Sciencemadness Discussion Board

Enthalpy Variation

HgDinis25 - 20-6-2014 at 14:08

Yesterday it was the day of the national exam of Physics and Chemistry A.
In one of the questions they gave us the following equation:

N2 (g) + 3H2 (g) <--> 2NH3 (g)

They also said the Enthalpy variation was -92kJ/mol.
Then they asked to choose the right sentence from the following:
1-The reaction releases 92 kJ for every two mol of Ammonia formed.
2-The reaction releases 92 kJ for every mol of Ammonia formed.

I selected the second one, but the right one was the first, acording to our Education Ministery. Many teachers wen't crazy about it, including mine, beacuse they had taught it to the students that the second sentence was the right one.

Any help?

DraconicAcid - 20-6-2014 at 14:18

Quote: Originally posted by HgDinis25  
Yesterday it was the day of the national exam of Physics and Chemistry A.
In one of the questions they gave us the following equation:

N2 (g) + 3H2 (g) <--> 2NH3 (g)

They also said the Enthalpy variation was -92kJ/mol.
Then they asked to choose the right sentence from the following:
1-The reaction releases 92 kJ for every two mol of Ammonia formed.
2-The reaction releases 92 kJ for every mol of Ammonia formed.

I selected the second one, but the right one was the first, acording to our Education Ministery. Many teachers wen't crazy about it, including mine, beacuse they had taught it to the students that the second sentence was the right one.

Any help?


The first one is correct. It's -92 kJ/mol, the "per mol" being per moles of reaction, not ammonia. There are two ammonias formed per reaction.

92 kJ will be given off a) per mol of N2 consumed, b) per three moles of hydrogen consumed, or c) per two moles of ammonia formed.

If "many teachers" were unhappy about it, they'd better learn more about it before trying to teach it again.

[Edited on 20-6-2014 by DraconicAcid]

aga - 20-6-2014 at 14:49

How was this Taught to you ?

More exactly, what was the Process that was taught to figure it out ?

HgDinis25 - 20-6-2014 at 14:56

Quote: Originally posted by aga  
How was this Taught to you ?

More exactly, what was the Process that was taught to figure it out ?


By your question I believe you think the students (me) got it wrong (I may be mistaken on what you meant). Well, I just emailed my teacher and she confirmed that the right answer is the first (the one our class selected). She siad that it is 92kJ per mol of Ammonia formed.

Answering your question, it was taught just like that. Kj per mol of reactants formed, when the unity used was kJ/mol. If there were more than one product, then it would be any of them. She also taught that kJ/mol is differernt from just kJ and that Enthalpy variation could be expresed in any of those two units.

Well, I explained to here, back in the day we learned this (2/3 months ago) that I thought it meant the energy per mol of reaction (I used the same expresion draconic acid used) and she said in front of the entire class I was wrong (a few misunderstandings followed wich I'm not going to comment on) and I ended up believing her. She even said that she had consulted a University book that confirmed what she said.

Now I just don't know.

[Edited on 20-6-2014 by HgDinis25]

aga - 20-6-2014 at 15:01

So a misunderstanding then, but not yours.

It is a great shame that when contradicted by a student that the Teacher did not Check thoroughly.

However, it is very easy to be wrong, and very hard to check everything in real life.

Edit:
Next time you are told you are Wrong, do the maths, create the evidence, then decide for yourself.
This process may show that you are wrong, in whch case, say so.
If it shows you to be Right, go back and present your Proof, so the teacher may also learn.

The rest of your class (apart from Bob) will also have chosen the wrong answer.

[Edited on 20-6-2014 by aga]

HgDinis25 - 20-6-2014 at 15:08

Quote: Originally posted by aga  
So a misunderstanding then, but not yours.

It is a great shame that when contradicted by a student that the Teacher did not Check thoroughly.

However, it is very easy to be wrong, and very hard to check everything in real life,


So I guess this is a fact? I was starting to think there would be no objective answer, but it seems to be a pretty simple fact from what you've said.

Actualy, the teacher checked other teachers that confirmed her version of the facts (there are many who are against the answer considered right by the ministery). She also said that she checked her university book on the subject, that also pointed the enthalpy the same way she did.

I don't care if it's easy or not to be wrong, that exam is going to be very important to enter the best university in my country, I'm pissed that I aldeady got one question wrong because of my teacher (and the others who defended her) even after I told her she was wrong in class.

Anyway, would you give the same explanation draconic gave?

