Hard time understanding Chem passages

[axp107]

It's been a while since I've taken chem.. I always understood the math really well.. except when given a passage talking about a simple reaction taking place, it's hard for me to understand what is really going on..


I never really learned what some really basic terms even meant.

For example,

1) In a test tube, BaCl2 is added to a few drops of a sample of the unknown solution. No precipitate is formed. A few crystals of sulfamic acid are then mixed into the sample. No visible reaction occurs.

What is going on here... we're adding BaCl2. No precipitate is formed. What does this mean? Does it mean BaCl2(s) is not reformed...? or does it mean a "solid" PRODUCT is not formed..?

What does precipitate really even mean.

Sulfamic acid is mixed. How come there is no reaction?

2) Another part of the passage says Barium sulfate is insoluble while barium sulfamate is soluble.

What does this mean? That Barium sulfate does not break up into ions? while barium sulfamate does so successfully?.. Are we talking about water here?

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[scotties123]

this is dealing with solution chemistry, which I guarentee u will be on the mcat. i havent done a single practice exam yet that didnt have solution chem on it. When something precipitates its due to one of two things. Either its insoluble in the solvent (i.e. like dissolves like) and doesnt break into ions, or because the Ksp (solubility product) is exceeded, causing precipitate to be formed in order to maintain the solubility product. I highly suggest looking over the Solution chemistry chapter in whatever material ur using. I think EK does a nice job of explaining it.

 

[axp107]

Solution chem is by far my weakest area. On the EK 30 mins I've been doing well on every chapter except.. I got an 8 and a 6 on the solution and phase change chapters respectively :S

 

[koopa_troopa]

That passage is testing solubility, and more specifically the common ion effect (part 2). Precipitate just means a solid. And when they say no visible reaction has occured, its most likely because they do not see any precipate or color change.

 

[axp107]

Ok I guess what gets confusing about precipitation/solubility problems is...

they say you've got a CaOH2 solution is water.. lets say

It disassociates....

then you add HCL or something...

what happens now?

What is the rxn?

Combining acids/bases with solubility scares me

 

[smeagol]

Ok I guess what gets confusing about precipitation/solubility problems is...

they say you've got a CaOH2 solution is water.. lets say

It disassociates....

then you add HCL or something...

what happens now?

What is the rxn?

Combining acids/bases with solubility scares me

I understood solubility rules much better after taking Gen Chem Lab II. Words like 'precipitate' didn't have much meaning to me until I saw for myself what it was.

Anyways, as .Ca(OH)2. dissociates as you say, it forms calcium and hydroxide ions in an aqueous solution.

As you add HCl, you now have all of these in your aqueous solution.
.
Ca2+ + Cl- + H+ + OH-.

The cations and anions will 'bump' into each other to form molecules. Now, what happens after they 'bump' into each other determines the type of reaction that you have.

All cations will react with all anions.

So: .Ca2+ will react with Cl- .., making ..CaCl2..
and
..Ca2+ will react with ..OH-.., making ..Ca(OH)2 ..again.

Now this is where the concept comes in. Both ..CaCl2..and Ca(OH)2.. are completely soluble in water. So as they form these molecules, they will immediately dissociate again into their respective ions.

Because they dissociate into their respective ions again, the interaction is coined "no reaction." A precipitate reaction would have occurred if there was an ion that would precipitate with calcium, such as the sulfate ion (but that is not the case here).

Now regarding the other ions:
..
H+ + OH-. --> .H2O.

This will make water. Although water DOES dissociates into the hydronium & hydroxide ions, the formation of water from these ions greatly exceeds its want to dissociate in an aqueous solution.

.H+ ..+ ..Cl-.

This is what you had added earlier. These ions will recombine to form HCl, but as soon as it does so, HCl (being a strong acid) will immediately dissociate back into its respective ions.

So the overall equation is this: .Ca(OH)2 + HCl .=. H2O + CaCl

Since CaCl does NOT form a precipitate, the reaction is classified as an acid/base.

The net ionic equation then is simply the formation of water: ..H+ + OH- = H2O.

Edit: well that sucks, the subscripts/superscripts didn't work out.

 

[Green Pirate]

good post smeagol, but what are some ways of detecting whether a compound will be soluble/insoluble? For example, you stated that Ca(OH)2 and CaCl2 would dissociate into their respective ions. I guess we might know this from prior experience, but if faced with a less familiar compound, how do we know whether it dissociates or not?

 

[smeagol]

http://intro.chem.okstate.edu/1515SP01/Database/Solub.html

I only know the solubility rules from memorizing tables like the one above (did a quick search on yahoo). EK has a list of some solubility rules in section 4-8 (5th edition). I think that one would need to know a lot of the basics. Exceptions to rules listed above will probably be provided for in the passage.