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Zinjanthropos
Post  Post subject: Chemistry Question  |  Posted: Mon Mar 27, 2017 1:08 pm
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If I mix two chemicals together, will they always react the same way? Someone told me that temperature can trigger different reactions so I was wondering if a + b doesn't always equal c.

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PhDemon
Post  Post subject: Re: Chemistry Question  |  Posted: Mon Mar 27, 2017 1:25 pm

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It is true that under the same conditions you would expect to see two chemicals reacting in the same way to give the same products, but the devil is in the detail... That "same conditions" is quite important. You change the temperature, pressure, concentration of one (or all) of the chemicals or the solvent used you may see a different reaction or different amounts of product formed...

Whether a reaction is feasible or not depends on thermodynamics, how quickly it occurs depends on kinetics, different conditions change the "feasibility" and "rates" of reactions. This can be calculated but it would not be easy to explain in forum posts, a lot of the undergrad chemistry course is concerned with this stuff!
The basics are here: https://en.wikipedia.org/wiki/Chemical_thermodynamics and here: https://en.wikipedia.org/wiki/Chemical_kinetics

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Last edited by PhDemon on Mon Mar 27, 2017 1:44 pm, edited 1 time in total.


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PhDemon
Post  Post subject: Re: Chemistry Question  |  Posted: Mon Mar 27, 2017 1:43 pm

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One example I was just teaching today is the reaction of sodium hydroxide with a haloalkane...

If the sodium hydroxide is dissolved in water you produce an alcohol via nucleophilic substitution, if the sodium hydroxide is dissolved in ethanol you produce an alkene via an elimination reaction...

Another is the combustion of hydrocarbons... burn a hydrocarbon completely in oxygen you will get water and carbon dioxide, reduce the oxygen concentration and you will get some carbon monoxide formed as well.

Or here: http://www.bbc.co.uk/schools/gcsebitesi ... rev3.shtml play with the slider on the graphic to show how changing the temperature affects the yield of the product.

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marnixR
Post  Post subject: Re: Chemistry Question  |  Posted: Mon Mar 27, 2017 1:59 pm
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another example is the difference in exhaust gases from petrol and diesel engines : because the latter operate at a higher temperature diesel exhaust fumes contain appreciable amounts of NOx, whereas the lower temperature for petrol enipgines hardly produces any

the reason being that the oxygen and nitrogen reaction requires a certain minimum temperature to take place

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Zinjanthropos
Post  Post subject: Re: Chemistry Question  |  Posted: Mon Mar 27, 2017 3:25 pm
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PhDemon wrote:
It is true that under the same conditions you would expect to see two chemicals reacting in the same way to give the same products, but the devil is in the detail... That "same conditions" is quite important. You change the temperature, pressure, concentration of one (or all) of the chemicals or the solvent used you may see a different reaction or different amounts of product formed...


I think this is what I'm looking for. Same conditions would mean mixtures have exactly the same quantities plus all other circumstances. Your saying a change in one or more conditions will affect results regardless of same quantities. If the ratio between two chemicals doesn't change but the amounts do, other conditions remaining the same, would this make a difference in reactions? (If I mixed 10 of this and 10 of that, or 1000 of this and a 1000 of that.)

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PhDemon
Post  Post subject: Re: Chemistry Question  |  Posted: Mon Mar 27, 2017 3:45 pm

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Zinjanthropos wrote:
Your saying a change in one or more conditions will affect results regardless of same quantities.


Yes, even if all quantities are kept the same changing the conditions can change the yield of products (or even which products form).

Quote:
If the ratio between two chemicals doesn't change but the amounts do, other conditions remaining the same, would this make a difference in reactions? (If I mixed 10 of this and 10 of that, or 1000 of this and a 1000 of that.)


If the ratios are the same the absolute amounts will not matter for reactions of pure substances that are solid or liquid with the same surface area to volume ratio (read caveat below) as the reaction stoichiometry will not change if the other conditions are constant. What will change is the actual amount of product made (the percentage yield will be the same) and the amount of heat energy given out or taken in by the reaction as this is a molar quantity.

CAVEAT: it will matter if the chemicals are in solution because changing the amounts will change the concentration or if they are gases as changing the amount will change the pressure.

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Zinjanthropos
Post  Post subject: Re: Chemistry Question  |  Posted: Mon Mar 27, 2017 4:09 pm
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PhD, I'm impressed. I hope that when you teach this to your students that they learn it, just so they never have to ask these types of questions. :) Obviously chemistry is alien to me and I hope you don't mind providing answers. I have another one.....