Edit: Yes my entire class failed thar question -there was a big argument between me and my teacher in that class. I pointed out a few proofs, including an example of the reaction of Hydrogen with Oxygen to form Water. Long Story. I didn't got your Bob (joke ?) thing. Is my humor rusty?


[Edited on 20-6-2014 by HgDinis25]

DraconicAcid - 20-6-2014 at 15:13

Quote:
Well, I just emailed my teacher and she confirmed that the right answer is the first (the one our class selected). She siad that it is 92kJ per mol of Ammonia formed.

Answering your question, it was taught just like that. Kj per mol of reactants formed, when the unity used was kJ/mol.


No- it's per mol of whatever has a coefficient of one.
Quote:
If there were more than one product, then it would be any of them. She also taught that kJ/mol is differernt from just kJ and that Enthalpy variation could be expresed in any of those two units.


This is also wrong. kJ/mol is different from kJ, but the difference is that kJ/mol is a characteristic quantity for that particular reaction. kJ is how much heat is evolved/absorbed in a given experiment, and will depend on the quantities used.

Sadly, most first-year university textbook authors are too smegging lazy to make this distinction, and write "kJ" when they mean "kJ/mol".
Quote:
Well, I explained to here, back in the day we learned this (2/3 months ago) that I thought it meant the energy per mol of reaction (I used the same expresion draconic acid used) and she said in front of the entire class I was wrong (a few misunderstandings followed wich I'm not going to comment on) and I ended up believing her. She even said that she had consulted a University book that confirmed what she said.

Well, now you can take the judgement of the Minister, and tell her that you were right all along.

HgDinis25 - 20-6-2014 at 15:22

Quote:

Well, now you can take the judgement of the Minister, and tell her that you were right all along.


Sadly that won't give me back the 0.5 points in 20 already lost in the exam.
It's really a shame the quality of classes and teachers my country has because this isn't even close to be the first time something like this happen. Is this worldwide or just happening here?

DraconicAcid - 20-6-2014 at 15:27

Quote: Originally posted by HgDinis25  
Quote:

Well, now you can take the judgement of the Minister, and tell her that you were right all along.


Sadly that won't give me back the 0.5 points in 20 already lost in the exam.
It's really a shame the quality of classes and teachers my country has because this isn't even close to be the first time something like this happen. Is this worldwide or just happening here?

That's probably world-wide. I know some of the high school teachers in my neck of the woods keep telling their students that evolution is a ridiculous idea.

aga - 20-6-2014 at 23:20

Never be afraid to be 'Out of Step', provided that you have firm grounds to believe you're right.

The ryder to that is that sometimes you Will be proven wrong, so need to be able to suck up the embarassment too.

Check out the Solomon Asch Conformity Experiment
http://www.youtube.com/watch?v=NyDDyT1lDhA

(the Bob joke is that there's always someone who gets the wrong answer in a class - Bob - who accidentally got it right that time)

blogfast25 - 21-6-2014 at 04:55

It was a silly question to begin with. Many very decent chemists will express reaction heat per mol of reaction product and that makes perfect sense.

Would anyone really express the reaction heat of:

2 Al + 3 O2 === > Al2O3 as - 838 kJ/mol to accommodate the coefficient 2, when every decent textbook uses the value of - 1676 kJ/mol, that is the formation enthalpy of alumina? I hope not.

When education systems ask silly questions like that you have to wonder about their priorities. Students should be able at least to justify their choice of answer and be awarded points if that choice was logical, not dictated by a bureaucratic rule.
Quote: Originally posted by DraconicAcid  
That's probably world-wide. I know some of the high school teachers in my neck of the woods keep telling their students that evolution is a ridiculous idea.


I profoundly disagree with you. Education systems in Western Europe have made enormous progress.

Perhaps you're referring to a certain 'Gr**test Nation' but that country has been going bonkers for decades IMHO.

[Edited on 21-6-2014 by blogfast25]

HgDinis25 - 21-6-2014 at 07:00

Well, I'm in Portugal (wich is on the far west of Europe) and I can assure you that our educational system is completly screwed up. And now with the help of the good and friendly European Union, Portugal has been investing all the education funds in the so called "Profesional courses". They are intendend to make the stundents finish education when they're 18 so they can immediatly go to the work market. Fundings to universities or the scientific courses of high school? Nop, they're not worth it.

aga - 21-6-2014 at 07:00

Quote: Originally posted by DraconicAcid  
some of the high school teachers in my neck of the woods keep telling their students that evolution is a ridiculous idea.

Evolution is a ridiculous idea, and has been religiously disproven time and again.

There is simply no way that evolution can happen on a flat earth around which the universe revolves.

