A mixture (no pun intended) of Questions (1 Viewer)

[airie]

1. Concentrated H₂SO₄ is used as a catalyst in dehydration of ethanol to form ethene and water, we all know that. And that dilute H₂SO₄ can be used in hydration of ethene. But my teacher found some source saying that conc H₂SO₄ can used as a catalyst in hydration of ethene as well. Question is, why would you use conc if dilute suffices? Wouldn't you wanna save materials? Or are there any difference in how they aid the rate of reaction?

2. This one's weird. And bugging me for a while. If you've got ethanol, ethene, and conc H₂SO₄ mixed in a closed system and heated, what do you get? More ethanol, more ethene, or more butane? I've got a range of answers from people...

3. In general, solubility of solids (in water) increase with increasing temperature, but that of gases decrease. Why?

Spoiler

I'm thinking that for gases, given more energy as temp increases, they would have a greater tendency to break the bonds with water molecules, and escape into the air. But why doesn't it work for solids? Dissolution in general is exothermic. Then why don't solids dissolve less in warmer water?

4. Why doesn't the value of the equilibrium constant change when volume or pressure (strictly speaking, gas partial pressure, and only for equilibria involving gases) or conc of reactants/products change? Why does it change with temperature?

Spoiler

This is my guess, someone see if it's on the right track: when the temp is changed, neither the total amount nor the conc of reactants/products are changed, but the position of the equilibrium is shifted, thus K_eq is changed; looking at the expression of K_eq and relate to (delta)H of the forward reaction, one can determine if the contant is increased or decreased. When the volume/pressure is changed, though, the conc of ALL chemicals in equilibrium changes, and it somehow shifts such that K_eq remains the same. Same thing happens after the equilibrium shifts to counteract changes in conc of reactants/products. I'm not sure of the exact mechanism that makes K_eq remain constant though. Someone give an explanation?

5. In the first steps of the prac determining the sulfate content of a fertiliser, a few drops of HCl is added to the fertiliser dissolved in water, before it is heated and Ba²⁺ added blah blah blah... What's the HCl for? The teacher said it's basically to ionise/dissociate all constituent molecules into ions. I'm wondering how exactly does this work.

6. Tc-99m is used in medicine, and my teacher said that Tc-99 cannot be used. It's because of the long half-life of Tc-99 (which is something like >0.2 million years, compared to 6 hours of Tc-99m), which is a beta emitter. Problem though: Tc-99m decays into Tc-99 anyway, so what's the point? Or am I missing something?

Spoiler

Could it be that beta particles Tc-99 emits are of low energies and thus of little harm to humans? And because of this, it's not useful in med cos you'd need easy detection of the radioisotope in the body? Still, surely you'd not wanna be exposed to beta particles, however low their energies might be...

7. Just wondering if anyone could explain what are geometrical and optical isomers, and what's the difference. I've wiki-ed them but I'd like a clarification

(Weird. A bazillion questions pop up when I look at chem. But when I look at physics, I just go, "Yep, yep, this all works..." Hey, I can't control the number of questions that come up in my head...)

 

[Forbidden.]

For 5) I thought it was Molybdenum-99 that decayed into Technetium-99m and the latter isotope decays to half its mass after 6 hours.
But I know its definitely a gamma emitter.

 

[airie]

Half its mass? What does Tc-99m actually decay into?

EDIT: I take it that you meant #6, btw

And just to add for #3, yeah some dissolutions are endothermic, like NaCl, NH₄Cl, and some other ammonium compound I think... But for the exothermic ones, like NaOH and H₂SO{sub]4[/sub] and stuff, does increasing the temp decrease their solubility?

 

[alcalder]

3. In general, solubility of solids (in water) increase with increasing temperature, but that of gases decrease. Why?

Spoiler

I'm thinking that for gases, given more energy as temp increases, they would have a greater tendency to break the bonds with water molecules, and escape into the air. But why doesn't it work for solids? Dissolution in general is exothermic. Then why don't solids dissolve less in warmer water?

Because solids are being broken into their associated ions and attaching to the water molecules. The higher the temperature the more energetic the collisions and more likely to dissociate ions. (I think - it's been a while.)

4. Why doesn't the value of the equilibrium constant change when volume or pressure (strictly speaking, gas partial pressure, and only for equilibria involving gases) or conc of reactants/products change? Why does it change with temperature?

Spoiler

This is my guess, someone see if it's on the right track: when the temp is changed, neither the total amount nor the conc of reactants/products are changed, but the position of the equilibrium is shifted, thus K_eq is changed; looking at the expression of K_eq and relate to (delta)H of the forward reaction, one can determine if the contant is increased or decreased. When the volume/pressure is changed, though, the conc of ALL chemicals in equilibrium changes, and it somehow shifts such that K_eq remains the same. Same thing happens after the equilibrium shifts to counteract changes in conc of reactants/products. I'm not sure of the exact mechanism that makes K_eq remain constant though. Someone give an explanation?

