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OXIDATION NUMBERS WORKSHEET - Emp Form/Mol Form

in this video we're going to go over oxidation numbers and how to find it so let's say if we're given the element in zinc what is the oxidation number of zinc now the first rule that you need to know is that the oxidation state of any pure element is always zero so the oxidation state of oxygen gas as a pure element is zero fluorine gas as the pure element is zero even phosphorus as a pure element is zero so there's no charges and it's only one pure element is not a compound the oxidation state will always be zero so that's the first rule you need to keep in mind now the second thing is the oxidation state of ions the oxidation state of the zinc two plus ion is basically the charge of what you see there it's positive two the oxidation state of the Fe plus three I am is simply positive three now sometimes you might have diatomic ions for example the mercury two plus ion individually each mercury ion has an oxidation state of one because there's two of them so you need to write an equation Q mercury Adams has a net charge of positive two so if you divide both sides by two you can get the individual oxidation state of each mercury particle which is plus one so there's another example this is the peroxide ion to find the oxidation state of each oxygen atom in this ion you can write an equation is two oxygen atoms with the total charge of negative two so individually each oxygen atom has a charge of minus one so that's the oxidation state of oxygen individually in the peroxide honor this is the superoxide iron so if you want to find the oxidation state you need to divide the total charge by two so each oxygen atom has a net charge of negative 1/2 so two of them combined will have a net charge of negative one so keep this in mind anytime you have a pure element the oxidation state will always be zero and if you have an ion let's say if it's a mono atomic ion the oxidation state is the same as that ion now let's talk about compounds whenever you have fluorine inside a compound when it's not a pure element fluorine is always going to have a negative 1 oxidation state fluorine is the most electronegative element when oxygen is in a compound it's going to have a negative 2 oxidation state unless it's bonded to fluorine or unless you hear the name peroxide or superoxide whenever you hear the name peroxide oxygen has a negative 1 oxidation state if you hear the word superoxide it has a negative 1/2 oxidation state if you hear the word oxide then the oxidation state is negative 2 which is 9 at the seventh time now hydrogen will have an oxidation state of plus run when bonded to a nonmetal when bonded to a metal hydrogen will have an oxidation state of negative one and really the key is electronegativity hydrogen is more electronegative than most metals that's why it bears a negative charge but hydrogen is usually less electronegative and then most nonmetals and so that's why there's a positive charge so typically the element that's more electronegative is the one that usually carries the negative charge now let's work on some examples what is the oxidation state of magnesium and chlorine in this compound by the way most halogens are usually negative one chlorine technically has a negative one charge like fluorine if we write an equation mg plus 2cl this whole compound is neutral so therefore the total charge is zero now if chlorine has a negative 1 oxidation state that means magnesium has to have a positive 2 oxidation state you can literally solve it and it makes sense magnesium is an alkaline earth metal which typically has a positive 2 charge go ahead and find the oxidation states of aluminum and fluorine in this example well we know that fluorine is negative 1 in a compound always an aluminum based on where it's located in a periodic table it's typically positive 3 within an ionic compound and you could solve it to Al plus 3 F should add up to 0 because then that charge is 0 so each fluorine atom has an oxidation state of negative 1 so now we got to add 3 to both sides so aluminum has an oxidation state of positive 3 is another example find the oxidation state of vanadium and oxygen and is kapha so this is called vanadium oxide so whenever you hear the word oxide oxygen has a negative two charge so we got two vanadium atoms plus five oxygen atoms with a net charge of zero so each oxygen atom has an oxidation state of negative 2 5 times negative 2 is negative 10 and then add 10 to both sides so 2 V is equal to 10 next divide both sides by 2 so 10 divided by 2 is 5 and so the oxidation state of vanadium is positive 5 now let's go over some examples containment polyatomic ions consider sulfate what is the oxidation state of sulfur and sulfate well no oxygen is usually negative 2 so let's write an equation sulfur plus 4 oxygen atoms has a net charge of negative 2 so each option atom has an oxidation state of minus 2 and 4 times negative 2 that's negative 8 next we need to add 8 to both sides negative 2 plus 8 is positive 6 so this is the oxidation state of sulfur and sulfate