HgDinis25 - 21-6-2014 at 07:03

Quote: Originally posted by aga  
Quote: Originally posted by DraconicAcid  
some of the high school teachers in my neck of the woods keep telling their students that evolution is a ridiculous idea.

Evolution is a ridiculous idea, and has been religiously disproven time and again.

There is simply no way that evolution can happen on a flat earth around which the universe revolves.


Of course it is a ridiculous idea. Everyone knows women were made from a rib taken of a man...

blogfast25 - 21-6-2014 at 08:04

HgDinis25:

Portugal may be a particular case, I don't know.

But here in Britain I've followed the secondary education of my daughter very closely and found it to be much better than what I had experienced. She's now finished her first year at Uni and my impression is that that too was better organised and educationally worth more than my own Uni experience.

Despite that, education remains a political wedge issue, resulting in many false complaints, often driven by parents' own ideological biases ('state schools' v 'private schools' for instance).

[Edited on 21-6-2014 by blogfast25]

DraconicAcid - 21-6-2014 at 12:08

Quote: Originally posted by blogfast25  
Perhaps you're referring to a certain 'Gr**test Nation' but that country has been going bonkers for decades IMHO.


Sadly, no...I'm well to the north of where you would expect such nonsense.

aga - 21-6-2014 at 13:58

Quote: Originally posted by HgDinis25  
Everyone knows women were made from a rib taken of a man...

Preposterous !
It is Obvious that Women are Heavenly, and made of much better stuff than spare man-parts.
Perhaps you refer to mass-produced Thai lady-boy copies.

[Edited on 21-6-2014 by aga]

HgDinis25 - 21-6-2014 at 16:27

Aga, you sir have a exquisit sense of humor that I adore... hehe

Blogfast: The usual debates about education need to be made by people who have real arguments to explain, not the common non sense you pointed out. But the holes in education here go well beyong subjective debate: they twist up facts so their hill trained profesors may teach the hill trained students even more twisted up facts.

And it isn't generalized to my location. Here we have national final exams of Physics and Chemistry A (they join Physics and CHemistry in the same subject, I don't know if it's the same in other countries). The average on the past 5 years has been negative, meaning that the average student doesn't learn the subject and doesn't pass it. This alone should point out the incompetence of our educational system.

blogfast25 - 22-6-2014 at 04:55

Hg:

Here at GCSE level (up to about 15), it's just science (one course, one exam). At A level (about 15 - 18) it splits up in the various sciences with separate exams. In itself that indicates the 'Evil EU' cannot be blamed for Portugal's educational woes because if they were the result of EU directives we would be compelled to follow the same rules.

Dany - 22-6-2014 at 06:25

The first answer is the correct one simply because the variation in heat of formation is given for the formation of two moles of NH3. Thus, The heat of formation released by the formation of one mol of NH3 will be -46 kJ/mol which is the value found experimentally (-45.9 kj/mol)
see,

http://webbook.nist.gov/cgi/cbook.cgi?ID=C7664417&Units=...

Dany.

blogfast25 - 22-6-2014 at 10:09

Quote: Originally posted by Dany  
[...] variation in heat of formation [...]


You meant the reaction enthalpy, right? There can be no 'variation in heat of formation': the heat of formation of a pure substance is fixed in fixed conditions of temperature and pressure, it can't vary.

[Edited on 22-6-2014 by blogfast25]

HgDinis25 - 22-6-2014 at 11:47

Quote: Originally posted by blogfast25  
Hg:

Here at GCSE level (up to about 15), it's just science (one course, one exam). At A level (about 15 - 18) it splits up in the various sciences with separate exams. In itself that indicates the 'Evil EU' cannot be blamed for Portugal's educational woes because if they were the result of EU directives we would be compelled to follow the same rules.


Not so fast blogfast. You ironically described EU as "evil" because of my argument being similar to other arguments given by people that just want to blame everything bad that happens in the world in the EU.

England, and the UK, are not currently being helped by the FMI or Troika. Because Portugal is, they receive a certain amount of funds that must be used for education purposes only.

It has been regulated by EU that such funds should be used in the development of the "Profesional courses" I spoke of earlyer. This being said, there is no investment in developing the regular courses that lead to University or in instructing the teachers that teach such courses.

Who's to blame? Well, Portugal obviously. But it is the EU that made the regulation about the money given to the portuguese education. I'm not against it by the way. I'm against it's uses.

blogfast25 - 22-6-2014 at 12:53

Hg:

The development of professional courses need not be a bad thing in and of itself. We have many 'diploma courses' here, designed for those who can't/don't want a Uni degree. A country full of degree people but with few people skilled in specific trades isn't a great thing either.

