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Balancing Redox Reactions
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0:01
in this video we're going to focus on balancing redo reactions so first we're going to um work on just simple reactions between metals and metal cations then we're going to work on balance and reduxx reactions under acidic conditions and in basic conditions so let's begin now the first thing you want to do is separate the reaction into two half reactions so let's start with zinc zinc metal converts into the zinc 2 plus I now the first thing you want to do is you want to balance the atoms so the
0:36
zinc atoms are balanced we have one zinc atom on the left side one zinc Ion on the right once all the atoms are balanced the next thing you want to do is balance the charges we have no charge on the left but a plus two charge on the right side so we got to add two electrons to balance it you always want to add the electrons to the side with a higher charge so this half reaction is balanced by the way this half reaction is known as oxidation whenever you have electrons in
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the right side um electrons are lost zinc gave away electrons so it's oxidation now let's look at the other half reaction we have ag+ converted into AG so we have a plus one charge on the left zero on the right so we need to add one electron to the left side to balance the charges now the next thing we need to do is make sure the number of electrons are equal anytime you have electrons on the left side it's a reduction
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reaction now before we can add these two we need to make the number of electrons in both half reactions equal so we're going to multiply the second equation by two and now we can add them so we have if we add bring this down we have zinc plus 2 ag+ cuz we got to multiply by two the two electrons will
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cancel and on the right side we have zinc 2+ and we're going to multiply that by two so 2 AG so now that reaction is Balan let's try another example for the sake of practice so let's say we have magnesium uh 2+ with Fe let's say it converts to um actually hold
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on fe2+ let's say it converts to mg 2+ and Fe now feel free to pause the video and try this example yourself and see if you can get the answer and then unpause it when you're ready so let's start with mg so the Magnesium atoms are already balanced on both sides so we have a charge of zero on the left a charge of two on the right and so we need to add two electrons to the side with the higher charge now we have fe2+ on that side and Fe on the right side so the left side has a higher
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charge so we're going to add two electrons to balance it now the electrons are already equal so these will cancel and we can simply add them which means that this is already
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balanced let's try another example A Plus CU 2+ and a
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3+ plus CU now this reaction isn't balanced because the charges are different even though the the number of atoms are the same so let's start with
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aluminum we need to add three electrons to the right side to balance the charge for aluminum so that's it for that half reaction and this half reaction is known as oxidation now for the second one we have copper 2 plus on the left and copper on the right side the left side has a higher charge so we need to add two electrons to it and now so the second half reaction which is the Redux reaction or the reduction reaction that half reaction is balanced so now what we need to do is make sure
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the number of electrons are are the same so the least common multiple of three and two is six so we're going to multiply this equation by two and this equation by three and at that point the number of electrons will
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equal so we have two Al 2 al3+ and six electrons and here we have six electrons 3 cu2+ 3 CU and now we're going to add the two equations so as you can see these cancel and then we're left with 2 al+ 3 cu2+ and on the right side we have 2 Al 3++ 3 CU and now it's balanced notice that the total charge on the left side is the same as the total charge on the right side 2 * 3 is 6 3 * 2 is 6 both sides sides have a total charge of plus six so now we're going to focus on
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balancing Redux reactions under acidic conditions so let's say if you're given this
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reaction how would you balance it so under acidic conditions you are allowed to add H+ or H2O um to balance the reaction let's start with zinc so we have zinc metal converting into zinc 2+ so we know we need to add two electrons to this half reaction to balance it so that reaction is done now let's look at the next example the second half reaction we have bromate converting into bromide now we can't add electrons yet because we have to balance the number of
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atoms we have three oxygen atoms on the left side so what we need to do is add three uh water molecules so the oxygen atoms are balanced but notice that we have six hydrogen atoms on the right side so we need to add six H+ ions on the left side so now the atoms are balanced now we need to look at the charges so if we calculate the total charge on the left side it's 6 * 1 that which is plus 6 and then we need to add this Nega 1 on on bromate so 6 - one that's a total
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charge of + 5 on the left side on the right side we only have a charge of minus one so if you subtract the two numbers if you take the larger number minus the small number five minus 1 is equal to 6 so what this tells you is that you need to add six electrons to the side with the higher charge which is the left side and now that half reaction is balanced however the number of electrons are unequal so we got to multiply this by three so we have three zinc atoms E3 zn2+ plus 6