let's look at another example phosphate go ahead and find the oxidation state of phosphorus in phosphate so once again oxygen is still negative 2 so we got a phosphorus atom plus 4 oxygen atoms and then that charge is negative 3 based on what we see here so it's going to be P plus 4 times negative 2 and 4 times negative 2 is negative 8 and then add 8 to both sides so negative 3 plus 8 that's going to be positive 5 and that's the oxidation state of phosphorus let's look at another example let's try Nightrain and also glory as perchlorate go ahead and find the oxidation state of nitrogen and chlorine in these two pilots Emig ions so we have a nitrogen three oxygen atoms and that's going to equal in that charge of negative one so always negative 2 3 times negative 2 is going to be negative 6 and negative run plus 6 if we add 6 to both sides that's going to be positive 5 so that's the oxidation state of nitrogen and for chlorine in per chlorine it's going to be CL plus 4 oxygens equals in that charge of negative 1 so this is going to be 4 times negative 2 which is a negative 8 and then negative 1 plus 8 that's going to give us an oxidation state of positive stuff so now I know how to find the oxidation states of elements within compounds and polyatomic ions now I want you to understand the concept of electronegativity and how it relates to oxidation numbers electronegativity increases towards fluorine on a periodic table so as you go up into the right the electronegativity increases so let me give you some values of common elements so let's say hydrogen is somewhere in the corner over there and then we have boron carbon nitrogen oxygen fluorine chlorine bromine iodine phosphorus and sulfur hydrogen has an electronegativity value of 2.1 for boron is 2.0 carbon is 2.5 and then 3.0 3.5 floorings the highest is 4.0 phosphorus it's two point one is the same as hydrogen sulfur is 2.5 chlorine the street for no and this is 2.8 iodine is 2.5 so keep these values in mind so here's a question for you what is the oxidation state of oxygen and flowing in oxygen difluoride now oxygen has an electronegativity value of 3.5 fluorine is 4.0 so which one is more electronegative electronegativity is the ability of an atom to attract electrons to itself so flowing is going to pull on electrons in this molecule it's going to have a stronger Pole than oxygen so flowing is going to acquire a partial negative charge whereas oxygen is therefore going to acquire a partial positive charge because fluorine pulls on the electrons stronger than oxygen can so in this example oxygen will not have its typical charge of negative 2 the only time oxygen will have its oxidation state of negative 2 is if it's the most electronegative element in that compound if it's not then it's going to have a positive oxidation state keep in mind any time flame is in the compound it has an oxidation state of negative 1 and the reason for that is because fluorine is the most electronegative element on a periodic table so now we can solve for oxygen so o plus 2 F should have a net charge of zero because there's no number here so fluorine is negative 1/2 times negative 1 is negative 2 so if we add 2 to both sides oxygen is going to equal positive 2 which makes sense because it's partially positive in this particular example now let's look at two other examples hydrochloric acid and sodium hydride chlorine has an electronegativity value of 3.0 hydrogen is 2.1 and sodium it's like 1 point something I'm not sure what the exact number is it could be like 1.5 1.7 but I know it's less than 2 so in this example hydrogen there's a partial positive charge chlorine there's a negative charge because chlorine is more electronegative than hydrogen so therefore chlorine is going to have its oxidation state of negative 1 which is typical of most collisions hydrogen is going to have an oxidation state of plus 1 as you mentioned before whenever hydrogen is bonded to a nonmetal the oxidation state is usually positive 1 now what about in sodium hydride well we know that sodium is an alkali metal which always have a positive run charge so therefore sodium is going to have an oxidation state of +1 but hydrogen has an oxidation state of negative 1 typically when hydrogen is bonded to a metal it usually has a negative Run oxidation state and it makes sense because hydrogen is more electronegative than most metals so it usually bares the partial negative charge that's why it has a negative oxidation state sodium has the positive charge still it has a positive oxidation state and so you can use electronegativity to help you determine what the oxidation state will be so let me give you another example BH Stream what is the oxidation state of boron and hydrogen feel free to try that one now hydrogen has an electronegativity value of 2.1 and boron is 2.0 now as boron a metal or nonmetal in this example hydrogen is more electronegative so hydrogen bares the partial negative charge or boron bears the partial positive charge so therefore hydrogen it's going to have its oxidation state of negative one because it's more electronegative than boron so then this