[darkling235]

This is probably a very rookie question but I'm trying to understand chemical reactions better. I understand how to write down the equations and solve them but now I'm wondering how to tell if any reaction at all will occur. Long story short, I'm trying to figure out how I can tell if and how two random chemicals will bond. I don't expect an actual answer to my question but if someone could tell me what topic to research so I can understand it, I'd be grateful.

I understand that if I put Na and Cl together, they'll form table salt. I understand they'll do this because Na has 1 valence electron and Cl has 7, etc etc.

But what about chemicals like Sulfur and Oxygen? These do bind and form SO2, but how can I tell that that's what they do? They both have 6 valence electrons so I understand that they can "covalently" share those 2  electrons to fill their outer shell, but what's the rule that tells me that they do this?

I keep being given chemicals and told to explain what the end results are. But how do I know that they will react at all? There must be some rule that says Chemical X and Chemical Y will interact to form a molecule, or dissociate, or oxidize or something. I have to believe there is some law or set of laws that will let me figure out if two chemicals will interact and if so under what circumstances (tempreture, pressure, etc). If you have to learn every single reaction between 2 chemicals by rote, the science of chemistry would seem to be impossible to learn.

If anyone could just tell me what area to research to figure this out I'd appreciate it, as none of my books even mention the concept.

[Arkcon]

I have to believe there is some law or set of laws that will let me figure out if two chemicals will interact and if so under what circumstances (tempreture, pressure, etc). If you have to learn every single reaction between 2 chemicals by rote, the science of chemistry would seem to be impossible to learn.

Difficult perhaps, but not impossible.  It would take a couple of years of dialog to teach you all the rules, that is what a chemistry class is for.  Try giving us some examples, that you can and can't solve, and we'll see what we can help you with.

[darkling235]

Well I'm just sort of enjoying a hobbey at the moment. I'm primarily an herbalist but studying one subject always seems to lead to studying another. After several years of extractions and solutions I've decided to learn more about actual chemical structures. At the risk of asking a particularly dumb question is there a name for these rules or sub-rules in chemistry that determine bonding and or reactions? I'm asking just so I would know what to specifically research. Just grabbing random chemistry books or reading articles on chemistry would be much easier if I could skim through the contents and find out if they address these specific questions.

[syd]

Study bonding theories and electron orbitals, and you will see why SO2 ends up being a relatively stable molecule.  Unfortunately, nothing in chemistry ends up being as cut and dry as one valence electron readily bonds with an atom that has 7 and needs one more:).

[darkling235]

After studying herbs and hollistic medicines for ten years, I'm completely in agreement that what seems simple at first gets ridiculously complicated with hundreds of sub rules and exceptions once you actually delve into the discipline. Still it's good to have an idea what to start studying.
Thanks

[darkling235]

I'm about to ask some probably stupid questions. Please bear with my naivete. I'm trying to understand

Ok I've been working on my rules and I think I have a fairly solid grasp of the basic ideas (please muffle your laughter at that comment).

I've been doing problems and I've consistently gotten right answers, I can do Lewis structure, find bonds, count electrons, and fill shells. In short I feel like I have a pretty good understanding of HOW chemicals will bond, unfortunately this requires me to have an initial idea that they will bond and a vague idea how. Right now I'm trying to understand why CO2 is formed sometimes and CO1 othertimes.

I've been studying electron shells lately and I think I have the basic idea that the electrons will always choose the easiest method to fill its valence shell. Carbon is (i think) 2s^2   2p^2.

It would really like to fill those missing 2 electrons in the 2nd P shell. so it will bond with any nonmetal that will provide 4 electrons to that shell.
CO2 for example has

O-C-O

Carbon will bond in this case because Oxygen will share two electron each giving Carbon its full 2 p^6. So 2 electrons would be shared with each oxygen atom.

What kind of confuses me is why CO2 is more energetically favorable than CO1? Is it much more expensive in terms of energy to have 4 bonds rather than two? The obvious answer may well be yes but I'm not really sure. Can someone explain why? On a hunch I might guess that since Oxygen's electron structure is  [HE] 2s^2 2p^4, the P shell has 2 free electrons and one bonded pair of electrons.