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Rons so now we can add the two equations so the electrons will cancel and then everything on the left side of that line simply falls down to the left side so we have 6 H+ plus bromate plus 3 ZN yields 3 Z n2+ Plus BR R minus + 3 H2O and now that half reaction is balanced let's try another one under acidic conditions
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okay feel free to pause the video and try this example so let's begin let's start with aluminum so all we need to do is add three electrons to balance the charge and that's it for that half reaction next let's look at hypochloride now for this one we have to balance the number of atoms before we can balance the charge we have one oxygen atom on the left so that means we got to add H2O to the right side now we have two hydrogens on the right side so we got to add 2 H+ on the left
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side so now the number of atoms are balanced now we got to balance the electrons the total charge on the left side is Let's see we have 2 * 1 which is two plus this negative 1 charge so we have a total charge of plus one on the left side on the right side we have a total charge of minus one so if we subtract the two numbers starting with the larg number the largest minus the smallest so 1 minus1 gives us two what this means is we got to add two electrons to the left
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side which is the side with the higher charge okay so now these half reactions are balanced so what we need to do next is to make sure the number of electrons are equal as we said before the leaston multiple of two and three is six so we need to multiply this by two so we can get six electrons and we need to multiply this equation by three so we can get six electrons so let's rewrite what we have so we have two a and that yields two al3+ with uh six electrons
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now on the bottom we have six
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electrons plus six H+
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ions we have two hypochlorite
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ions and we have three chloride
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ions and three H2 molecules so now we can add these two uh half reactions so the number of electrons will cancel so we get 2 Al plus 6 H+ and two hypochloride ions
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that turns into 2
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al3+ three chloride
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ions and three H2
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molecules so now every atom is balanced we have two aluminum atoms wait hold on this is supposed to to be a three 3 CL minus so let's fix that that's a three okay now everything is balanced we have three chlorine atoms two aluminum atoms on both sides six hydrogen atoms three oxygen atoms and let's make sure the total charge um adds up so 6 * 1 is a 6 plus3 that's a total charge of positive3 on the left side on the right side 2 * 3 is 6 and we have 3 * 1 which is -3 6 +3 is
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3 so the left side and the right side have the same total charge let's try one more example under a cdic conditions and then we'll try a few problems under basic conditions
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okay so go ahead and try this example so let's start with
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Fe so the atoms are balanced so we need to balance the number of electrons so all we got to do is add three electrons to it so now we can move on to the reduction part so we have the same number of Manganese atoms but we have four oxygen on the left so we need to add four H2O molecules on the right side now we have eight hydrogen atoms on the right side so we need eight H+ ions on the left side so next we got to balance the number of electrons here we have a total charge of 8
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plus1 which is POS 7 and here the total charge is plus two 7 - 2 is 5 so we're going to add five electrons to the side with the higher charge so now that half reaction is balanced the least common multiple between three and five is 15 so we're going to multiply this equation by five and this by
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three so now we have 5 Fe 5 fe3+ and 15 electrons and below that we have 15
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electrons three permanganate ions 24 H+
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ions three mn2+ ions and 12 H2O
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molecules so now we can add those values so we have 5 Fe plus three prominate ions 24 H+
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ions five
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fe3+ three of those and 12 H2O molecules okay let's check check to see if we balanced it correctly we have five iron atoms 24 hydrogen atoms 12 * 2 is 24 three manganese atoms and here we have 12 oxygens and 3 * 4 is 12 oxyen now let's add the charge so 5 * 3 is 15 3 * 2 is six so we have a total charge of positive 21 on the right side 3 * - 1 is -3 and 24 * 1 + 24 which is POS 21 so the charge is balanced as well so that's how you can balance Redux reactions under acidic conditions
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balance the atoms first by adding hydrogen or water and then balance the charges by adding electrons then add the two equations whenever the electrons are equal and then you're done so now let's try to balance these uh problems under basic conditions so let's start with this example let's say if we have iron metal plus the hypochloride ion and let's say that turns into iron 2 hydroxide plus chloride so let's start with Fe so we have Fe turnning into fe2 now under basic conditions we we can