Le Chatelier's Principle syaye that the shift in equilibrium will be in the direction that minimises or reduces the effect of the change and thus the same relative equilibrium is established with changes of concentration, pressure and volume. The reason there is no change with pressure or volume changes is because by increasing pressure you either have to reduce the volume of the container or increase the amount of substance. Therefore, the system re-establishes the equilibrium.

However, temperature, as you rightly say, shifts the equilibrium more one way or the other and thus the constant will change.

6. Tc-99m is used in medicine, and my teacher said that Tc-99 cannot be used. It's because of the long half-life of Tc-99 (which is something like >0.2 million years, compared to 6 hours of Tc-99m), which is a beta emitter. Problem though: Tc-99m decays into Tc-99 anyway, so what's the point? Or am I missing something?

Spoiler

Could it be that beta particles Tc-99 emits are of low energies and thus of little harm to humans? And because of this, it's not useful in med cos you'd need easy detection of the radioisotope in the body? Still, surely you'd not wanna be exposed to beta particles, however low their energies might be...

What does Technetium-99 do once it gets into the body?

Once in the human body, Tc-99 concentrates in the thyroid gland and the gastrointestinal tract. The body, however, excretes half of the ingested Tc-99 within 60 hours. It continues to excrete half of the remaining Tc-99 every 60 hours that follow. After 120 hours, only one-fourth of the ingested Tc-99 remains in the body.

For more info on Tc-99, go to: http://www.epa.gov/radiation/radionuclides/technetium.htm

​

7. Just wondering if anyone could explain what are geometrical and optical isomers, and what's the difference. I've wiki-ed them but I'd like a clarification

The optical isomers are mirror images of each other.
http://www.elmhurst.edu/~chm/vchembook/images/209mirror.gif

Geometrical isomers are isomers that have the same molecular formula but the atoms are in different non-equivalent positions to one another. Geometrical isomers occur as a result of restricted rotation about a carbon-carbon bond.
http://mooni.fccj.org/~ethall/stereo/square.gif

 

[wrxsti]

OMG i find what is said about chem... its like ... yep yep.... and with PHYSICS its lik WHAT THE????? totally opposite of you.... :S

 

[airie]

Thanks alcalder

#3...

Because solids are being broken into their associated ions and attaching to the water molecules. The higher the temperature the more energetic the collisions and more likely to dissociate ions. (I think - it's been a while.)

Why doesn't the same thing happen with gases then? What makes some dissolutions endothermic but others exothermic?

Spoiler

Last night this friend explained to me that dissolution of NaCl, for example, is endothermic because of the energies involved in the bondings: basically, the energy released when the molecule dissociates is less than that input for the ions to bond with water molecules, which are packed around the ion. This is because energy is needed to arrange the particles orderly ie. in a predictable way, instead of randomly floating around...He wasn't sure what happens with, say, NaOH, though, of which the dissolution is exothermic...

I'm thinking maybe it's to do with the strength of the bonds within the solute molecules and between the solute and water molecules, and whether the solute molecules are polar - thus whether strong bonds are established with water molecules?

Basically, what exactly makes gases tend to escape into the air, instead of bonding with water molecules, when given more energy (as a result of more successful collisions, or due to formation of bonds in a higher energy state)? Why is this energy used as KE, instead of in forming bonds?

Btw...for exothermic dissolutions of solids, increasing temp would result in decreased solubility, right? But as far as we know, are dissolutions of solids generally exothermic, or endothermic?

And, are there any gases that ABSORB heat when dissolved in water?

#4...

The reason there is no change with pressure or volume changes is because by increasing pressure you either have to reduce the volume of the container or increase the amount of substance.

So either way when pressure or conc of reactants/products are changed, you eventually change the conc of ALL chemicals in equilibrium in the SAME way (either all increased or decreased after re-establishment of equilibrium, compared to before the change). And cos K_eq is a ratio of the product of conc of products to that of the conc of reactants, this means the numerator and the denominator are changed in the SAME way.

Yeah, I figured all that. (When I was asking the teacher at school if that's right though, she said we learn the simplified version of equilibria for the HSC, the mechanism is actually more complicated. But she never got back to me explaining how exactly does it work...) But I was actually wondering how do the conc of reactants and products shift in such a way that K_eq remains exactly the same.

... Or maybe it's just one of those observations? That, "just is"? Like, "constant K_eq" is basically this law that all equilibria obeys, provided that they are at the same position?

Would it be right to even say that K_eq pinpoints the position of an equilibrium?

#6...

What does Technetium-99 do once it gets into the body?

Once in the human body, Tc-99 concentrates in the thyroid gland and the gastrointestinal tract. The body, however, excretes half of the ingested Tc-99 within 60 hours. It continues to excrete half of the remaining Tc-99 every 60 hours that follow. After 120 hours, only one-fourth of the ingested Tc-99 remains in the body.

For more info on Tc-99, go to: http://www.epa.gov/radiation/radionuclides/technetium.htm​

Why don't we just use Tc-99 to start with then? Is it to do with the ease of detection (as beta emission of Tc-99 is of low energies)? Or some other reason that makes Tc-99m more preferable over Tc-99?

