Chemistry question...No, not homework help

Factual Questions
1

Ivyboy is having a problem understanding chemical equations. Since I have managed to forget most of my high school chemistry, and Ivylad doesn’t understand this either, we turn to you in helping Ivyboy understand this concept. For example:
MnO[sub]2[/sub]
H[sub]2[/sub]O[sub]2[/sub] (Aq) ----------------------> H[sub]2[/sub]O (L) + O[sub]2[/sub] (G)

This equation shows the effect of a catalyst (manganese 4 oxide) on hydrogen peroxide. The product gains an oxygen atom. How is this possible when the law of conservation of mass clearly states that matter is neither created or destroyed in a chemical reaction, just rearranged?

2

Well, they sort of left out a step. Each H[sub]2[/sub]O[sub]2[/sub] molecule liberates one oxygen (O) atom. These monatomic oxygen atoms then quickly combine to form diatomic oxygen molecules (O[sub]2[/sub]). As written, the equation is innacurate.

3

Or *inaccurate - much like my typing. :smack:

4

Actually, the equation (omitting reference to the catalyst) ought to be either:

H[sub]2[/sub]O[sub]2[/sub] --> H[sub]2[/sub]O + O: (monatomic oxygen radical)

or

2H[sub]2[/sub]O[sub]2[/sub] --> 2H[sub]2[/sub]O + O[sub]2[/sub]

5

Q.E.D., you can spel gud! :slight_smile:

Ivylass, is he coping with “Moles” O.K.? It’s the simplest concept in the world, once you get it – and one of the toughest to get a handle on.

6

Great. We copied it straight from the textbook.

In the paragraph above the equation, it says

and then they show the equation I typed earlier.

7

Oh, hush, you! I haven’t finished my morning coffee yet. Yeah, that’s the ticket.

8

Ivyboy says Moles are 6.24 x 10[sup]28[/sup] and that they can be used for any quality. So it’s a unit of quantity, like saying I have a dozen of something? You can say you have a mole of something?

If so, he understands it.

9

The equation accurately shows what’s happening in the reaction, they just didn’t bother to balance it. That’s done fairly often, depending on the purpose of including the equation.

10

The bit of the text you quoted is correct, albeit, a bit vague for scientific purposes. It should more properly have read “the compound manganese (IV) oxide catalyzes the decomposition of an aqueous solution of hydrogen peroxide into water and monatomic oxygen.” And the equation should be one of the versions Polycarp posted. I prefer the first one, personally, since it shows what is happening on a fundamental level.

11

A mole is 6.022 x 10[sup]23[/sup] particles.

12

Yeah, the equations Polycarp gave are correctly balanced. The equation you copied from the book gives the general idea what reactants and products are involved, but not how much of each. That’s why the book called it a “skeleton equation”. It’s not incorrect, per se, as long as they note that they didn’t do anything to balance it.

13

Okay, first off, this really hurts my head. But I press on for my son’s illumination…

Ivyboy asks if you can show all the steps to the skeletal equation.

14

Actually, thinking it over, it’s fine the way it was. It’s just the equation that needs to be balanced. Unbalanced equations are fine for professionals in the field, but in high school texts, it just leads to unnecessary confusion, IMO.

15

Ivyboy says :smack:

You know, with 10th grade chemistry textbooks, you would think they would make things as CLEAR AS POSSIBLE!

To expound further, how do you know where the elements end up? For instance:

2AgNO[sub]3[/sub] (aq) + Cu (s) ---------------> Cu(NO[sub]3[/sub])[sub]2/sub + 2Ag (s).

How do they know that the Copper will end up with the nitrate and not the silver?

16

That one’s not too bad if you think about it. The nitrate ion (NO[sub]3[/sub]) is a polyatomic anion. Metals can lose their electrons fairly easily to become cations. So, both the silver and the copper can become positively charged, and can combine with the negative nitrate. The only question is which nitrate compound is more favorable (the copper nitrate or the silver nitrate), and for now it may just be best to take it for granted that the reaction proceeds as written. (If you really wanna’ know which way the reaction will proceed, you can look up the various redox potentials in a table, but that’s probably more complex than you need.)

I’ll note that I am making some simplifications for the sake of understandability, and I apologize in advance for any errors this introduces.