is going to be B plus 3 H which is equal to zero so 3 times negative 1 is negative 3 so boron is going to have an oxidation state of positive 3 in this example now let's consider these two examples so Furyk acid or rather hydrofluoric acid and also sulphur dioxide now hydrogen has an en value of 2.1 sulphur is 2.5 an oxygen is 3.5 so in sulphur dioxide oxygen has the partial negative charge sulphur has the partial positive charge now in h2s hydrogen has the partial positive charge sulphur has the function negative charge 9 a periodic table we have elements like nitrogen oxygen fluorine typically nitrogen has a negative 3 charge oxygen - 2 fluorine negative 1 so for 2 sulfur usually has a negative 2 charge if if sulfur is the more electronegative element so looking at h2s hydrogen is brought in to a nonmetal that is more electronegative than itself so hydrogen is going to have the positive 1 oxidation state and there's two of them so sulfur in this example has its normal oxidation state of negative 2 so you can base your answer on a periodic table if sulfur is the more electronegative element now in so2 you can't do that because sulfur doesn't have the partial negative charge so you can't based a charge on a periodic table you can do so however for oxygen because oxygen is the electronegative element in that compound so you can use the negative two charge for oxygen so oxygen is going to have an oxidation state of minus two and to find it for sulfur it's going to be s plus two oxygen atoms equals zero so that's 2 times negative 2 which is negative 4 so sulfur is going to have a positive oxidation state of 4 due to the positive partial charge so elements that are less electronegative typically those are the ones you got to solve for the ones that are more electronegative you can find a charge based on a periodic table if they carry a negative charge now let's look at some other examples nh3 and no2 go ahead and find the oxidation state of each element now in ammonia hydrogen has an en value of 2.1 but nitrogen is more electronegative it's 3.0 so therefore nitrogen should have its normal charge of negative 3 if we write an equation n plus 3 H is equal to 0 hydrogen is going to have a positive run charge it's partially positive whereas nitrogen is partially negative so typically when hydrogen is bonded to a nonmetal it's usually +1 which means n has to be negative 3 so as you can see nitrogen is the electronegative element in this example and it has its periodic charge of negative 3 which you can find out in periodic table now in this case o is more electronegative so nitrogen is going to have a different oxidation state it's not going to be its natural oxidation state of negative 3 so in this example it's going to be positive for typically when you have elements like nitrogen sulfur phosphorus if they carry and the element that's more electronegative than itself those are the elements together solve for the one that usually has a partial positive charge now try these two examples methane and the carbon dioxide and methane hydrogen has a positive one charge hydrogen is less electronegative than carbon so it's going to be partially positive carbon is going to be partially negative so solve them for carbon we have C plus 4-h is equal to zero so that's 4 times 1 so C is negative 4 so when carbon is bonded to hydrogen carbon has a negative oxidation state when carbon is bonded to oxygen it's going to have a positive oxidation state when it's bonded to hydrogen it has a negative oxidation state so oxygen is negative 2 and there's two of them so carbon is going to have to be positive 4 in this example now sometimes you might have elements that have an average oxidation state that's not a whole number let's try these two c3h8 and fe3o4 in this example hydrogen is less electronegative than carbon so it's going to be positive 1 so if we write the formula 3 C + 8 H it's equal to zero so that's going to be 8 times 1 and if we subtract 8 from both sides 3 C is equal to negative 8 so carbon on average has an oxidation state of negative 8 over 3 now let's do the same thing for fv304 so we got 3 iron atoms and 4 oxygen atoms within that charge of zero so oxygen has an oxidation state of negative 2 so 4 times negative 2 that's negative 8 and if we add 8 to both sides when you get this so Fe has an oxidation state of 8 over 3 so 8 over 3 is about 2.67 now keep in mind an individual iron atom cannot have a charge of 2.67 it's usually a whole number like positive 2 or positive 3 because electrons and protons there they basically have numerical charges an electron has a charge of negative 1 a proton has a charge of positive 1 so a typical ion will have a decimal charge so what does it mean that the average oxidation state is 2 point 6 7 so what is meant by that in this compound there are three are in ions and four oxygen ions each oxygen has a charge of negative two so the total negative charge is negative 8 in order for the compound to be electrically neutral the total positive charge has to be positive 8 iron metal has two common oxidation states positive 2 and positive 3 now they all