Stankovic123's chem q's

[brightsky]

∆H = ∆U + ∆(PV)

∆(PV) can most definitely be negative. think of what happens when you decrease the pressure while holding the volume constant, or decrease the volume while holding the pressure constant.

a thought just occurred to me. what you are describing is the change in enthalpy. of course, delta H can be negative or positive, since enthalpy values can go up and down. but can enthalpy values be negative? the same applies to pressure and volume. whilst the change in pressure and the change in volume can be negative, it seems that pressure and volume values cannot.

[scribble]

well since you guys are finding Kc using concentrations and  concentrations have units, it does have a unit.
K is [A][ B]/[C] for the reaction C<=>A+B at equlibrium
and [A], [B ] and [C] is given in M (mol/L)
so the unit of K is M*M/M or M

if youre finding K for the reaction C<=>2A + 3B
K = [A]^2[ B]^3/[C]
so the units are M^2*M^3/M or M^4

and it's good to put the unit there because although A+B<=>C is the same reaction as 2A + 2B <=> 2C, the K values will be different and will have different units.


In uni, you learn more about K. but as far as VCE and you guys are concerned, K has a unit in terms of mol/L because thats what you're using to find it. and it doesn't make sense for the unit to disappear unless you have the same number of particles on both sides of the reaction and they cancel to give no unit.



edit: @ brightsky, yes enthalpy can be negative. that is what an exothermic reaction is. no pressure/volume cannot be negative. (unless you do accelerated math in which case volumes can be negative if you want them to be lolwhatislifemylecturerisahugetroll)

[lzxnl]

I'll just firstly say that for the Gibbs free energy equation, delta G = -RT ln K
Therefore K does not have a unit. It is actually defined in terms of chemical activities which are approximated numerically by concentrations in mol/L or by partial pressures in bar.

I'm pretty sure you can't have zero internal energy; even by cooling things down to absolute zero, quantum effects prevent absolute cessation of motion. Therefore zero enthalpy can't exist.

[brightsky]

hmm..i'm probably being a little anal here, but change in enthalpy does not equal enthalpy. enthalpy is a measure of the 'chemical energy' of a system. change in enthalpy is, as the name suggests, the change in chemical energy of a system. for an exothermic reaction, the system begins at a higher enthalpy than it ends up, so the enthalpy change is negative. for instance, the enthalpy of a system may begin at 10, when only reactants are present, and drop down to 5, when the products are formed. (values are arbitrary here.) the initial enthalpy is 10; the final enthalpy is 5. the change in enthalpy is -5.

[scribble]

^mhm you're right, sorry I read what you were saying wrong. we use ΔH so much, i kind of always imagine we're talking about it and not just enthalpy. XD so no, a system can't have a negative entropy

[lzxnl]

I just read your post about negative volumes. Lol. Them integrals. Messing with our understanding of measure since forever.

[scribble]

ahaha. we were finding the area of a 3D parallelogram/parallelopiped and it came out to be negative and the lecturers like, so the volume will be positive because you don't want a negative volume. then he pauses and goes ...but in another world that is much more natural than this one, negative volumes do exist!! but let's not get into that, its beyond the scope of this course.
>this man
>wish i knew what was going on in his head
>swear he says these things just because they sound cool

[brightsky]

ahaha. we were finding the area of a 3D parallelogram/parallelopiped and it came out to be negative and the lecturers like, so the volume will be positive because you don't want a negative volume. then he pauses and goes ...but in another world that is much more natural than this one, negative volumes do exist!! but let's not get into that, its beyond the scope of this course.
>this man
>wish i knew what was going on in his head
>swear he says these things just because they sound cool

oh yes but remember volume of parallelopiped does not EQUAL the triple scalar product; it is the absolute value of it. actually coming to think of it, how do you even define volume? i remember my maths teacher once posing us the challenge of defining area. at the end of the lesson, he told us that the definition of area takes hundreds of pages to write down, and that mathematicians are still debating over the definition. trololol

[lzxnl]

We're debating maths in a chemistry topic?