(Wow. That's...another heap of questions...)

lol wrxsti XD I don't know, either I don't get chem, or I'm not thinking in physics. Or both. <.< ... Which...sucks. Whichever way you look at it

 

[xiao1985]

1) About the biggest myth in HSC Chemistry. You were taught to use conc H2SO4 as the catalyst for dehydration. The reality is: any acid can do that job of the catalyst just as well. The reason why conc. H2SO4 is chosen is due to the fact that conc. H2SO4 also (just happens) to absorb water (dehydrating agent). Consequently, it will push the equilibrium to the product side, yielding more product.

Conversely, ANY acid can catalyse hydration of ethene, not just dilute H2SO4. (Hence in some sources of HSC chemistry, you actually see H3PO4 acting as the catalyst as hydrating)

So to your question: conc. and dliute doesn't influence the rate of reaction to a non-neglible amount.

2) Should I assume there's water present? (Otherwise you cannot hydrate further ethene) It all comes down to the dH of the reaction and entropy (which you won't have learnt yet) My speculation is that more dehydration will occur (because more synthesis reaction tend to be exothermic) and heating a system up will favor the side with more moles of substance (ethene +water comparing to ethanol)

and no, you won't bet butane... ever.

3) I think it has entropic reasons behind it. Higher temperature favors the system with more disorder. Solid -> dissolved solid = more disorder. dissolved gas -> gas = more disorder.

For a HSC interpretation: think this way: hotter = molecules tend to "bump" more. If solid "bumps" more it is more likely to be dissolved (escapes from lattice). If dissolved gas bump more, it's more likely to escape from water.

Additional Q: for enthothermic dissolution, clearly increasing temp -> more dissolved. for exothermic dissolution, it's a completing effect of enthalpy cause less to dissolve and entropy cause more to dissolve.

5) HCl is used to dissolve previously undissolved solids (such as carbonates and oxides)

6) With a long half life of >.2 million years, the energy intensity emitted is likely to be very very low (something like 1 beta particle per minute or s th...) Try to detect 1 electron per minute is not .... possible. Therefore Tc99 is not suitable to be used as a tracer. Its parent, Tc99m, however, has a half life of 6 hrs, and is a gamma emitter. Half life of 6 hr means 2x10^6 photons per minute (again, I didn't do the calc. just in illustration) which is MUCH easier to detect and track

7) aka stereoisomer... i'd assume alcalder explained it well.

 

[airie]

1) About the biggest myth in HSC Chemistry. You were taught to use conc H2SO4 as the catalyst for dehydration. The reality is: any acid can do that job of the catalyst just as well. The reason why conc. H2SO4 is chosen is due to the fact that conc. H2SO4 also (just happens) to absorb water (dehydrating agent). Consequently, it will push the equilibrium to the product side, yielding more product.

Conversely, ANY acid can catalyse hydration of ethene, not just dilute H2SO4. (Hence in some sources of HSC chemistry, you actually see H3PO4 acting as the catalyst as hydrating)

So to your question: conc. and dliute doesn't influence the rate of reaction to a non-neglible amount.

Yes, I know that any acid would do, the catalyst is just to provide the H+ here. But my question is, what's the effect of the conc of the catalyst? And if you say that (well, I assume that you didn't mean a double negative there...) the effect of the conc of the catalyst on the rate of reaction is negligible, why doesn't the industry just use dilute acid? Instead of conc acid? You'd save the catalyst material by using dilute, wouldn't you?

2) Should I assume there's water present? (Otherwise you cannot hydrate further ethene) It all comes down to the dH of the reaction and entropy (which you won't have learnt yet) My speculation is that more dehydration will occur (because more synthesis reaction tend to be exothermic) and heating a system up will favor the side with more moles of substance (ethene +water comparing to ethanol)

and no, you won't bet butane... ever.

lol, I did notice that it didn't mention water.

Spoiler

Tally so far: more people say ethene...

5) HCl is used to dissolve previously undissolved solids (such as carbonates and oxides)

So how can you be sure that most undissolved solids would be dissolved by HCl? What characteristic makes HCl suitable? Its strength?

Thanks xiao1985

 

[xiao1985]

1) Just conc because not only do you speed up the rate of reaction, you also produce more product (and H2SO4 is pretty cheap any way. It must be economically viable to waste abit H2SO4 to boost the yield)

5) Hmmm good question... I would suppose it also get rid of the stuff which you don't want. (eg, silver ion, (AgCl is solid), lead ion (PbCl2 is solid) etc). Reason been that you will need to add sulphate next to form a ppt... but sulphate ppt with alot of other stuff as well, namely Ag and Pb. If you do actually have some Ag and Pb in your sample, it will ruin your result...

whereas, upon addition of HCl, Ag and Pb ions will ppt out with Cl-, and separated through filtration. So you can be rest assured that your result is at least more accurate than not removing Ag Pb ions before hand.