The devil is in the detail, of course...

HgDinis25 - 22-6-2014 at 13:55

Quote: Originally posted by blogfast25  
Hg:

The development of professional courses need not be a bad thing in and of itself. We have many 'diploma courses' here, designed for those who can't/don't want a Uni degree. A country full of degree people but with few people skilled in specific trades isn't a great thing either.

The devil is in the detail, of course...


Of course, there must be an equilibrium between both (this is an oversimplification of the work market). Anyway, thing is that if you only invest in one, the other will loose value in the international market and will get outdated shortly before losing credibility.

aga - 22-6-2014 at 14:27

From an Employer's perspective, what is required is people who can reliably Do stuff.

A degree is nice, but when you need someone to bulk manufacture substance X, you'd prefer it if they
a) knew how
b) had experience
c) knew what the Financial effects of screwing it up were.

Getting the Right People has always been a major problem for me.
Personally i'd take the self-taught Enthusiast over a graduate any day.

Huge companes with HR departments could never do that, as their decision cannot be quantified easily, and that is what the HR department needs to cover their asses.

In a Perfect world, a Degree would be what a Qualified Enthusiast got - a recognition of Excellence in a subject someone Wants to do, and was already doing it anyway, such was their passion for the subject.

Edit: a Degree in General Studies would not exist in my model, although Surfing would be ok.

[Edited on 22-6-2014 by aga]

blogfast25 - 23-6-2014 at 05:43

Quote: Originally posted by aga  
Personally i'd take the self-taught Enthusiast over a graduate any day.

Huge companes with HR departments could never do that, as their decision cannot be quantified easily, and that is what the HR department needs to cover their asses.



In most jobs that I've held personally, getting someone with the right education (over the autodidact) is the only thing an employer can do. This has little to do with '*rse covering': autodidacts mostly lack the math and other theory related skills to do these jobs. There are reasons why people spend up to 10 years (I include in that secondary education) getting their science/engineering degrees: science is HARD and hard to self-teach.

The only autodidacts that can rise through the ranks of a technical/science/engineering department are those who stand out, go on courses and embrace educational programs often offered by their companies.

Sorry aga, but for all your projected 'down to earth realism', yours is a cloud cuckoo land statement. Please don't take that personally.

[Edited on 23-6-2014 by blogfast25]

[Edited on 23-6-2014 by blogfast25]

aga - 23-6-2014 at 06:03

Differences of opinion IMHO.
I agree that a good grounding gained by formal education is very much an essential.
What i meant was that an Enthusiast who genuinely loved the subject, and was doing it anyway, Then goes and studies would be the winner for me over the Higher Grades Academic who chose the subject cos his dad said they had to.

I will take it personally, so it's a Duel at Dawn, 30 paces, portable Collider each.
Last Hadron Standing will decide the matter.

blogfast25 - 23-6-2014 at 12:23

Quote: Originally posted by aga  
Then goes and studies would be the winner for me over the Higher Grades Academic who chose the subject cos his dad said they had to.




I think when you caricature the argument it loses even more traction in my opinion: who on earth studies, say physics, 'cos his dad said they had to'??

When I did my engineering course there were a few 'fils a papa' that needed a piece of a paper so they could follow in Dad's footsteps as CEO/Chairman/Big Cheese of the Family Firm. Needless to say these people never had to seek employment though. They didn't make good students either...

aga - 23-6-2014 at 13:06

I have encountered a few who were learning X cos they had to (result: deficient)

Also met many who were Experts cos they finished the course (result: deficient)

Heart wasn't in it, which is basically the crux of my argument.

When it is formal study, backed by a pre-existing Passion, it is unbeatable (result!)

Now if I had studied better, i could maybe have summed that up at the start ...

Edit:
What time does the sun rise where you are ?
Got a plane ticket to book.

[Edited on 23-6-2014 by aga]

blogfast25 - 24-6-2014 at 04:18

Quote: Originally posted by aga  
What time does the sun rise where you are ?
Got a plane ticket to book.

[Edited on 23-6-2014 by aga]


We are so far North the sun practically doesn't set in summer! ;)

Sunrise in Bridlington today at 4.28 am.

[Edited on 24-6-2014 by blogfast25]

aga - 24-6-2014 at 11:16

Quote: Originally posted by blogfast25  
Sunrise in Bridlington today at 4.28 am.

That's a tad to early for me.
High Noon then ?
Is that free in your diary ?
Wait. I think i have a Space mission at Noon.
Yes. Best get my people to talk to your people ...