Occasionally I've wondered about this: When I look at the Periodic Table I see each element has a different atomic number(protons, electrons). Why is there nothing in between two elements? Or is there, even theoretically? We can't synthesize something that would fit in that slot because the physics doesn't allow it or what?

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PhDemon
Post  Post subject: Re: Chemistry Question  |  Posted: Mon Mar 27, 2017 4:36 pm

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No problem ;)

In the periodic table elements are listed by atomic number, this is the number of protons the atom has in it's nucleus. As protons are discrete particles you cannot have fractions , just a whole number of protons. Because of this there cannot be anything between elements that have consecutive atomic numbers. Any new elements will be at the end of the table after all of the others as elements with atomic numbers 1 to 118 have been discovered... There can be no gaps.


As an aside, this was the great thing about Mendeleev's original periodic table. Even though he based it on atomic mass not atomic number (which isn't quite right but protons hadn't been discovered back then) he spotted gaps between the elements known at the time and predicted theoretically the existence of a number of elements that were later discovered.

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Zinjanthropos
Post  Post subject: Re: Chemistry Question  |  Posted: Wed Mar 29, 2017 5:10 pm
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Multi part question regarding odors/smells and humans:

1. In a non-polluted state, does air smell and if it does, have we evolved not to notice it?
2. Does oxygen have an odor or what if anything of air's elements actually has an odor?
3. How many elements have an odor?

I realize at some times its subjective, noses differ. Other creatures may smell something we don't. I understand air is comprised of different percentages of elements.

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PhDemon
Post  Post subject: Re: Chemistry Question  |  Posted: Wed Mar 29, 2017 7:05 pm

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1. Dry air is (roughly) 78% nitrogen, 21% oxygen with most of the remaining percent being argon. None of these have a noticeable odour. This could be because we have evolved so this is the case otherwise we would be walking around holding our noses! In polluted air the main "smellies" are probably ozone (its name comes from a Greek root meaning "smelly"!), NO2 (it honks!) and various hydrocarbons/organic molecules.

2. Pure oxygen does not have an odour, but its allotrope ozone (O3) does. See above.

3. I'll revisit this tomorrow if I think of any more but off the top of my head (and from personal experience!) chlorine, bromine and iodine are all odoriferous (the name bromine is from a Greek root meaning "stench") and sulphur pongs... Other elements (nitrogen, osmium, selenium etc.) are famous for making smelly compounds but AFAIK the pure elements are odourless.

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Zinjanthropos
Post  Post subject: Re: Chemistry Question  |  Posted: Thu Mar 30, 2017 9:01 pm
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I thought humans can't smell air or water because it's more important for survival to smell/detect the bad stuff they may contain. Not sure if there's any truth to that.

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iNow
Post  Post subject: Re: Chemistry Question  |  Posted: Thu Mar 30, 2017 9:15 pm
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You have to remember what smell is. It's a response to certain molecules entering our nose and interacting with certain receptors.

We have receptors preferentially "tuned" to detect some things and not to notice others. Clearly, there was an evolutionary benefit to being able to smell and avoid things that might harm us or to smell and approach things that might help us (i.e. food).

We also desensitize when surrounded by a smell for too long, but that's much more related to nerve cells firing and then getting tired / no longer firing with the same frequency or force, as it were.

That said, we're primarily visual animals. Smell is ancestral and deeply connected to our memories and emotions, but it's not our strongest sense as it might be in other animals like sharks or bears, for example.

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marnixR
Post  Post subject: Re: Chemistry Question  |  Posted: Thu Mar 30, 2017 9:45 pm
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agree with iNow - the receptors in your nose are always saturated with nitrogen, oxygen and humidity
hence the brain ignores those signals because they're always there
what's important to the brain is things that change, in order to catch things that are out of the ordinary

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Zinjanthropos
Post  Post subject: Re: Chemistry Question  |  Posted: Fri Mar 31, 2017 4:10 pm
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marnixR wrote:
agree with iNow - the receptors in your nose are always saturated with nitrogen, oxygen and humidity
hence the brain ignores those signals because they're always there
what's important to the brain is things that change, in order to catch things that are out of the ordinary


So there probably is a smell for many things our brain doesn't bother with. Does chemistry cancel chemistry in living organisms to benefit their survival? If so I wonder how the brain does it, and with what chemicals? Are there chemicals that can totally cancel or neutralize the effects of other chemicals? I don't mean overpower, just an even saw-off between two chemicals.

