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A few chem questions unclear to me...


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1, How do you test a substance whether it is or not water, chemically?

 

2, When Water and Phosphorus Tribromide are reacted, what reagent is water here?

 

 

 

above are the questions that apear in my exam, and I have already done, but just want to see how you other will solve it.

 

Albert

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1, How do you test a substance whether it is or not water, chemically?

First, of course, if it is not a liquid at room temp, then it is not water. If you want to test whether a sample contains water, one can heat it and collect the vapor. Next, one can start adding certain anhydrous salts (e.g. anhydrous CuSO4 or anhydrous CoCl2). Anhydrous CuSO4 turns from white to blue on addition of water or when it is placed in a stream of humid vapor. Anhydrous CoCl2 turns from blue to pink, when water is present.

These just are some thoughts, but certainly there are many other compounds, which can be used to detect water.

 

2, When Water and Phosphorus Tribromide are reacted, what reagent is water here?

I would expect it to hydrolyse and the main reaction to be

 

PBr3 + 3 H2O --> H3PO3 + 3HBr

 

The HBr dissociates further in water, forming H(+) and Br(-). H3PO3 also is an acid, it is a fairly weak diprotic acid and can dissociate up to HPO3(2-).

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but for woelen's method as well.

 

I mean, if you dip anhydrous salt in the solution, it is the same thing.

 

And by the way, if you dont know, boil at 100c might also boil alcohol as well.

 

Albert

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  • Freeze it and see wether it expands or contracts. (water expands)
  • Try to burn it (water doesn't burn)
  • Check it's PH (water has a PH of 7)
  • Give it to someone you don't like and tell them to drink it (water wont kill them)

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I just cant find, in chemistry, there is an unique method for testing water.

 

For example, gas, hydrogen, we use to use pop test, which a splint would make a pop noise, and oxygen, in more abundant, will make the glow brighter.

 

However, not only hydrogen will make that noise. I think any other gas fuel is apposite.

 

ALbert

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the prob with electrolysis is that the soln could be 97% sulphuric acid and still give give you your Hydrogen and Oxygen.

 

the PH test would be a good start, as would specific gravity :)

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In all of chemistry, I don't think there is any 'one' test that will 100% positively identify a substance. Multiple tests are always needed. For water, one would be dissolving an ionic compound in the liquid, and dissolve a known amount until saturated. Table salt, for instance, will dissolve to a certain extent in water which is different for that of ethanol, methanol, isopropanol, etc. With the temperature of the liquid known, the amount of salt added until saturated known, you should be able to tell that it's conclusively water just by that data. In addition, your nose will play a powerful role in determining what it is since water is odorless and all other liquids that I know of have some type of odor to them.

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I`de trust Specific gravity and PH test for a starter, as for smell I can think of dozens of chems that would pass that test and kill you at the microgram level :(

 

evaporation to Crystal would also be another I`de employ, as Jdurg says, there is no one particular test (unless you happen to have spectral analysis equipment handy).

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So exasperating,

 

in my exam,

 

I thought oxygen gas is a base

 

for the reaction, of carbonic acid, I wrote

 

H2CO3 + O2 ---> H2O + CO2 + O2

 

,which sopposed to be simply without the oxygen

 

Darn

 

More over, I assimlated lime water as a indicator of oxygen, but actually it is for CO2.

 

:-(

 

Any way, do you guys know why Lime Wire react with CO2??

 

Albert

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So exasperating' date='

 

in my exam,

 

I thought oxygen gas is a base

 

for the reaction, of carbonic acid, I wrote

 

H[sub']2[/sub]CO3 + O2 ---> H2O + CO2 + O2

 

,which sopposed to be simply without the oxygen

 

Darn

 

More over, I assimlated lime water as a indicator of oxygen, but actually it is for CO2.

 

:-(

 

Any way, do you guys know why Lime Wire react with CO2??

 

Albert

Lime water is a solution, containing Ca(2+) ions and OH(-) ions. OH(-) ions react with CO2 as follows:

 

 

2OH(-) + CO2 --> H2O + CO3(2-)

 

With calcium ions in the water a precipitate is formed:

 

Ca(2+) + CO3(2-) --> CaCO3 (ppt)

 

So, bubbling CO2 through lime water produces a white cloudy liquid. If a lot of CO2 is bubbled through this, then a white precipitate is formed, which after some time settles at the bottom. You can even demonstrate that you exhale CO2, if you breathe out some air and bubble this through lime water.

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The answer to why most substances react is in the thermodynamics. When water/hydroxide react with carbon dioxide to form carbonic acid/carbonate, the reaction occurs because a more stable product is formed. If you think of the structure of carbonate, you see that there is resonance stabilization which makes the product have a lower free energy at certain conditions.

