Sciencemadness Discussion Board

Some stuff my teacher said and I'm not sure if they are real...

Gui316 - 26-4-2012 at 11:23

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?

Hexavalent - 26-4-2012 at 11:48

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.

barley81 - 26-4-2012 at 11:53

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.

phlogiston - 26-4-2012 at 11:56

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.

Gui316 - 26-4-2012 at 12:01

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?

GreenD - 26-4-2012 at 12:03

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.

DJF90 - 26-4-2012 at 12:06

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.

MrHomeScientist - 26-4-2012 at 12:21

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.

Magpie - 26-4-2012 at 12:31

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.

Polverone - 26-4-2012 at 12:44

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.

phlogiston - 26-4-2012 at 13:55

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.

DerAlte - 26-4-2012 at 14:40

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