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add water and hydroxide like under acidic conditions we could have added H+ and water so what we need is we got to add two hydroxide ions so the atoms are balanced the charge on the right is zero the charge on the left is -2 so the right side has a higher charge and these numbers differ by two so we're going to add two electrons to the right side so now let's start with the uh other half reaction hypochloride converted into chloride now if we add hydroxide we change the
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number of oxygens and hydrogens so take a minute think of how you would balance that second half reaction feel free to pause the video and try yourself whenever you get to this point it's easier if you add water to the side that has more oxygen atoms so let's begin by adding one water molecule to the left side notice that we have two hydrogens on the left two oxygen on right whenever they're the same that's good we can balance it so we're going to add two hydroxide ions on the right side
19:06
so the number of atoms are balanced now we get to balance the charges we have a total charge of negative one on the left side and here that's minus1 that's 2 * 1 so -2 so we have a total charge of minus three on the right side this is1 this is -2 so 1 and3 they differ by
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two so we got to add two electrons now which number is higher negative 1 org3 negative 1 is higher on the number line so we got to add it to the left side if you have the electrons here on the right side for the other one it has to be on the left they both can't be on the same side in one reaction electrons are given away and the other half reaction receives the electrons now fortunately the electrons are equal so we don't have to multiply the half reactions by anything so now we can add
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everything so notice that the electrons cancel but also the hydroxide ions cancel as well we have two on the left two on the right so now we can add the half reactions on the left side we have H2O plus the hypochloride ion plus Fe and that's going to convert into CL minus and fe2 and that's it that half reaction is now balanced so let's try another example so let's say if we have iodide plus chlorate and let's say that converts into the triiodide ion and chloride so let's start with i
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so we have IUS converting into I3 minus so first let's balance the atoms we have three iodine atoms on the right side let's put a three in front of on the left side so the total charge on the left side is minus three on the right side minus one these numbers they differ by two and negative 1 is larger than3 on the number line so we're going to add two electrons to the right side so that half reaction is balanced now let's move on to the next one so we have chlorate converted into
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chloride so let's balance the number of oxygen atoms so remember we want to add water to the side that has more oxygen atoms because we have three oxygen atoms we're going to begin by adding three water molecules notice that we have six hydrogen atoms and six oxygen atoms on the left side so what that means is we need six hydroxide ions so the atoms are balance the total charge on the left side is 1 on the right side we have 1 + -6 so we have a total charge of7 these numbers they differ by
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six if we take the large number minus the small number-1 - -7 is the same as 1 + 7 and you get uh positive 6 so1 is larger than7 so we're going to add six electrons to the left side the side with the higher
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charge so now we need to multiply this equation by three so that the number of electrons are the same so if we do that we're going to have nine iodide ions three triiodide ions and six
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electrons so now let's add the two equations these those will cancel so we have have three H2O plus one chlorate ion and nine iodide
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ions one
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chloride three triiodide ions and that is it no wait six hydroxide almost forgot that
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one okay so let's check out work so the iodine atoms are equal nine on the left 3 * 3 is 9 we have one chlorine atom on both sides 3 * 2 is six six hydrogen atoms we have three oxygen atoms there another three here so a total of six and let's add the the charge this is NE 1 here we have uh 9 * 1 so 9 +1 is -10 here is 3 * 1 so that's -3 plus this minus charge plus -6 -3 +6 +1 is -10 so the charges are balanced as well now let's try another example
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so let's say if we have sulfite plus this nickel hydroxide ion and it converts to nickel and sulfate go ahead and balance the reaction under basic conditions so let's start with sulfite sulfite converting into sulf
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so we need to add water to the side that has more oxygen atoms so we're going to add an H2O
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molecule on the right side now the question is how many H2O molecules should we have because we already have some oxygen atoms on the left side so think about this feel free to pause the video and figure out how many oxygen atoms do we need on the other
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side it turns out that we only need one water