Screw definitions of areas. Leave that to the pure mathematicians; I'm fine with using my depravedly unrigorous notion of area, volume and length. Saying that area is an integral works for me.

[scribble]

them fucking mathematicians

by the way, am i right in thinking that production of a chemical of choice is no longer examinable? but you still have to learn it because theres a sac or something like that?

[Mao]

I just thought that potential energy would have an effect on the collision. Because when an energy profile is determined, that takes into account chemical energy, as far as I know? 

"Only successful collisions, where the energy of the collision is greater than the activation energy, allow a chemical reaction to progress"
That was pg 251 of towards the bottom.

Btw appreciate the help

It's definitely activation energy, not chemical energy.

Consider an exothermic reaction. No matter how large the subsequent deltaH is, its rate is determined by Ea. Changing deltaH does not affect Ea here.

Also consider an endothermic reaction, in this case Ea includes deltaH, but the overall energy barrier is still Ea.

The activation energy is the relative energy of the activated complex/transition state/intermediate state of a reaction pathway. i.e. if a reaction is to take place, it must form this "intermediate" that have half-broken bonds and so forth. Note that it is a relative energy with respect to the energy of the reactants, so you don't need to consider the total chemical potential of the original reactants.

It is not practical to talk about internal energy vs enthalpy here.

[Mao]

I'm pretty sure you can't have zero internal energy; even by cooling things down to absolute zero, quantum effects prevent absolute cessation of motion. Therefore zero enthalpy can't exist.

hmm..i'm probably being a little anal here, but change in enthalpy does not equal enthalpy. enthalpy is a measure of the 'chemical energy' of a system. change in enthalpy is, as the name suggests, the change in chemical energy of a system. for an exothermic reaction, the system begins at a higher enthalpy than it ends up, so the enthalpy change is negative. for instance, the enthalpy of a system may begin at 10, when only reactants are present, and drop down to 5, when the products are formed. (values are arbitrary here.) the initial enthalpy is 10; the final enthalpy is 5. the change in enthalpy is -5.

A common issue with energy and enthalpy is that it is often interpreted like "temperature", that they have a minimum/absolute zero. The point is, energy and enthalpy are all defined relative to some reference state. They generally don't have a "zero" point.

[zvezda]

It's definitely activation energy, not chemical energy.

Consider an exothermic reaction. No matter how large the subsequent deltaH is, its rate is determined by Ea. Changing deltaH does not affect Ea here.

Also consider an endothermic reaction, in this case Ea includes deltaH, but the overall energy barrier is still Ea.

The activation energy is the relative energy of the activated complex/transition state/intermediate state of a reaction pathway. i.e. if a reaction is to take place, it must form this "intermediate" that have half-broken bonds and so forth. Note that it is a relative energy with respect to the energy of the reactants, so you don't need to consider the total chemical potential of the original reactants.

It is not practical to talk about internal energy vs enthalpy here.

So are you saying that because the potential energy is relatively constant for the reactants, its the change in kinetic energy which is the deciding factor?

Wouldnt heinemann need to mention chemical energy when mentioning the amount needed to reach activation though?

[zvezda]

I was thinking about the effect of catalysts on equlibrium position, and wouldnt adding a catalyst result in a momentary increase in the rate of the forward reaction compared to the backwards reaction?
Heinemann says that "it has been shown experimentally that they increase the rate of forward and back reactions equally".

[psyxwar]

I was thinking about the effect of catalysts on equlibrium position, and wouldnt adding a catalyst result in a momentary increase in the rate of the forward reaction compared to the backwards reaction?
Heinemann says that "it has been shown experimentally that they increase the rate of forward and back reactions equally".

They do, it's just that since the equilibrium position is already on the right everything is happening faster, so your getting more product at a higher rate. The back reaction is also happening faster, but the overall reaction is again forward. The only thing a catalyst does is increase the rate at which the whole process happens.