Redox reactions?

Question

I'm having problems understanding oxidation and reduction, I'm not sure how to assign oxidation numbers. Like with compounds like H20 I know that hydrogen is +1 and oxygen is -2...but in reactions involving compounds with three or more elements I get confused, despite reading the rules for assigning oxidation numbers. Can someone explain what I do for reactions like:
a)2AgNO3+Cu----->Cu(NO3)2+2Ag
For this one I know that O is -2 so the sum of Ag and N has to be +6 to balance the compound's charge. But how do you figure out what charge N has and what charge Ag has?

b)3AgNO3+K3PO4---->Ag3PO4+3KNO3
Again, I know that O is -2, but how do I figure out the rest?

Also I'm a bit confused as to why when I write out ionic formulas...
Like for c) why is it:
3[Ag+NO3] and not 3[Ag+N+O3]?

These might seem like dumb questions but I'm pretty new at this and I'm having a bit of trouble understanding it...Thanks in advance for explaining it to me (I didn't get the textbook).

Note: I have the textbook...but by "get" it I mean I didn't understand that chapter of it.

Answer 1

NO3 - is a common polyatomic ion, which means that it the nitrogen here is commonly stuck to three oxygens. When you see NO3 (AKA nitrate ion) attached to something else, you know that the entire NO3 ion has a -1 oxidation number.

That is why your book writes it as Ag + NO3.

So in AgNO3... Since NO3 has a -1 oxidation number, we know that Ag must have a +1 oxidation number so that the number for the whole molecule is 0.

Thus, you know Ag is +1 and O is -2. You can now figure out the oxidation number for N, by doing some simple algebra, which should be + 5.

Answer 2

You have to know some of the players. For instance, NO3- is the nitrate ion. Oxygen nominally has a -2 valance (the bonding is covalent with the nitrogen and oxygen) so N has a +5 valance. Unless the reaction indicates that the nitrate ion is an oxidizer, it stays as nitrate ion. Silver ion is almost always +1.

1. In this reaction, the silver is being reduced from +1 to elemental state (zero) while the elemental copper is oxidized to the +2 valance state.
2. This is NOT a redox equation. All that is happening is that silver and potassium are swapping cations.
3. Again, NO3- is an ionic group. With each, one Ag+ is associated. You need the 3 because the PO4-3 ion, another important cation has a -3 charge. From our rule with oxygen in ions, P has a +5 valance in phosphate.

Again, become familiar with the cation groups, such as phosphate, nitrate, hydroxyl, sulfate, and the like.

Answer 3

A metal like Ag will always be +1. Usually you can look at the elements in a formula like AgNO3 and say that O has to be
-2 and Ag has to be +1 (its only oxidation state as a cation) then figure out the N by doing the math.

Also remember that the sum of the oxidation numbers in a polyatomic ion must be equal to the overall charge on the ion.

So if phosphate (PO4) is -3, then P has to be +5

If nitrate (NO3) is -1, then N has to be +5

Here is more info about assigning oxidation numbers:

http://www.fordhamprep.org/gcurran/sho/sho/lessons/lesson52.htm

Answer 4

a) You have to know from prfevious study that silver is Ag+ and nitrate is NO3-. In NO3-, O is -2, and three O's are -6. The charge on NO3- is -1, so N must be +5.

b) This is not an oxidation-reduction equation. It does not matter that P is +5. It only matters that PO4 3+, Ag+, K+, and NO3- balance out.

c) If the question is, "Write the ionic reaction for silver nitrate plus sodium chloride yields sodium nitrate and silver chloride," you write down:

Ag+ + Cl- ===> AgCl

This is because the interaction of silver and chloride ions is the only thing that matters.

Answer 5

NO3 is a polyatomic ion, an covalent molecule that has a charge...NO3 is -1, so u kno O is -2, so total O charge is -6....the entire molecule is -1 so N has to equal +5....since there is 1 NO3- molecule, u kno Ag has to be +1 b/c overall compound is neutral....same w/ Cu, since the compound has 2 NO3-, Cu has to be +2
u have to remember the polyatomic ions tho...

or u can use the periodic...
group 1(starting w/ Li) is +1,
group 2 (Be) is +2....
transition metals are usually +2/+3 but there are exceptions....
group 3 (B) is +3
group 4 (C) is iffy as either +4, +2, or -4
group 5 (N) usually -3, but there are exceptions
group 6 (O) usually -2, but there are exceptions
group 7 (F) usually -1 , are exceptions