Gui316
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Some stuff my teacher said and I'm not sure if they are real...
Well, that happens quite often. My teacher says something that I'm compelled to refuse, but I just wanted to check if my thoughts are correct before
getting into more trouble and being an arrogant smart-***
She said once that arsenic will not sublime upon heating on atmosphere pressure, she said that it would pass through the liquid phase first. I have
always thought that arsenic will become a gas without passing through the liquid phase upon heating...Is that correct? I'm not sure, I've done some
little research on it, but since she is my teacher, I really don't know who is correct.
And today she said that every reaction can go backwards, except combustion reactions. Then she wrote that sodium oxide will react with water to form
sodium hydroxide, and sodium hydroxide will became sodium oxide if cooled down or electrolysed. I think that she might have made a mistake, but I'm
not sure...
Na2O + H2O <----> 2NaOH
Is that correct?
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Hexavalent
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With regards to every reaction being reversible, no, of course not. In quite a few reactions a product is formed which is completely unreactive or
unreactive with the leftover materials.
A reaction is usually reversible if it is unfavorable, or if the ending materials can push it back to the starting materials. Also, if a reaction is
pushed heavily by lets say an acid or base (an unreactive reactant with a strong acid or base) and forms an intermediate which is not favorable (like
a carbocation that doesn't have anything to react with immediately) then it will "fold" back pushing back the electrons and using the conjugate
acid/base to donate or take a proton.
Sodium oxide will react with water to give sodium hydroxide, but I've never heard of the reverse happening with cooling or electrolysis.
This is from my, comparatively, medium experience. It's kinda hard to explain here, but I'm sure the more seasoned chemists can offer better words.
"Success is going from failure to failure without loss of enthusiasm." Winston Churchill
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barley81
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Arsenic sublimes. It does not melt at normal pressure. It would take much higher pressure to get arsenic to liquefy. Your teacher isn't right about
that.
Sodium oxide isn't formed from sodium hydroxide easily. Electrolysis makes oxygen gas, water, and sodium metal. I suppose if you isolated the sodium
and oxygen you could combine them to make sodium oxide. Cooling won't make sodium oxide either.
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phlogiston
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You are right in both cases.
From wikipedia:
| Quote: | | Arsenic (and some arsenic compounds) sublimes upon heating at atmospheric pressure, converting directly to a gaseous form without an intervening
liquid state at 887 K (614 °C).[2] The triple point is 3.63 MPa and 1,090 K (820 °C).[9][2] Arsenic makes arsenic acid with concentrated nitric
acid, arsenious acid with |
And confirmed by a more reputable source, the CRC handbook of chemistry and physics, which says '613 subl' (and for the melting point gives 817 C at
28 atm)
Na2O + H2O will give NaOH.
Cooling down a solution of NaOH will simply yield solid NaOH crystals if the solution is concentrated enough (it becomes saturated due to the
cooling).
Cooling down solid NaOH will just give cold NaOH.
Electrolysing a solution of NaOH will give hydrogen and oxygen if inert anodes are used. If Na2O would form, it would react immediately with the water
to form NaOH again, as per her own equation.
Electrolysing molten NaOH will give sodium metal, water and oxygen.
Sounds like your teacher has a bit to learn still.
-----
"If a rocket goes up, who cares where it comes down, that's not my concern said Wernher von Braun" - Tom Lehrer |
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Gui316
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I'm grateful for the fast answers. But something really annoys me... How can I discuss about those subjects with her in a peaceful way? I mean, it
would sound extremely arrogant from me to tell her that what she said is not completely accurate. How should I deal with these kind of situations? I
really don't want to sound arrogant, but i don't want let her teach things that are not accurate either...That is one thing that I don't want in my
conscience...So, how do we deal with that?
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GreenD
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Quote: Originally posted by Gui316  | | I'm grateful for the fast answers. But something really annoys me... How can I discuss about those subjects with her in a peaceful way? I mean, it
would sound extremely arrogant from me to tell her that what she said is not completely accurate. How should I deal with these kind of situations? I
really don't want to sound arrogant, but i don't want let her teach things that are not accurate either...That is one thing that I don't want in my
conscience...So, how do we deal with that? |
Might not be able to - she may get pissed if you call her out.
Just respond with what you know.
ʃ Ψ*Ψ
Keepin' it real.
Check out my new collaborated site: MNMLimpact.com
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DJF90
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Alot of what is taught, even at higher levels, isn't completely accurate. I have found that often it is a simplification that makes it easier to teach
and for the students to understand.
