. Okay. Uh which actually gets
• 24:00 - 24:30 oxidized at the end. Okay. Now the next one for reducing agent is to use iron 2+. Okay. Um so basically the iron 2 will then um react when you add sodium hydroxide in you'll get precipitate. So is from the color of the precipitate then we can conclude or say that this reducing agent is present. Okay. All right. So with that um we come to the end. So what I
• 24:30 - 25:00 want all of us to do now is actually to make sure you complete your assignment okay um electronically in Google classroom before you attempt the questions in your workbook. So what you have to submit at the end of the lesson is really the online worksheet that will be put up on Google Classroom for you to emphasize or for you to reinforce what we talk about during lecture.
• 25:00 - 25:30 Okay. All right. with that. Thank you