Can someone please help me on Gas evolved questions?

[AsherLev]

Can anyone please explain the general principle behind which gas is preferentially evolved during a reaction. I always seem to have trouble knowing which substances will react to produce what...is there a general trend/rules im not getting? For example, on an AAMC test, sodium carbonate was mixed with HCl, and Carbon dioxide was said to evolve...i answered sulfur dioxide...any help would be greatly appreciated! Thanks!

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[AsherLev]

anyone please...?

 

[Tussis]

Just for the record, the OP is referring to Passage VI on CBT-8

In all honosty, I guessed. So I would also like something of an explanation for this one. The reason I made the guess to CO2 is that it is [seemingly] perhaps the most common gas ever evolved in an acid/base reaction - or at least the gas that gets the most notoriety.

 

[mdgolfer84]

I think this is the question where you have to look at the precipitate.

I believe the precipitate was Nickel CO3. and they put acid and the precipitate went away.

So my understanding was that the CO2 came out as gas since the precipitate was dissolved.

The acid is used as a catalyst.. and catalysts are not used up in a reaction.

I'm not sure if thats what you were looking for but thats how i answered the question

 

[MundaneMD]

Can anyone please explain the general principle behind which gas is preferentially evolved during a reaction. I always seem to have trouble knowing which substances will react to produce what...is there a general trend/rules im not getting? For example, on an AAMC test, sodium carbonate was mixed with HCl, and Carbon dioxide was said to evolve...i answered sulfur dioxide...any help would be greatly appreciated! Thanks!

Check this out, especially card #4. http://www.chemmybear.com/apch08sc.pdf

Just for the record, the OP is referring to Passage VI on CBT-8

In all honosty, I guessed. So I would also like something of an explanation for this one. The reason I made the guess to CO2 is that it is [seemingly] perhaps the most common gas ever evolved in an acid/base reaction - or at least the gas that gets the most notoriety.

Here is how I would come up with an answer:

HCl + Na2CO3

1) HCl is a strong acid, so it will dissociate in solution. H+ and Cl-
2) Na2CO3 is a salt. It will dissociate into ions in solution. 2Na+ and CO3--
3) Since its an acid/base reaction, keep an eye out for water and a salt.

2Na+ + 2Cl- = 2NaCl (Salt)
2H+ + CO3-- = H2O (Water) + CO2 (Gas)

Hope that clears up any confusion.

(note to other chem majors: yes, the mechanism I described for this reaction is over-simplified, but the answer is the same in the end regardless. I'm just trying to make it easier to understand )

 

[axp107]

Can anyone please explain the general principle behind which gas is preferentially evolved during a reaction. I always seem to have trouble knowing which substances will react to produce what...is there a general trend/rules im not getting? For example, on an AAMC test, sodium carbonate was mixed with HCl, and Carbon dioxide was said to evolve...i answered sulfur dioxide...any help would be greatly appreciated! Thanks!

wait a min.. unless there's something I'm missing (some other compound in the rxn you're not giving...) how can sulfur even appear where there's no sulfur to react?

 

[mrmilad]

As a rule of thumb whenever I see Carbonates or Bicarbonate I would go with CO2 forming as a gas specially when water or acidic environment are present.

 

[RPedigo]

If there's carbonate in the solution, that's CO3--

CO3-- can pick up a proton [H+], resulting in HCO3-

HCO3- can pick up another proton, resulting in H2CO3

H2CO3 is carbonic acid, which has an equilibrium with carbon dioxide gas and water:

H2CO3 <--> H2O + CO2

This reaction is very important-- biologists should also be familiar with this equilibrium, which is expedited by carbonic anhydrase, to carry CO2 in the blood.

 

[AsherLev]

To tell people why i was thinking it might be SO2-. Here is the exact question:



A class is given a demonstration of solution chemistry and solubility equilibria.
A 6.57-g sample of NiSO4•6H2O (molar mass 262.84) is dissolved in enough water to make 50.00 mL of a green solution, and 7.15 g of Na2CO3•10H2O (molar mass 286.14) is dissolved in enough water to make 50.00 mL of a colorless solution. The two solutions are mixed, and a green precipitate forms. The resulting slurry is divided into two equal portions. To one portion is added an excess of 6 M HCl, which results in the disappearance of the precipitate and a rapid evolution of a gas. To the second portion is added a few milliliters of 6 M NH3. The precipitate dissolves, and a blue solution forms.



So the mixture of the 2 solutions has NiSO4 and Na2CO3 . So thats why i was confused as to why when HCl was added, the carbonate evolved and not another gas. Answer choices were



. According to the information in the passage, the gas that evolves is: A
) sulfur dioxide. B
) sulfur trioxide. C
) carbon dioxide.
According to the passage, the gas is given off when HCl is added to one portion of the slurry. HCl reacts with CO32– to form CO2(g). Thus, C is the best answer.


D
) carbon monoxide.

 

[axp107]

I never really understand what the big dot in the middle means..

NiSO4•6H2O

does that meant NiSO4 in water? is that it? would it just dissasociate into Ni2+ and SO4 2- ?

 

[Mayowa1994]

From my understanding sulfate is soluble in the solution so it won’t precipitate and evolve as a gas. Carbonate however is insoluble and will evolve

 

[BiigNerd]

For an explanation via youtube, search for "Double Replacement Reactions Chemistry, Acid Base Neutralization, Gas Evolution, Precipitation" by The Organic Chemistry Tutor. Your point of interest will be around the 10 minute mark.

I discovered him back when I took GC and I literally never went to class lectures since doing so.