In this video, we're going to talk about oxidation and reduction reactions. And here's the example we're going to start with. Magnesium plus oxygen gas produces magnesium oxide.
In an oxidation-reduction reaction, or simply a redox reaction, electrons are being transferred from one element to another. Now, perhaps you found or watched my video on oxidation numbers. If you have, then you know that the oxidation number of any pure element is zero. Now, in magnesium oxide, magnesium, being an alkaline earth metal, has a charge of... plus 2. Oxygen has a charge of minus 2. Notice that the oxidation state of magnesium went from 0 to plus 2. So the oxidation state increased.
Whenever the oxidation state goes up, the substance is said to be oxidized. In the case of oxygen, the oxidation number decreased from 0 to negative 2. Whenever the oxidation number decreases, the substance is being reduced. Now, you need to know that oxidation always occurs with a loss of electrons.
Reduction is associated with a gain of electrons. Metals, they like to give away. electrons they like to form metal cations as they give away electrons they will acquire a positive charge nonmetals like oxygen they like to acquire electrons and so they will develop a negative charge the substance that is oxidized is known as the reducing agent and the substance that is reduced is known as the oxidizing agent Metals are reducing agents because they will cause the other substance to be reduced.
Nonmetals, like oxygen gas, fluorine, they're oxidizing agents because they cause another substance to be oxidized. And that's where you got to see it. So these are some terms that you want to be familiar with when dealing with redox reactions. Now let's go back to that reaction. The reaction between magnesium and oxygen gas.
Now magnesium is changing into the magnesium ion. This magnesium ion has a plus two charge and in order to become a plus two cation it has to lose two electrons. Now oxygen, I'm going to write atomic oxygen because this reaction is not balanced. Individually oxygen acquires two electrons and turns into oxide. Now because magnesium lost electrons, this reaction, which is known as a half reaction, that's the oxidation part of the reaction.
Anytime a substance loses electrons, it's being oxidized. This second half reaction is the reduction part, since oxygen is acquiring electrons. whenever a substance gain electrons or if the oxidation number decreases its reduction now for half reactions anytime you have electrons on the right side it's going to be an oxidation half reaction whenever the electron electrons are on the left side, it's a reduction half reaction.
That's another way in which you can see it too. Consider this reaction. Zinc metal reacts with hydrochloric acid to produce hydrogen gas and zinc chloride.
identify the substance that is oxidized and the substance that is reduced and also identify the oxidizing agent and reducing agent now if you get a question like this on the test to find a substance that is oxidized or reduced always look at the reactants it's one of these two don't look at the products Now let's find the oxidation states of everything. The oxidation state of any pure element is always 0. Now whenever hydrogen is bonded to a nonmetal, it's going to have a positive 1 oxidation state. chlorine has to be minus 1. Now in this one too, chlorine is still negative 1 which means zinc has to have a plus 2 charge. Now the oxidation of zinc changes from 0 to positive 2. So zinc is being oxidized. Hydrogen changes from 1 to 0, so HCl as a substance is being reduced, even though only the hydrogen portion of that substance is being reduced.
So typically, if they ask you for the substance, even though it's only the hydrogen element that's being reduced, you would say the whole substance is being reduced, if you have to choose an answer on a multiple choice test. Now, the substance that is oxidized is the reducing agent. And typically metals tend to be reducing agents.
The substance that is reduced is the oxidizing agent. And this is a common question that you might see on a typical chemistry exam. Now let's try one more example for the sake of practice.
Methane reacts with oxygen gas and it produces carbon dioxide and water. So feel free to... to pause the video identify the oxidation states of every element in this reaction and then find the substance that is oxidized reduce and identify the oxidizing agent and reduce an agent so let's identify the pure elements which is only oxygen gas that's going to be zero now we said that whenever hydrogen is bonded to a non-metal it's going to have a plus one oxidation state And typically, when oxygen is found in a compound, the oxidation state is negative 2, except when it's bound to fluorine, or except when it's in the form of peroxide or superoxide. It's different. Whenever you hear the word oxygen, you're going to think of the word fluorine.
the oxidation state of oxygen is negative 2. If you hear the word peroxide, it's negative 1. If you hear the word superoxide, it's negative 1 half. Now we've got to find the oxidation state of carbon. So let's start with methane.
So we have one carbon atom and four hydrogen atoms, which has to add up to zero because methane is neutral in charge. Now each hydrogen atom has an oxidation state of one. So therefore, C plus four must equal...
zero therefore carbon has to have an oxidation state of negative four in methane now let's calculate the oxidation state of carbon in co2 so it's C plus two oxygen atoms which equals a net charge of zero and each oxygen has a charge of negative two so in this case carbon is going to have an oxidation state of positive four So, carbon changes from negative 4 to positive 4. Therefore, the oxidation number of carbon is increasing, which means carbon is oxidized, or technically, methane is oxidized. Now, hydrogen doesn't change. But oxygen changes from 0 to negative 2. So therefore, oxygen gas, the oxidation number is decreasing, so it's being reduced. Which means that methane is the reducing agent, and oxygen gas, it's being reduced, which makes it the oxidizing agent. So keep in mind, even though the element carbon is being oxidized, you would still describe the whole substance methane as being oxidized because carbon is part of methane.