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molecule so notice that we have five oxygen atoms on the right side two hydrogen atoms on the right but we already have three oxygen atoms five is two and so we only need two hydroxide anons to balance it notice that if we add two hydroxide ions on the left side we have five oxygen atoms on both sides and two hydrogen atoms in the last example the number of water molecules that we added was equal to whatever this number was but in that case it won't work in this example
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because we already have three so we need to do a quick method to find out how many water molecules you need to add to the right side subtract these two numbers 4 minus 3 is 1 so you need to add one H2O molecule then the number of hydroxide ions you need is equal to the number of hydrogen atoms that you have on the right side since we have two hydrogen atoms we need two hydroxide ions and now it's balanced the total charge on the left side is4 this is -2 that's -2
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so4 on the right side the total charge is minus two and they differ by two so the right side has a higher charge -2 is higher than4 on the number line so we're going to add two electrons so that half reaction is balanced now let's look at the other
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example so clearly we can see that we need three hydroxide ions on the right side and the total charge on the right is uh -3 on the left1 so we need to add two electrons to the left side since it has the higher charge and negative 1 and3 differ by two so the number of electrons are the same which is good so we can just add everything now notice that we have three hydroxide ions on the right side two on the left so if we subtract both sides by two hydroxides this disappears and we
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have one left over
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here so now let's add everything on the left we have have a sulfite ion plus the nickel hydroxide
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ion and we have
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sulfate plus nickel plus water and plus a hydroxide ion okay let's check our work so we have one sulfur atom one nickel atom both sides we have three oxygen atoms here and another three so six here we have four oxygen atoms plus one plus one that's six and we have three hydrogen atoms two here plus one that's three the total charge on the left side -1 + -2 is -3 -2 + 1 is -3 so everything is
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balanced so let's try another example so
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chromate plus magnesium converting into magnesium
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hydroxide and chromium hydroxide so let's start with
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magnesium so we can see that we need two hydroxide on the left and the total charge on the left side is minus 2 it's zero on the right side so Zer is higher than -2 so we're going to add two electrons to the right side so now let's balance chromate so we have C4 2 minus and CR O3 now this one is going to be more complicated than usual because we already have hydrogen present so feel free to pause the video and try to balance it this one is going to be harder than the other
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ones so we already know that we should add water to the side that has more oxygen atoms generally speaking that's if there's no hydrogen atoms present but should we use that same technique in this case what would you say so initially let's try this Trion error let's say if we add water to the left side that means we have five oxygen atoms and two hydrogen atoms now we want the number of oxygen hydrogen atoms to be equal the number of oxygen hydrogen atoms in the
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right side are equal and whenever you see that that means this technique will work you see how we have four oxygen atoms here let's add four water molecules so on the left side we have eight hydrogen atoms and eight oxygen atoms which is basically saying we have eight hydroxide ions and we have three hydroxide ions on the right side 8 minus 3 is five so we need five hydroxide ions on the right side notice that it's balanced we have eight hydrogen atoms on both sides eight oxygen atoms on both
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sides well fortunately that worked out pretty well so now let's balance the charges so we have minus two on the left side Min - 5 on the right side so -2 is larger than 5 and if we subtract them -2 minus5 is plus three so we need to add three electrons to the left side so we got to multiply this by three and this by two so we can get six electrons on both sides so now we have six hydroxide ions plus three magnesium atoms and three mg2 and six
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electrons so here we have six electrons eight water molecules two chromate ions and two chromium hydroxide formula units and 10 hydroxide ions so now we can add them so these will cancel notice that these two are similar so we're going to subtract both sides by six hydroxide ions so that's going to cancel and this is going to be reduced to four so it's going to be 3 mg Plus 8 H2O plus two chromate ions three mg O2 to CR O3 and four hydroxide ions so now it's balanced let's make sure so we have
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three magnesium atoms on both sides 16 H2O molecules on the left I mean 16 hydrogen atoms here we have 3 * 2 which is six hydrogens 2 * 3 which is 6 that's 12 + 4 so that's 16 we have two chromium atoms two chromium atoms there and now for the oxygen we have eight oxygen here plus another eight that's 16 3 * 2 is 6 2 * 3 is 6 that's 12 + 4 16 everything is balanced and then finally the charges so 2 * 2 is4 and 4 * -14 so everything is balanced in this video so that's the end of this lesson
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and and so now you know how to balance basic redox reactions redox reactions under acidic conditions and under basic conditions and you've seen
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