UP/DOWN, Up, Up

Is it impossible to share the paired electrons in that first shell? Again the obvious answer may be yes but I'd just like to get my supposition confirmed. I'm working this out as I go after all.

Does it have something to do with the way Oxygen travels in bonded pairs? I currently don't have a theoretical reason why O2 would bond with anything at all since their covalent bonds should ensure their valences feel happy with 8 electrons.

Of course CO1 does form in highly energetic reactions like burning things. Can I assume from this that given enough energy the first electron pair in an Oxygen atom WILL bond to the carbon?

Then again, I'm forced to wonder why CO1 wouldn't become unstable and break down after the temperature dropped.

Or could CO1 be formed by first the high energy of the reaction breaking the 2 links in the Oxygen molecule?

Can anyone help explain these phenomenon to me? I'm getting kind of confused as I try to understand these reactions. Thank you for bearing with me.
Thanks a lot

[syd]

Lewis structure has limits because of its simplicity.  I can tell you what I know..

Are you familiar with the fictitious charge you can assign to atoms in a lewis structure?  It's basically like this: valence electrons - (lone electrons +1/2(bonding electrons)) = charge on that atom.

In CO, its lewis structure has has carbon triple bonded to one oxygen atom, with each having a lone pair of electrons around them.  Therefore, the carbon has a -1 charge and the oxygen has a +1 charge, making a net 0 charge on the molecule.

In CO2, each O atom is double bonded to the carbon, and all atoms hav 0 charge, which is a favorable setup in lewis structures.  So you'd think co2 is the more preferable setup and it is.  However, I think you're along the right track when you say it takes a lot of energy to make the CO bond.  It ends up being stable though because (I think) the triple bond it makes in the reaction takes a lot of energy to break.  Atoms will only rearrange themselves when it is energetically favorable.  In other words, CO won't easily switch into CO2 because the benefits outweigh the costs in what is gained by forming the CO2 bonds.   It also costs some energy just to form a bond

[James Newby]

Try looking up Hess' Law if you are interested in the energy values.  Carbon monoxide will go to carbon dioxide but you have to force it with either harsh conditions and/or often expensive catalysts.  If you imagine the energy level of CO as a hole, you need a lot of energy to force it out of the hole and back down into the more stable CO₂ hole.  (bizarre metaphor!)

To continue your study of bonding theory, it may be good to look up Aufbau principle and the  Pauli principle.  This bonding principle is the reason Quantum mechanics was needed so expect some rather intense maths.

Is it impossible to share the paired electrons in that first shell? Again the obvious answer may be yes but I'd just like to get my supposition confirmed. I'm working this out as I go after all.

In general its only the outer electrons that do the bonding.  As you go down the periodic table some of the inner ones begin to play a role but that shouldnt bother you at the moment.

Some trickier Lewis structures would be Boron trifluoride (BF₃), the carbonate ion (CO₃^2-) and the nitrate ion (NO₃^-)

[darkling235]

If I understand what your saying correctly, its that certain bonds between certain electrons are more expensive to make (and therefore less favorable) than others. Is there a rule or equation to use to figure out what the path of least resistance is? I'm having some trouble with Hess's law so I'm not sure if thats supposed to be it.
Thanks

[James Newby]

You have to put in energy to break bonds ie go to a higher energy state.  Then when you make bonds you get energy out again.  Each bond has a certain amount of energy associated with it so if you break the bond it requires x amount of energy.  When the bond is formed you then get x amount of energy back out.  Bond strength is usually directly related to the amount of overlap between orbitals.  This about comparing the overlap between carbon and iodine compared to carbon and fluorine.

If you want to see if a reaction will proceed or not then that is a different equation.  Look up Gibbs free energy, Atkins physical chemistry is probably the best book for it.

The path of least resistance is a very tricky question as there are so many variables.  Hess Law will tell you if the reaction is exothermic/endothermic and how much energy you have to put in to get the reaction to go, and Gibbs free energy will tell you if the reaction will spontaneously proceed or not.

Its very hard to answer questions about physical chemistry as there are many different factors!

[darkling235]

Ok let me just see if I have these concepts down. I think I'm still missing a link...probably more than one actually but I'm studying this stuff in my free time after all :=).

If I have 2 chemicals, let's call them A and B.