Now the next thing that you need to be able to do is you need to take a look at a reaction. and tell if it's an oxidation reduction reaction. So consider this reaction aluminum metal reacts with copper chloride to produce aluminum chloride and copper metal.
Is this reaction a redox reaction? So this reaction is a single replacement reaction, and if there's a transfer of electrons, then it's going to be a redox reaction. Aluminum has an oxidation state of 0, but in AlCl3, it has an oxidation state of plus 3. So if the oxidation number changes, then it's a redox reaction.
All single replacement reactions are redox reactions. Now, if you don't want to look for the oxidation numbers, there's a quick way to tell. if something is going to be a redox reaction.
If you see a pure element on one side and then that element being part of a compound on the other side, it's always going to be a redox reaction. There's a transfer of electrons. So let me give you two examples and I want you to determine if it's a redox reaction or not. So both of these examples are synthesis reactions, also known as combination reactions.
Sometimes, a combination reaction can be a redox reaction, and sometimes it won't be. Now, if you look at the first example, there are no pure elements. All we have is compounds.
When you see that, chances are it's not a redox reaction. But if you look at the second example, we have a pure element, zinc, and then that same element is in a compound. So the second example is a redox reaction. And let's prove it. So let's start with the first example.
Oxygen has a negative. 2 charge. Magnesium in magnesium oxide has a plus 2 charge. Hydrogen has a positive 1 charge.
Now, magnesium hydroxide is composed of Mg plus 2 and 2 hydroxide ions. But as you can see, magnesium still has an oxidation state of positive 2. Now, let's focus on hydroxide. Hydroxide is oxide plus hydrogen.
If you want to solve it, you know that hydrogen is going to have a positive 1 charge. And oxygen is going to have a negative 2 charge. When you add up negative 2 and 1, you're going to get the net charge of a hydroxide ion, which is negative 1. So in magnesium hydroxide, oxygen is still negative 2. and hydrogen is plus 1. So notice that the oxidation state of magnesium did not change. It's still positive 2. The oxidation state of hydrogen is still the same.
It's positive 1. And the oxidation state of oxygen has not changed. If the oxidation state does not change, that means there was no transfer of electrons, and so this is not a redox reaction. Now let's analyze the second example. Zinc and chlorine has an oxidation state of zero. In zinc chloride, chlorine has a negative one oxidation state, zinc has a positive two oxidation state.
So notice that zinc is being oxidized. Therefore there is a transfer of electrons which makes this reaction a redox reaction. Now consider these two decomposition reactions.
When mercury oxide is heated, it decomposes into mercury and oxygen gas. And if you were to heat calcium carbonate, it will decompose into calcium oxide plus carbon dioxide gas. So, which of these decomposition reactions is a redox reaction?
Looking at the first example. We have oxygen as a pure element on the right side, and then that same element is within a compound. So the first example is a redox reaction. In the second example, there are no pure elements. What we have are just compounds.
So the second example is not a redox reaction. That's a quick way to tell if it's a redox or not. So now I'm going to give you a list of reactions and determine which ones are redox and which ones are not. So feel free to pause the video if you want to.
So the first example, is it a redox reaction? Combustion reactions are always redox reactions. Here we have a pure element on one side and then that same element is inside a compound. So the first example is a redox reaction.
What about the second example? Notice that there are no pure elements in this example, so this is not a redox reaction. Acid-based neutralization reactions are not redox. Anytime you have a double replacement reaction, it's never going to be a redox reaction. But all single replacement reactions will be a redox reaction.
The third example is a combination reaction. There are no pure elements, so therefore this is not going to be a redox reaction. The last one is a double replacement reaction, specifically a precipitation reaction.
And as we can see, all we have are just four compounds in this reaction. There are no pure elements, so it's not a redox reaction. So to review... All combustion reactions are redox reactions. All single replacement reactions are redox reactions.
Now, synthesis and combination reactions, sometimes they're redox, sometimes they're not. And the same is true for decomposition reactions. Double replacement reactions are never redox reactions. So that includes acid-base reactions, precipitation reactions, and other types of double replacement reactions. And a quick way to tell is if you see a pure element on one side, and then the same element within a compound on the other side, it's going to be a redox reaction.
That's the easiest way to tell. So that's it for this video. If you want to find more chemistry videos, just check out my channel. And you can find other videos on physics, calculus, algebra, trig, pre-cal, and other stuff like that. So thanks for watching.