can't be positive 3 because 3 plus 3 plus 3 is 9 and they can't all be positive 2 because 2 plus 2 plus 2 is 6 so some of them is positive 2 and some are positive 3 so the question is how many iron ions to have a +2 charge and how many have a +3 charge in order to get up to eight two of them has to have a positive 3 charge and one of them has to have a positive 2 charge if you average the numbers 2 3 and 3 and divided by 3 that's going to be 8 over 3 which averages out to 2 point 6 7 so whenever you get a decimal value what it really means is that that's the average oxidation state individually some more positive 3 and some are positive 2 so the individual ions should have a numerical oxidation state so when you have multiple of them the average could be a no value because these they don't all have to be the same they can be different so hopefully this makes sense in terms of why some oxidation states have a decimal value now let's try the polyatomic ions that have three different elements in it go ahead and find the oxidation state of every element in that polyatomic ion so oxygen has an oxidation state of negative two hydrogen is positive one when it's bonded to nonmetals so usually so all we're going to do is find sulphur so H plus s plus three oxygen atoms as in that charge of negative one so hydrogen is one oxygen is negative 2 and so 3 times negative 2 that's negative 6 and then 1 plus negative 6 is negative 5 so now let's add 5 to both sides negative one plus five is positive four so in this example sulfur has an oxidation state of positive four go ahead and try this one k2 cro4 find the oxidation state of every element in that example so we have two potassium atoms a chromium atom and four oxygen atoms now we know oxygen is going to have an oxidation number of negative two potassium is an alkali metal which all of them have a positive one charge chromium is the transition metal and it has a variable charge so that's what I'm going to solve for so this is going to be 2 times 1 plus CR plus 4 times negative 2 and all of that is equal to 0 so 4 times negative 2 that's negative 8 and 2 plus negative 8 is negative 6 so therefore in this example chromium has an oxidation state of positive 6 try this one potassium bicarbonate find the oxidation state of carbon in this example so we know oxygen is going to be negative to potassium and alkali metal is plus 1 and hydrogen hydrogen is actually bonded to the oxygen in bicarbonate if you were to draw the Lewis structure so therefore hydrogen is bonded to a nonmetal through a covalent bond and so it's going to be plus 1 so we have K plus h plus c plus 3 o and that's equal to 0 so K is positive 1 hydrogen is 1 an oxygen is going to be 3 oxygen is negative 2 but we're going to multiply that by 3 so 1 plus 1 is 2 3 times negative 2 is negative 6 and then 2 plus negative 6 that's negative 4 so in this example carbon is positive for potassium bicarbonate you could break it up into two ions k+ and hco3 minus so just by looking at k+ that tells you that cain has an oxidation state of positive 1 now bicarbonate is basically the sum of the hydrogen ion and the carbonate ion so therefore you can see that hydrogen in this example also has a positive one charge and then from this you can find the oxidation state of carbon you can say c plus 3 own has in that charge of negative 2 then you have to add six to both sides so negative two plus six is positive four so if you understand the ions and all the polyatomic ions you can break it down and efficiently to see that hydrogen has a positive one charge in this example and the same is true for K so that's why it's good to know the polyatomic ion Chi now have two more examples fully BR CL 3 and IBR 5 find the oxidation state of every element in this example so most halogens like fluorine chlorine bromine iodine they typically have a negative 1 charge but both bromine and chlorine can't be negative so which one is negative and which one is positive keep in mind bromine has an electronegativity value of 2.8 chlorine is 3.0 iodine is 2.5 so in this example chlorine bears the partial negative charge bromine is partially positive so therefore chlorine is going to have its natural oxidation state of negative 1 bromine we need to calculate it so it's going to be BR plus 3 CL and that's equal to 0 so this is going to be 3 times negative 1 and so we can see that bromine has an oxidation state of positive 3 now in the second example bromine is going to carry the partial negative charge iodine carries the partial positive charge if it's written correctly usually the electropositive element is written first the electronegative element is there in second so the one that you see on the right side is usually the one that carries the natural charge that can be found on the clear example so in this case bromine is going to have its natural oxidation state of negative 1 so iodine it's going to have an oxidation state of positive five in this example so hopefully you understand the relationship between electronegativity and oxidation numbers so that's it for this video thanks for watching and have a good day