17

Okay, in simplest English, the manganese dioxide works catalytically (without undergoing permanent change itself) to cause the hydrogen peroxide to break down into water and monatomic oxygen radicals (MORs). The monatomic oxygen then unites to form normal diatomic oxygen molecules. Because you need two MORs to make one oxygen molecule, you need to write the values of the equation so that you have enough oxygen to produce the ultimate result: either saying (my first equation) that the first step of the process is to produce the MORs, or, preferentially, saying (my second equation) that two moles of peroxide produces two moles of water and one mole of oxygen.

A mole is a quantity of a substance (usually measured in grams) that is equivalent to the molecular weight of the substance in the unit of measure. In other words, it’s how much of that substance contains Avogadro’s number of molecules of that substance (when talking grams), or a multiple of it (when talking another unit of measure).

Industrial equations might substitute tons for grams; descriptions of chemical cellular processes might be talking about micrograms in each cell. But in any chemical process, one mole is equivalent to the amount of the substance it takes to make up one “unit” of that substance from the reaction POV.

Like this: the elementary school class goes to the ranch, and the kids are treated to horseback rides. They need to have as many horses available as there are kids. Obviously, 30 horses weigh much more than 30 third graders – but they’re in one-to-one correspondence.

To get a chemical equation to balance, you need to put as many of each component on one side as you have on the other. This may mean that you need twice, three times, or one-and-a-half times as much of a given starting substance as you have of one of the products, as we had here: it takes two moles of peroxide to produce two moles of water plus one mole of normal oxygen.

You need equal numbers of each kind of atom on each side of the equation. If one product is something that doesn’t match the quantities (as for example the O[sub]2[/sub] in the equation), you need to alter the quantities to produce a result measurable in whole units of that product. (I suppose you could write the equation as

H[sub]2[/sub]O[sub]2[/sub] —> H[sub]2[/sub]O + 0.5O[sub]2[/sub]

but the custom is not to work with such fractional units.)

18

One final question, and then I will send Ivylad to his chemistry teacher for more intense tutoring. I thank you all for your help.

Does the number on the coefficient go back to the charges on polyatomic ions?

19

I’m not sure I entirely understand the question, but lemme’ give it a shot.

Charges on polyatomic ions are (typically – in chemistry, there’s almost always several exceptions to the rule) fixed. Going back to the silver/copper nitrate example you gave, the nitrate ion will always have a charge of negative one. (To prove this, Ivylad can draw a Lewis diagram of NO[sub]3[/sub], if he’s gotten to them yet, satisfying the octet rule on each atom, and then compare the result to the total number of valence electrons in one nitrogen and three oxygens.)

In the example, the silver ion has a charge of positive one, so one silver can combine with one nitrate to produce a neutral molecule. (Electrically neutral is almost always a good thing. :wink: ) The copper, on the other hand has a charge of positive two, in the example. (We’d have to determine this experimentally, but it’s not that difficult to do.) So, to create some thing neutral, we need two nitrates for the one copper. This gives us AgNO[sub]3[/sub] on the left of the equation, and Cu(NO[sub]3[/sub])[sub]2[/sub] on the right.

Now, by the law of conservation of mass, we need two nitrates on the left hand side, so we have to give a coefficient of 2 to the silver nitrate species. Similarly, we’ll only need one copper atom on the left side, and two silver atoms on the right. So, in this sense, there is a relation between the ion charge and the reaction coefficient, but it’s stoichiometrically determined, and therefore will depend heavily on the particular reaction being studied.

I hope that makes sense. If not, please let me know.

20

From my (British) point of view, I’d balance the equation as:

H[sub]2[/sub]O[sub]2 (aq)[/sub] —> H[sub]2[/sub]O sub[/sub] + ½ O[sub]2 (g)[/sub]

In my chemistry classes at university we’d usually try to keep it simple, and use fractional units if it made things clearer.

Obviousaly you don’t get “half an oxygen molecule” floating around - all it means is that you need two hydrogen peroxide molecules to produce one oxygen molecule.

But as others have said, if you only want to show reactants and products, you often wouldn’t bother balancing it (although in a school textbook I would argue that this is not good practice).