Calculate Oxidation Numbers - Video Tutorials & Practice Problems
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Calculating Oxidation Numbers is the first pivotal step in understanding redox reactions.
Calculate Oxidation Numbers
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Calculate Oxidation Numbers
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Now the oxidation number is an important idea as we make our way towards oxidation and reduction reactions, otherwise known as redox reactions. Now first, we're gonna say that the oxidation number itself is an element's ability to gain, lose, or share electrons when alone or in a compound. And we're gonna say when it comes to oxidation numbers, it's important talk about the natural state of an element. Now for an atom in its natural state, also called its standard state, state, its oxidation number or oxidation state is equal to 0. Now remember we have our periodic table here, we know that with our periodic table, there are charges that are unique to different groups and we know that certain elements exist in certain forms in nature. Now when it comes to group 1 a, we know that the charge is plus 1. Group 2 a is plus 2. 3 a is plus 3. N a, we tend to on 4 a, we tend to skip because some elements have varying positive charges, so we're just gonna skip that. We know that group 5 a is minus 3, then minus 2, minus 1. And remember everything is trying to become noble gas. They're perfect, so they tend not to have charges. If you don't remember this or you haven't seen my videos on it, and you wanna explore this a little bit further, make sure you take a look at my videos on periodic table charges. Now, also remember we've talked about natural states of elements in the past as well. So when it comes to elements of the periodic table, we have our diatomic molecules. Those are hydrogen, which is h 2, nitrogen, oxygen, fluorine, chlorine, bromine, and of course, our iodine. Also remember that phosphorus tends to exist as p 4 in its natural state when found in nature. And that sulfur is s 8, and since sodium is right below sulfur, it is se8. These are the natural forms of these different elements when they're found in nature. And remember, all the others, I'm not writing them in because in nature, they exist as monoatomic atoms. This, you can also remember, refresh your memory by taking a look at my periodic table videos when we talk about the natural states of elements. But right now, just realize that if you find an atom in its elemental state or natural state or standard state, whatever term you're most comfortable with, its oxidation number will be equal to 0.
Elements in Natural State have an oxidation number of zero:H2, N2, O2, F2, P4, S8, Cl2, Se8, Br2.
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example
Calculate Oxidation Numbers Example 1
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Which of the following choices represents a natural state with an oxidation number equal to 0? Alright, so remember when you are in your natural state as an element, your oxidation number or oxidation state will equal 0. If we take a look here, we have Na3, Cl, He and Mn 4. Remember that when it comes to chlorine, Chlorine exists as a diatomic molecule, so Cl 2 is its natural state. So, this would not have an oxidation number equal to 0. Here, Na 3 and Mn, these are 2 metals. Remember, these metals exist as monoatomic elements in their natural states. So, this wouldn't work and this wouldn't work. The answer here is c because helium here is shown in its natural state and as a result of this, would have an oxidation number equal to 0. So here, option c is the correct answer.
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Calculate Oxidation Numbers
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Now when an element gains a charge, it's no longer in its standard or natural state, so its oxidation no will no longer be equal to 0. Now, ions, recall, an ion is an element or compound with a positive or negative charge. Remember, your positive ions are called cations and your negative ions are called your anions. Now for a monoatomic ion, the oxidation number is equal to its charge. So if they gave us for example the aluminum ion, aluminum is in group 3 a so its charge is 3 plus. If we see a charge present for that mono for that element, that is also its oxidation number. Okay. Well, remember, it has to be in its ion form for the charge and oxidation number to equal one another. Okay? That's not always going to be the case. We're going to see later on, they're going to be cases where a particular element, its oxidation number can range widely based on what it's connected to. But for right now, if you see an ion, you see its charge, that charge is equal to its oxidation number. So keep that in mind as we click on the next video and attempt an example question.
Oxidation number of monoatomic ions equals to their charge.
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Calculate Oxidation Numbers Example 2
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So here it says, which of the following elements would have the most positive oxidation number based on its ionic form? Now, if you've watched my videos on periodic table charges, you'll remember that the periodic table has some key things we need to remember when it comes to particular elements and their charges. For example, silver. Silver is a transition metal. Most transition metals have multiple charges. Silver on the other hand does not. Remember, silver is always plus 1 for its oxidation number. Scandium. Scandium is also a transition metal. It's in group 3b, remember? So group 3b elements. They tend to have a charge of plus 3. Next we have sodium. It's in group 1 a, so its charge is plus 1. And then sulfur is in group 6 a, so its charge is minus 2. We're looking for the most positive oxidation number, and that would have to mean that option b is the correct choice. It has the biggest positive value of plus 3. Remember, if you haven't gotten a chance to look at my periodic table videos on charges, and you just wanna go and take a look at the different types of elements and their charges, I highly suggest that you go back and take a look. Because when it comes to charges of elements, it's gonna form a foundation for a lot of later topics we're going to cover in chemistry 1 and chemistry 2. Alright. So just remember, in this question the answer would have to be option b, it has the most positive charge and therefore the most positive oxidation.
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Problem
Problem
Which of the following elements would have the lowest oxidation number?