 

As for the identifying water question, I would look at the boiling point and refractive index. Those are fairly good for identifiying unknown liquids, assesing their purity, and they're also pretty easy experiments to run. All you need for a determination of the boiling point is a simple distillation apparatus and the determination of the refractive index is very easy with a refractometer. If you really want to go overboard and unambiguously identify the compound as water, you can obtain an IR or NMR spectrum of the compound or feed it into a mass spectrometer.

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The answer to why most substances react is in the thermodynamics. When water/hydroxide react with carbon dioxide to form carbonic acid/carbonate' date=' the reaction occurs because a more stable product is formed. If you think of the structure of carbonate, you see that there is resonance stabilization which makes the product have a lower free energy at certain conditions.

 

QUOTE']

 

thermodynamics??

 

Is there way to calculate instead of verbal description??

 

Albert

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The answer to why most substances react is in the thermodynamics. When water/hydroxide react with carbon dioxide to form carbonic acid/carbonate' date=' the reaction occurs because a more stable product is formed. If you think of the structure of carbonate, you see that there is resonance stabilization which makes the product have a lower free energy at certain conditions.

 

QUOTE']

 

thermodynamics??

 

Is there way to calculate instead of verbal description??

 

Albert

Yes, one can compute the energy contained in a certain molecule. This energy can be regarded as potential energy (that is the best macroscopic equivalent I can think of). The computations, however, are extremely complex. They require quantum mechanics. With lots of simplifications one can reason about how stable molecules will be and this reasoning frequently provides nice rules of thumb, but if one really wants good estimates of the free energy in molecules (and relatied to this, the stability) then detailed QM computations need to be done. There are software packages for doing this kind of computations, but these are not suitable for the beginning chemist and for more complex molecules than e.g. CH4, NH3, etc. quite some computational power is needed.

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Woelen is not kidding. My freshman year of college I did a research project on the conformations of Green Fluorescent Protein. (Published in the Journal Of Chemical Physics "Photoisomerization of green fluorescent protein and the dimensions of the chromophore cavity." Chemical Physics, 270 (2001): 157-164.) We did numerous calculations to derive what the most stable conformation of the protein was. With the molecular mass of the protein being so incredibly high, it literally took weeks to calculate the energy of ONE conformation. Some of the calculations actually took months to finish up using numerous SGI workstations. I'll never forget the feeling of disgust when we finished a VERY long calculation only to look at the model and see that we accidentally had a pentavalent carbon atom in there. hehe.

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The furtherest my chem teacher has taught about reaction equation's system is about chantelier's principle.

 

are all reaction equations revirsible??

 

btw, to YT, it does exist, it is a weak acid, when CO2 dissolves in water. My teacher has done a test. Just place your water, lid open in air. After a while, test the water's pH. But, of course, it depends on the conditions, for eg, temperature, any catalyst, pressure, etc.

 

However, can all the factors of a reaction equation sum to one thing: Energy?? More pressure, more kinetic energy of particles, more temperautre, more heat energy??

 

Albert

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The furtherest my chem teacher has taught about reaction equation's system is about chantelier's principle.

 

are all reaction equations revirsible??

No' date=' not all reactions are reversible. E.g. burnt wood and the gaseous reaction products cannot revert to wood and oxygen.

 

btw, to YT, it does exist, it is a weak acid, when CO2 dissolves in water. My teacher has done a test. Just place your water, lid open in air. After a while, test the water's pH. But, of course, it depends on the conditions, for eg, temperature, any catalyst, pressure, etc.

No, the acid does not exist as such. The acid H2CO3 cannot be isolated. When CO2 is dissolved in water, then indeed the liquid becomes a little acidic, but the real reaction mechanism from CO2 and water to the acid is not really simple. The net equilibrium will be something like, without the H2CO3 species present in solution:

 

CO2 + H2O <---> HCO3(-) + H(+)

 

 

Another example of such a 'troublesome' acid is sulphurous acid (H2SO3). This acid also does not exist in the free state, nor in solution. No one has ever observed the species H2SO3. In water, when SO2 is dissolved, then the species SO3(2-), HSO3(-), SO2, S2O5(2-) and HS2O5(-) can be observed.

 

So, observing that a liquid is somewhat acidic does not always mean that the corresponding acid exists!

 

However, can all the factors of a reaction equation sum to one thing: Energy?? More pressure, more kinetic energy of particles, more temperautre, more heat energy??

No, this is exactly what makes the computations of chemical reactions and structures so difficult. When a molecule or set of molecules need to be modelled, then the number of states for even very simple systems can be incredibly large. The same energy in a system can represent many many different configurations.

 

In practice, one also frequently observes, that reactions are not 'clean'. Especially, when the different energy levels for differnet compounds are very close to each other, then a reaction can produce many different compounds (there are many competing reactions). An example of this is reduction of HNO3 by zinc. This reaction can yield NO, NO2, NH4(+), NH3OH(+), N2O, all in one 'dirty' mix. What is formed precisely, depends on temperature, pressure, concentration of reactants, etc.

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