there a 3 popular very different ways this topic is taught...
1) leo the lion says grr...
2) oilrig..
3) "reduction = reduction in charge"
at YA, you will see all three of those methods... ok? so let's go through them one at a time...
*** leo the lion says grr ***
Loss of Electrons is Oxidation... (LEO)
Gain of Electrons is Reduction... (GER)
notice the acronym doesn't exactly follow the acronym "LEO say GRR"?
notice that this really isn't intuitive.. a lion and ger/grr in a chemistry class or lab?
notice that you still have to translate lost of electrons into gain in charge?
*** 2. OILRIG ***
Oxidation Is Loss... (OIL)
Reduction Is Gain...(RIG)
notice this isn't intuitive either..... an oilrig in a chemistry class or lab?
notice you're on your own to remember the entity being lost or gained is "electrons".. not charge
and again.. you still have to translate "loss of electrons" into "gain in charge"...right?
*** 3 "reduction = reduction in charge" ***
REMEMBER....
reduction means "reduction in charge"
and therefore it follows logically that
1) oxidation must mean an "increase in charge"
2) a "reduction in charge" happens with a gain of electrons (e's are negative right?)
3) an "increase in charge" happens with a loss of electrons.
notice this is easy to rememember
notice the rest is intuitive.
memorize this.!!!!
"reduction = reduction in charge"
"reduction = reduction in charge"
"reduction = reduction in charge"
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and for all the methods...
the agents are assistants. they assist the other species in whatever it's doing. Something that is reduced is assisting something else to oxidize. the species reduced is therefore the oxidizing agent. The species oxidized is the reducing agent. ok?
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your particular problem.... you need to start by identifying the charge on each atom...and then look for those that have changed from reactants to products..
H in H2O2 is in the +1 state... why? H is usually +1 unless it's bonded to itself or a lower electronegative species.
O in H2O2 is -1... why? 2 H's = +2.. ..to balance.. the 2 O's must be -2.. ie.. -1 each... and H2O2 is H-O-O-H...ok? Oxygen is almost always -2 except in the case where it's bonded to itself.
Pb in PbS is +2.. why? S is -2 and PbS has a zero charge.
S in PbS is -2.. why? S is usually -2 when it is a monoelemental anion.
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Pb in PbSO4 is +2.. why? SO4 is (-2).. it is the sulfate ion (think about H2SO4).. Pb must be +2 to balance
S in PbSO4 is +6.. why? 1x(+2) + 1x(?) + 4x(-2) = 0....
O in PbSO4 is -2.. why? O is usually -2.
H in H2O is +1
O in H2O is -2...
ok?
therefore...
H went from +1 to +1 and was unchanged
Pb went from +2 to +2 and was unchanged
and....
O went from -1 to -2 and was "REDUCED".. remember reduction is reduction in charge
S went from -2 to +6 and was "OXIDIZED".... increase in charge...
and...
since O was reduced it is the oxidizing agent
since S was oxidized, it is the reducing agent.
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got all that?
*** remember ***
reduction = "reduction in charge".... <<<< REMEMBER!... and the rest follow logically.
oxidation = increase in charge
reducing agent = species oxidized
oxidizing agent = species reduced.
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now look at the other two answers...
Bhimgiri is close but got lost on the interpretation of -1 to -2..charge change. That's not oxidation. it is reduction.. it occured by adding 1 electron.....
GER.. Gain of Electrons = Reduction.
RIG... Reduction Is Gain (in electrons)
and the species oxidized is never the oxidizing agent. agents are assistants. opposite.
TNH is also incorrect. oxidation ≠ gain of oxygen... it is "gain in charge".. loss of electrons.. by definition. You don't have to have oxygen in the reaction to have a redox reaction. And lead.. wasn't oxidized.. nor reduced. Same for hydrogen. The oxygen was "reduced" the sulfur was "oxidized" in this reaction.
I chalk both those errors up to trying to apply either LEOGER or OILRIG and not "reduction = reduction in charge".
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questions about any of this?