Equilibrium (2 Viewers)

[Aerlinn]

I've been reading about equilibrium, and I have some questions.
-If a reversible reacting system is NOT at equilibrium and must eventually reach this, are the irreversible (one way) reactions the ones (of all the reactions) that actually reach completion?

-Are open systems examples of these irreversible reactions that never reach equilibrium?

-When a system is at equilibrium, the temperature of the system is constant... these are my theories as to why: Any reaction= either exothermic or endothermic. If the forward reaction =exothermic, the back reaction= endothermic and vice versa. Exo releases heat, endo absorbs heat, and so in a system where the two reaction types are occuring at the same rate the temp thus remains constant. Correct me if I'm wrong.

-These are supposedly some things that tell you a system is at equilibrium:
---In gaseous systems the temp, vol and pressure= constant.
---There are no colour changes
I don't know why though... any insight would be good

-In exothermic reactions the energy is released as products are being formed. Is that because little energy is need to create new bonds to form the products and the rest, most of it, is released? ... or not?

-Why the lower the enthalpy the more energetically stable the species?
In endothermic reactions, reactants are supposedly more energetically stable than the products. Huh?

-Have a question which says 'the unit kJ/mol for delta H represents the energy released/absorbed in kJ per MOLAR AMOUNTS as specified in the balanced equation...
---What does it mean by 'molar amounts'?

-Why is it that, in exothermic reactions, lower temps are required to INCREASE yields...?

-Catalysts increase the reaction rate of both forward/reverse reactions to SAME extent... This is my interpretation of this. (ignore my bad drawing, just imagine that there's a same distance gap between the top of both curves compared to the curve with the catalyst) Main thing is: is it right/wrong?
http://www.freewebs.com/shrouds/endoexo.htm

 

[xiao1985]

a) theoretically, there are no irrevisible reactions... every reactions should be reversible to an extent... but practially speaking, alot reactions do reach completion and hence irreversible... so yes, your understanding is right in a practial sense...

b) i'm not sure what you are referring to in the question. it is possible for a reaction in an open system to not reach equilibrium, yes... but that goes for both reversible and irreversible...

c) you can look at it this way... adding in "in a closed system" (ie you cannot add/take away heat from the system

d) when there are no visible /measureable changes, a system would have reached equilibrium... and you can use the same arguement you used for c to argue volume, pressure and colour (concentration of reactants and products)

e) energy is needed to break bond... energy is produced when bonds are formed... which means exothermic reactions means the energy formed when bonds are formed is greater than the energy need to break the bond...

f) for first question, think this way: if you have a soccer ball on top of the hill (high GPE) it tend to roll down the hill (to have lower GPE) everything in nature tend to want to lower its energy

and you are right, enthalpically speaking, endothermic reaction has the reactants more stable than products... but later on you will learn entropy - a measure of randomness as well... the reaction's progress is really a mixture of both entropy and enthalpy

g) think exothermic as having heat as a product... so you will have:

A + B <---> C + heat

so if you decrease heat, you'd push equilibrium to the right due to le chatelier's principle... hence maximise C (product)

h) you are absolutely, 100%, totaly, right...

 

[Aerlinn]

a) I wonder why the theoretical is different from the practical?

b) K, so to clarify, it's possible for a reaction in an open system or irreversible/reversible reactions to reach equilibrium or reach completion... hmmm...

c) Not being able to take away heat makes sense. Is my theory also right? Or you
know, I could be completely off the track...

d) So, to follow on from what you said, ie. If there are no visible /measureable changes you can just say/ (or assume?) that volume, pressure, colour, concentration of reactants and products all are constant?

e)

energy is needed to break bond... energy is produced when bonds are formed... which means exothermic reactions means the energy formed when bonds are formed is greater than the energy need to break the bond...

K, so, it means simply with exo:

the energy formed when bonds are formed is greater than the energy need to break the bond

NOT

the energy is released as products are being formed

? To interpret that bigger jump in the graph. But the second quote comes from a handout... is it supposed to be wrong/right... Or, to make it better worded, should it say, there is an 'overall output of energy' rather than 'energy is released'... ?

f) I see. What does GPE stand for?
Is the fact that endothermic reaction have the reactants more stable than products due to the fact that, to take in the first point, reactants have lower enthalpy...?
Entropy sounds interesting

 

[xiao1985]

a) in theory, a ball should never stop bouncing (as it should always bounce back up, no matter how minute) but in practice, it does stop after a while... theory only works on papers, and practical works in real life... and in chemistry hsc, we usually talk about in practice

b) err... is that a question?!

c) your theory is right

d) volume, pressure, colour etc are properties that may be measured... so if there are no visible changes, naturally you can assume they are constant

e) energy are ALWAYS released when bonds are formed, doesn't matter if it's exo or endo reaction. just that for exo, energy required to break the bond is less than energy liberated from making a bond... and vice versa for endo (though i have to say, it's not entirely necessary to know this for hsc)

f) GPE = gravitational potential energy... correct for saying reactants have lower enthalpy

and yes, entropy is interesting... if you dont' have to be examined on that =(

 

[Aerlinn]

a) That makes sense ^^
b) Sort of. I had the impression somehow open systems are 'practically' (to quote) irreverible... are they...?
d) K. Where you said colour relates to concentration of reactants and products, I don't understand. huh?
e) Always worth knowing things... ^^ You say enery is liberating from making bonds. In products, for eg, wouldn't you need to incorporate energy into the bonds of those products when making the bond? Or maybe some is incorporated and some is released...

 

[xiao1985]

B) in certain circumstance, for eg, decarbonating a soft drink, in a open system, yes, it's practially irreversible

c) for eg, A + B <--> C , where A is colourless, B is pink and C is colour less... how pink the reaction chamber is will relate to the concentration of B... though that being said, i don't recall saying colour is related to concentration...

but yes, as longas the reactants and/or products carries a colour, then at equilibrium, you shouldn't see a colour change...