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marnixR
Post  Post subject: Re: Chemistry Question  |  Posted: Fri Mar 31, 2017 6:36 pm
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our brain is primed to notice things that change, that are noteworthy
more of the same it just ignores, presumably trying to avoid overload of the sensory manipulation

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PhDemon
Post  Post subject: Re: Chemistry Question  |  Posted: Fri Mar 31, 2017 9:47 pm

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I'm not a biologist and this may be wrong but there is no selection pressure to detect stuff that is always there. Human noses have probably evolved to detect smells that are useful or harmful. If back in the day everyone could smell "air" they would be out-competed by those that could smell food or danger. Just via natural selection the receptors that respond to neutral molecules that are around all the time would not benefit an organism and mutations that ignored them could be selected for. This may explain why stuff that is everywhere like water and air is odourless..

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iNow
Post  Post subject: Re: Chemistry Question  |  Posted: Sat Apr 01, 2017 12:01 am
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As marnix notes, we simply desensitize to things. When I notice this stuff, it's often when I walk into someones home for the first time. I'm used to the smells in MY house, but the smells in THEIR house always seem much more potent and powerful (even though I'm sure they're perfectly normal overall).

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Zinjanthropos
Post  Post subject: Re: Chemistry Question  |  Posted: Mon Apr 17, 2017 4:10 pm
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Next question: I remember someone buying me a chemistry set for Xmas when I was about 10. Without any formal training or knowledge I never had any accidents but did see some peculiar things. For some reason I remember putting strings in a solution and having something crystallize on them.

However I've always wondered if those old gift chem sets had potential for something really disastrous like explosions, poisonous vapors, corrosive acids, etc. Do they still sell them or is that heavily legislated these days?

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PhDemon
Post  Post subject: Re: Chemistry Question  |  Posted: Mon Apr 17, 2017 5:11 pm

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The crystals on strings will work with almost any saturated solution but borax is an old favourite in chemistry sets.

As to your other question the potential for really dangerous accidents with a chemistry set is minimal, they are much more heavily regulated and the chemicals in them are pretty lame nowadays but even with the old ones you'd have to try hard to do much damage... In the one I had as a kid the most dangerous thing was the spirit burner (I set fire to my bedroom curtains :lol:) used to heat stuff up and the copper sulphate which is a bit toxic if you eat it!

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Zinjanthropos
Post  Post subject: Re: Chemistry Question  |  Posted: Mon Apr 17, 2017 6:46 pm
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PhDemon wrote:
In the one I had as a kid the most dangerous thing was the spirit burner (I set fire to my bedroom curtains :lol:) used to heat stuff up and the copper sulphate which is a bit toxic if you eat it!


You've never set your hair on fire? Do they still use that image for a chemist who's experiment went wrong, you know the one where the hair is smoldering, spectacles broken, lab coat shredded and the face blackened by soot?

Speaking of smells, just what chemical is it that makes burnt hair smell so awful?

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PhDemon
Post  Post subject: Re: Chemistry Question  |  Posted: Mon Apr 17, 2017 7:05 pm

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I've never set my hair on fire but I've seen it happen to others... I did once set my tie on fire though!


As for burning hair IIRC there is a fair bit of sulphur in the amino acids in the proteins that make hair, sulphur compounds stink especially when they are on fire...

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Zinjanthropos
Post  Post subject: Re: Chemistry Question  |  Posted: Fri Apr 21, 2017 12:33 pm
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Next question: Hope this can be understood....Are there many chemical experiments or reactions that can't take place here on Earth? Along the same lines, would the same experiment yield the same results if they took place on different planets?

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PhDemon
Post  Post subject: Re: Chemistry Question  |  Posted: Fri Apr 21, 2017 12:40 pm

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Any reaction that is temperature or pressure dependent[1] would be different on different planets as they have different surface temperatures and pressures...

The feasibility of chemical reactions is related to the Gibbs free energy change of the process (https://en.wikipedia.org/wiki/Gibbs_free_energy ) you will see from the definitions on this page that G is dependent on both temperature and pressure so change the conditions and you change the feasibility of the reaction.

Does that answer your question?

[1] which is most of them.

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Zinjanthropos
Post  Post subject: Re: Chemistry Question  |  Posted: Fri Apr 21, 2017 12:54 pm
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I understand different conditions but are there experiments that chemists want to perform here on Earth but cannot? Or perhaps can be conducted elsewhere? I know simulated environs exist but what if it can't be done?

