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Topic: How do you determine the phase of compounds when writing reactions?  (Read 13285 times)

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Offline TommyAtkins

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For example:

A solution of silver nitrate is added to a solution of potassium bromide. I am supposed to write the equation out and include phases.

AgNO3 + KBr --> AgBr + KNO3

My guess is that silver nitrate and potassium bromide are both aqueous because the problem says they are solutions. How do I know what the phase products are though? Am I to assume it is just aqueous because the reactants were aqueous?

Also, are all solutions aqueous? The only time a reactant would be (l) for liquid would be if it was water right?

I apologize if the question is stupid. I am going into AP Chemistry next year and I'm doing the summer assignment for it. I forgot most of my chemistry over the summer.

Offline Ben Bob2

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Re: How do you determine the phase of compounds when writing reactions?
« Reply #1 on: August 10, 2015, 04:43:58 AM »
There is a list of rules for determining solubility. I remember using this source in general chemistry (I now have them written on the back of my periodic table  :)): https://www.youtube.com/watch?v=dCIRAPzHTfA
If your product isn't soluble, mark it as a solid in your equation. It will appear in your experiment as a precipitant.

All solutions are NOT aqueous: for example, large organic molecules like in butter aren't going to dissolve in water, so you'll need a different type of solvent. I don't recall having to deal with these much in General chem though.

Don't confuse liquid with aqueous. Think about mercury. It is liquid all by itself as a pure element, even out of water. An aqueous solution would have the mark "(aq)" by it.

I hope that helps.
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Offline TommyAtkins

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Re: How do you determine the phase of compounds when writing reactions?
« Reply #2 on: August 10, 2015, 05:47:51 PM »
Solubility rules give me a tremendous amount of trouble. Pretty much all of them have exceptions and confuse me.

Example:

According to my chart, sodium is completely soluble while hydroxide is not.

Is sodium hydroxide aqueous then? Solid? Do i base the phase off the metal or the non metal's solubility?

Offline Arkcon

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Re: How do you determine the phase of compounds when writing reactions?
« Reply #3 on: August 10, 2015, 07:45:27 PM »
Solubility rules are indeed problematical.  I'm sad to report that you simply have to study, try to memorize, and maybe pick up a hint as you go along.  For starters, look up compounds of sodium and potassium.  Wikipedia isn't a bad place to look, if your textbook is lacking.  You'll see, all simple compounds of sodium and potassium are soluble.  And for that reason, we use them preferentially, in high school chemistry courses, in industry, in analytical chemistry.

Now, lets address your specific question, and search for hints:

Quote
A solution of silver nitrate is added to a solution of potassium bromide. I am supposed to write the equation out and include phases.

AgNO3 + KBr --> AgBr + KNO3

You found your first hint, the reactants are solutions, so they're (aq).  That means they're not really there -- they exist as Ag+(aq), NO3-(aq), K+(aq) and Br-(aq).  Now what, if they all stayed as solvated ions, how can there be a reaction?  Something has to happen, somewhere in the products to drive the reaction forward.  An insoluble solid is just the ticket.  So is a gas bubbling away.  But what happens?  And to which one?
Hey, I'm not judging.  I just like to shoot straight.  I'm a man of science.

Offline Enthalpy

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Re: How do you determine the phase of compounds when writing reactions?
« Reply #4 on: August 11, 2015, 05:51:24 PM »
It's even unexpected to write the equation at all if one doesn't know whether the possible products are soluble.

Because, well, if all reactants and products were soluble, as they exist as K+ and NO3- and so on without a link between these, a reaction would mean nothing - just a different way to write the same bunch of ions.

Or in other words, writing as "AgNO3 + KBr" means putting many ions in the "equation" despite they don't react. All the action is between two ions that form a solid.

Offline TommyAtkins

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Re: How do you determine the phase of compounds when writing reactions?
« Reply #5 on: August 16, 2015, 11:47:22 PM »
Solubility rules are indeed problematical.  I'm sad to report that you simply have to study, try to memorize, and maybe pick up a hint as you go along.  For starters, look up compounds of sodium and potassium.  Wikipedia isn't a bad place to look, if your textbook is lacking.  You'll see, all simple compounds of sodium and potassium are soluble.  And for that reason, we use them preferentially, in high school chemistry courses, in industry, in analytical chemistry.

Now, lets address your specific question, and search for hints:

Quote
A solution of silver nitrate is added to a solution of potassium bromide. I am supposed to write the equation out and include phases.

AgNO3 + KBr --> AgBr + KNO3

You found your first hint, the reactants are solutions, so they're (aq).  That means they're not really there -- they exist as Ag+(aq), NO3-(aq), K+(aq) and Br-(aq).  Now what, if they all stayed as solvated ions, how can there be a reaction?  Something has to happen, somewhere in the products to drive the reaction forward.  An insoluble solid is just the ticket.  So is a gas bubbling away.  But what happens?  And to which one?

I sort of understand solubility rules now. NaOH would be soluble because all group 1a metals are soluble. Ammonium basically makes the entire compound soluble as well. There are still a few loopholes to these rules I don't understand. CaF for example. None of those elements are on my solubility chart or anyones I can find on the internet. I assume that when there are no rules regarding its solubility, it is simply insoluble.

I guess with the states, you just have to memorize them. Obviously Oxygen and Carbon are going to be gases. Water is the only liquid I've encountered in doing these problems. Others are either aqueous or solid and this is determined by solubility rules.

@enthalpy I am trying to grasp what you're trying to say to me. Essentially, you're saying that for two aqueous solutions to react, the ions that switch must form an insoluble compound as a product? I believe this is similar to what @Arkcon was trying to tell me. In order for a reaction to happen one of the end products must be solid. I vaguely remember learning about this in chemistry class. Something about a precipitate? I wish I could ask my teacher but it is still summer and he won't respond to any emails.

I'm looking over my entire page of problems ad I'm kind of noticing a pattern. Almost all of my reactions have a solid and an aqueous solution! Please correct me if I'm wrong but thats what I'm getting from you guys.

As always however, there were exceptions to this rule I found.

BaCl2 + 2Fe(NO3)2 --> Ba(NO3)2 + FeCl2

I balanced the equation on my paper but lets ignore that for now and focus on the states of everything in this reaction.

First two are aqueous. Barium Nitrate is aqeuous because nitrate is always soluble. Personally, I believe that the second compound is aqueous. However, if what I'm getting from you guys is correct, it should be solid right? It would be pointless for all reactants and products to be soluble like @Enthalpy said.

I get the general idea of what you guys are saying. Just don't understand why there are exceptions and why they exist.

Offline Enthalpy

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Re: How do you determine the phase of compounds when writing reactions?
« Reply #6 on: August 18, 2015, 05:30:18 AM »
If all M+, N+, X-, Y- are soluble, you can mix their water solutions, nothing happens.
M+ + X- + N+ + Y- stay the same, how boring.

A reaction happens (in this case, among ionic solution) when one potential product leaves the scene, generally precipitates because it's insoluble. The reaction is only between the ions that form the precipitate:
N+ + X-  :rarrow:  NX :spindown:

But it's more usual to write the other ions too, despite they don't participate:
M+ + X- + N+ + Y-  :rarrow:  M+ + Y- + NX :spindown:
or even,
MX + NY :rarrow:  MY + NX :spindown:
which makes sense only because NX precipitates, so to write this reaction, one must know in advance that one potential product is insoluble.

By the way, it's complete compounds that dissolve or not in water by ionization. Their anions and cations only tend to make a compound more or less soluble, but their combination determines it.

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