A
Indium, In
B
Strontium, Sr
C
Argon, Ar
D
Manganese, Mn
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concept
Calculate Oxidation Numbers
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Like we said before, oxidation numbers don't always correspond to real charges. And therefore, a list of rules will be necessary. Now, we're going to say when different elements are in a compound, these specific rules will be used to calculate our oxidation numbers. Now here, let's take a look at these specific oxidation number rules. When it comes to group 1 a elements, they will be plus 1 when connected to any other element. So that's something similar to charges of groups. And we're gonna see some similarities between oxidation numbers and charges, but then we're gonna see a huge deviation from that. Group 2a, plus 2 when they're connected to another element. Fluorine, it's minus 1 when it's connected to any other element. Now this is where things start to change. Hydrogen can be either plus 1 or minus 1. It's plus 1 when it's connected to nonmetals. For example, if hydrogen is connected to chlorine, or hydrogen is connected to oxygen, or it's connected to the nonmetal of nitrogen. In all these instances, since hydrogen hydrogen is connected to a nonmetal, its oxidation number will be plus 1. Now it's negative one when it's connected to a metal or boron. So for example, n a h or c a h 2 or b h 3. In these cases, hydrogen will be minus 1 for its oxidation number. Now oxygen is even more varied. We're gonna say here that oxygen, when it's not a peroxide or superoxide, its oxidation number is minus 2. When it's in its peroxide form, its oxidation number will be minus 1. Now what exactly is a peroxide? Well, a peroxide, we're going to say is when you have 2 group 1 a elements connected to 2 oxygens. So example, h 2 zero two, 2 hydrogens which are in group 1 a, 2 oxygens. This would be hydrogen peroxide, l I 202, lithium peroxide, or k202, potassium peroxide. Now if oxygen is a superoxide, its oxidation number is minus a half. What is a superoxide? A superoxide is when you have 1 group 1 1 a element with 2 oxygens. So here we could have potassium superoxide, cesium superoxide, sodium superoxides. So just remember oxygen can vary, so just look be on the lookout. Do you have a superoxide or a peroxide? If not, its oxidation number is minus 2. And then finally when we're talking about group 7 a, we're talking about chlorine, bromine, and iodine. They are going to be minus 1, except when they're connected to oxygen. In that case, we won't know what their new oxidation number will be, and we'll have to calculate it. So again, we use these specific oxidation number rules when we're talking about different elements connected together. So we're gonna have to utilize them to determine the oxidation number of any element given to us within a compound.
Specific rules exist for calculating oxidation numbers of group 1A and 2A elements, F, H, O and Halogens.
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example
Calculate Oxidation Numbers Example 3
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Here in this example it says, which compound has oxygen with the lowest oxidation state? If we take a look at the first one, we have n a o two. We have one group 1 a element with 2 oxygens. Remember, that pattern tells us we have a superoxide. And if you have a superoxide, that means the oxidation number of oxygen is minus a half. For the next one, we have c02. Now it's not a superoxide. For it to be a superoxide, we would need 1 group 1 a element with the 2 oxygens. Carbon is not in group 1 a, it's in group 4 a, so it's not a superoxide. Next, it's not a peroxide either. To be a peroxide, you need 2 group 1 a elements with 2 oxygens. So it's not a peroxide, it's not a superoxide, therefore its oxidation number is minus 2. For the next one, we have c s 202, that's 2 cesiums which are in group 1 a with 2 oxygens. That fits the definition of a peroxide. So this is a cesium peroxide, so oxidation number for oxygen would be minus 1. And then finally we have o 2, this is the standard or natural state of oxygen. Remember an element found in its standard or natural state has an oxidation number of 0. So out of all our choices, the one that has the lowest oxidation number, the one with the most negative value would be b, c02. So just remember, if we have specific rules for that given element, utilize them to find the correct answer.
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Calculate Oxidation Numbers
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Now when asked to determine the oxidation number of a non listed element within a compound, we're gonna follow the next 4 steps. Now remember, we've gone over the specific oxidation number rules for certain elements. This applies to elements that are not found within that list. So step 1, you're gonna treat the non listed element as x. You're gonna write you're gonna use the list to write the known oxidation number of other elements within that compound. For example, if you see oxygen is in that compound and and it's not a peroxide or superoxide, we know its oxidation number would therefore be minus 2. Step 3, if an element has a subscript, then remember to distribute it. And then step 4, add up the oxidation numbers, create an equation, and make it equal to the charge of the compound. So these are the rules that we're gonna employ when looking for non listed elements within a compound. And we'll see how it how we use utilize these 4 steps to get to our oxidation number for that particular element. Now that we've gone over them, click on the next video and let's take a look at an example question.
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Calculate Oxidation Numbers Example 4
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Here it says, give the oxidation number of the carbon atom in the acetate ion. Alright. So we're gonna say we're looking for carbon, so carbon is going to be our x. Hydrogen when it's with nonmetals is plus 1. And then oxygen here in it's not an ox, superoxide or a peroxide, so it's minus 2. So we have 2 carbons, each one is x, plus 3 hydrogens, each one is plus 1, plus 2 oxygens, each one is minus 2. This equation equals the charge of the ion which is minus 1. So all we have to do now is solve for x. So 2x+3 minus 4 equals negative 1. 2x minus 1 equals negative 1. So add 1 to both sides here. So here, you know, weird turn of events, you can see that x now equals 0. So here the oxidation number of carbon within the structure is 0. So there are moments where your oxidation number could be 0 and the element is not in not not in its natural state, such as in this example. So again, the oxidation number of carbon when we calculated it comes out to be
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