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MrHomeScientist
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Just present your evidence in a calm and logical manner, and she'll have to accept it if she's a reasonable person. Mention that what she said was not
completely correct, and if she argues direct her to the sources pointed out here. If she still gets annoyed, well, there's not much you can do beyond
showing the facts! Maybe approaching her after class rather than calling her out in front of other students would help avoid confrontation.
I agree with what everyone else has said, too. I could argue that she was correct, in a sense, about everything being reversible. Apply enough energy
and control your conditions well enough and you can do just about anything! Is that practical, though? In many cases, not at all.
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Magpie
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| Quote: Originally posted by Gui316 | | How should I deal with these kind of situations? I really don't want to sound arrogant, but i don't want let her teach things that are not accurate
either...That is one thing that I don't want in my conscience...So, how do we deal with that? |
My first thought was to go to her, in private, with your findings. Make sure your assertions are well supported with proper references (published
articles, books,etc). But I'm not sure this would be in your best interest. Your teacher probably is aware that she doesn't know what she is talking
about, but she is "the teacher" and has to act the part to keep her job. When you have to support yourself and your children you will learn how
important that job is.
So, even though you (through your taxpaying parents) are the customer, and are getting a terrible product, there's not a lot you can do. I recommend
doing what you are already doing, ie, learning indendently, and regarding every thing she says with scepticism.
The single most important condition for a successful synthesis is good mixing - Nicodem
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Polverone
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A video of arsenic sublimation may be more vivid: https://www.youtube.com/watch?v=Z5jXsz5Ym9A
All chemical reactions are formally reversible but many are not reversible in any practical sense. I would include the sodium oxide/water reaction as
an example. Some other non-combustion examples would be the coagulation of albumin or setting of urea-phenolic resins.
There are actually a couple of senses of "reversible" here that are worth separating. The one we're talking about is the reversibility of reactions
that sets up a non-trivial equilibrium in chemical systems. For example, the acid-catalyzed ester production from ethanol and benzoic acid stabilizes
before all the starting materials become products, because the products can also re-form starting materials. By manipulating the reaction conditions
you can force the equilibrium in favor of reactants or products.
Other reactions exhibit trivial equilibrium behavior; they overwhelmingly favor either the products or the reactants, and you don't have to worry
about reversibility. If you're evaluating a hydrogen peroxide solution by catalytic decomposition you don't need to account for oxygen combining with
water to re-form hydrogen peroxide.
The coagulation of albumin is also not reversible in the sense of establishing a non-trivial equilibrium. After the protein cooks there is no
manipulation of conditions that will cause it to revert to runny egg white. But you could use the cooked material as fertilizer or food for organisms
that chickens eat, indirectly reforming albumin in the chicken's egg.
PGP Key and corresponding e-mail address
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phlogiston
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A really good teacher might recognize your desire for knowledge and critical mind as signs of a really good student that shows some of the important
qualities needed get far in life. She really should be stimulating this kind of behaviour. These days, it is also rare to see people that can know
obscure facts like these by heart, while it really extremely useful to be able to memorise such things as it allows you to make connections in your
mind.
But if you suspect she would feel offendended if you point out the gaps in her knowledge, I suspect it may be better to simply swallow your desire to
correct her. Little good will come of it.
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"If a rocket goes up, who cares where it comes down, that's not my concern said Wernher von Braun" - Tom Lehrer |
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DerAlte
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There is no doubt that As sublimes at atmospheric pressure. Any reference will show this (eg Wiki on Arsenic).
I have no idea what the equilibrium constant is, but for all practical purposes (FAPP) the reaction of Na2O and H2O is not reversible in any closed
system. That is, the amount of Na2O remaining on adding to water will be vanishingly small - essentally all will be converted to NaOH.
Another example of FAPP non-reversible reactions is the action of an acid on zinc to produce hydrogen. But it is reversible, if only you could produce
the enormous pressure to reverse it.
Sometimes a reaction is reversible yet it is possible to complete the reaction in spite of
the reversibility.
If steam is passed over iron at a red heat the reaction
3Fe+4H2O <--> Fe3O4 + 4H2
can be carried to completion. If hydrogen is passed over iron oxide and the steam produced swept away, iron is regenerated. In effect, by removing the
steam from the reacting region we have reduced its concentration and it cannot back react with the iron. In these cases we violate the closed system
constion for equilibrium.
Der Alte
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Polverone
Now celebrating 21 years of madness
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Thread Split
28-4-2012 at 15:46 |
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