I can tell if, in theory, they will bond at all by building a valence diagram. If the shells allow for pair matching up, at some point I know these two chemicals will bond. The number and complexity of the bonds is throwing me a little right now but lets leave that aside for right now.

Having done that step, I can check Hess's equation (which I think is very cool now that I'm playing around with it a bit) and see if the reaction would take energy or release energy, irrespective of whatever interim steps it might take to accomplish the reaction.

I can use Gibbs equation (which I'm still working on, not sure I understand all the details yet) to see if sufficient energy exists in the "environment" to trigger the reaction or if I need to add some.


This is all very cool and thank you for pointing this stuff out to me but I find I'm still confused about bond numbers. I keep going back to Carbon and Oxygen

Carbon = 1s^2 2s^2 2p^2
And I could go further and say how the first two p's are filled in with 1 electron in each.
Oxygen = 1s^2 2s^2 2p^4
And Oxygen will have its first p shell paired and a single electron in each other p shell

Now I try to understand why this reaction would form CO1 in some cases and CO2 in others and I find myself only kinda getting it. Inductively I've noticed that CO1 only seems to form in cases where the energy supplied is high or the oxygen available is low. I'm not really sure which.

Is there a rule that lets me tell whether the product of mixing C and O2 will be CO1 or CO2. I'm not sure if this is a logical question but I'm trying to fill in some blanks here. And at the moment this is the really big question that still baffles me.

[Arkcon]

Now I try to understand why this reaction would form CO1 in some cases and CO2 in others and I find myself only kinda getting it. Inductively I've noticed that CO1 only seems to form in cases where the energy supplied is high or the oxygen available is low. I'm not really sure which.

Is there a rule that lets me tell whether the product of mixing C and O2 will be CO1 or CO2. I'm not sure if this is a logical question but I'm trying to fill in some blanks here. And at the moment this is the really big question that still baffles me.

Very good, you seem to have solved it on your own.  Carbon monoxide, CO (we don't use the subscript 1, it's always understood, by the way) does form in situations when there is insufficient oxygen, or more appropriately, when there is an excess of carbon, to reduce CO₂ (notice this forum has subscript buttons above), to CO.  You've also successfully intuited that CO is favored at higher temperatures.  http://en.wikipedia.org/wiki/Carbon_monoxide

[James Newby]

You have hit the nail on the head with one of your statements.  In the case of CO formation over CO2 its the amount of oxygen around.  IF you are after CO2 flood the reaction vessel with O2, If you want mainly CO add an excess of carbon.  The classic example is a bunsen burner.  With the tap closed the oxygen is limited so a yellow sooty flame is seen:

2CH4 + 2.5O2 -> CO + 4H20 + C

opening the tap floods the gas mixture with oxygen and a blue flame is seen:

CH4 + 2O2 -> CO2 + 2H2O

Notice the gas to oxygen ratio is double for the CO2 burning.  This is an incredibly simplified diagram, in real life its WAY more complicated

By bond numbers do you mean valency? ie the number of other atoms an atom will bond to.  Or do you mean will an atom form a single, double, triple bond?


Keep trying material and keep asking the questions.

AHHH! Arkcon has sneaked in! Sorry if i repeat anything

[darkling235]

A little bit of both. It's good to know that I had the basic idea right but I'm still struggling to figure out how to determine products. In the case of CO2 the oxygen molecules will bond to the Carbon and each will share 2 electrons giving the outer P shell in carbon its full set of 6 electrons (or 8 counting the s shell). However it can also bond with a single O atom in which case the O atom atom would have to share 4 electrons covalently to balance with carbon. I've heard that atoms don't like to share more than 3 though apparently they do sometimes. But I thought there should be an algebraic way of showing this.

If I wanted to calculate C + O2 (and this may be my first problem since I'm not even sure if this mixture would react or not. I've been trying to find enthalphy values for them but no luck so far. Assuming for the moment that given sufficient energy C and O2 will react) then I would try to finish the "equation." Thing is how many products are possible, CO, CO2, I haven't done the math out yet but I think a short lived CO3 would be possible as well. I'm kinda wondering if theres a way to tell when you throw 2 things into a pot and apply energy what the product would be. Or do you have to learn that by rote?