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PhDemon
Post  Post subject: Re: Chemistry Question  |  Posted: Fri Apr 21, 2017 1:02 pm

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In the lab you can create most conditions you need, we can get down pretty damn close to absolute zero on the temperature scale and can get hot enough temperatures that the problem is one of plasma physics rather than chemistry. On the pressure scale we can get pretty good vacuums and high pressure reactions are commonplace in industry. I suppose there may be times when we can't get to as high pressure as we would like (for example in making solid metallic hydrogen) but in those sorts of conditions no one would be able to live to do the experiment!

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Zinjanthropos
Post  Post subject: Re: Chemistry Question  |  Posted: Fri Apr 21, 2017 6:05 pm
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How about in the vacuum of space itself?

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PhDemon
Post  Post subject: Re: Chemistry Question  |  Posted: Fri Apr 21, 2017 6:09 pm

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Anything with a significant vapour pressure would instantly vapourize and as the matter density is so low and the molecules would be moving very slowly there would be few molecular collisions making chemical reactions tricky...

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Zinjanthropos
Post  Post subject: Re: Chemistry Question  |  Posted: Fri Apr 21, 2017 7:01 pm
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I was trying to lead up to this question: the first chemical reaction of the early universe (post BB)....any idea?

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PhDemon
Post  Post subject: Re: Chemistry Question  |  Posted: Fri Apr 21, 2017 7:14 pm

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Post big bang the only atoms knocking about would have been hydrogen and helium... No potential for any chemistry, the heavier elements were made in stars by nuclear fusion, chemistry was a late starter...

The earliest chemistry would have been H. + H. --->. H2. not very interesting ;)

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marnixR
Post  Post subject: Re: Chemistry Question  |  Posted: Fri Apr 21, 2017 7:27 pm
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it would have been a barrel of laughs if helium had had the ability to form chains :

HeHeHeHe....

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PhDemon
Post  Post subject: Re: Chemistry Question  |  Posted: Fri Apr 21, 2017 8:03 pm

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Chemical error, zero marks. (takes examiner's hat off) :lol:

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Zinjanthropos
Post  Post subject: Re: Chemistry Question  |  Posted: Tue May 30, 2017 7:46 pm
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Canada is tightening the noose around methane pollution. Pipelines and farting livestock being singled out. Ok, I added the fart part. Anyways, people I know we're discussing the best way to decrease the amount of methane going to atmosphere. Naturally methane does escape but commercial enterprise like oil and gas producers just vent it away seeing how it's lighter than air. My suggestion was to light it on fire at a burner and then capture the CO2. CArbon capture and storage technology is apparently way ahead of the game when compared to methane removal. Why is methane so hard to extract from the atmosphere, is it the specific gravity or just a tough chemical compound to break down?

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PhDemon
Post  Post subject: Re: Chemistry Question  |  Posted: Tue May 30, 2017 8:11 pm

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It's not that methane is a particularly tricky bugger, it's just that separating gases and trapping selected ones isn't easy to do cheaply... Methane isn't that reactive chemically so removing just methane from a gas stream is difficult and physical separation (other than liquefying and fractional distillation) take ages... As you say you normally just burn it and produce CO2 which as an acidic gas is easier to remove but it still won't be as cheap as the "fuck it" approach current in most industries, unless the fines are prohibitively large I'm guessing it will be ignored (cynical I know)... If you could use the energy produced as the leaked CH4 is burned AND capture the CO2 that would be a win/win.

In (almost?) all cases it will be cheaper to make fewer leaks happen than trying to fix it once it's up there...

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Zinjanthropos
Post  Post subject: Re: Chemistry Question  |  Posted: Wed May 31, 2017 12:18 pm
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Not sure if this is up your alley PH but yesterday we were using Windex to clean some windows when we ran out of the stuff. Is there any combination of common household liquids (chemicals) that can be used in its place? We tried vinegar and water and had reasonably good success but I kept wondering if there's something else.

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PhDemon
Post  Post subject: Re: Chemistry Question  |  Posted: Wed May 31, 2017 12:27 pm

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Vinegar and water is good, you could add a surfactant (a small amount of washing up liquid) if any greasy marks persist...

My mum swears by a product called Stardrops which is an ammonia based cleaner but I've never used it myself (and it would need to be used separately from the vinegar/water).

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iNow
Post  Post subject: Re: Chemistry Question  |  Posted: Wed May 31, 2017 12:58 pm
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Rubbing alcohol is often involved too

https://www.pinterest.com/explore/homem ... s-cleaner/

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PhDemon
Post  Post subject: Re: Chemistry Question  |  Posted: Wed May 31, 2017 1:59 pm

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Mum also claims rubbing the windows with balled up newspaper while cleaning makes the glass shinier... Not sure why, may be an old wives tale, but she has always done it...

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