Stankovic123's chem q's

[saba.ay]

Another question,
Why do we have both UV/vis and AAS. Aren't they effectively serving the same purpose in the same way? They both use energy from basically the same area of the electromagnetic spectrum, just that UV uses UV waves as well. So I guess, aside from the substances that don't absorb visible light waves (thus calling for the use of UV instead of AAS), why do we have these 2?
I know that AAS can analyze very small amounts of substance, if that contributes at all to this.
Help is much appreciated

I don't know if this will be a sufficient answer, but isn't AAS restricted to just metal ions? UV-Vis can be used for all coloured compounds/solutions and can also be used for colourless organic compounds which absorb UV light.

[zvezda]

I don't know if this will be a sufficient answer, but isn't AAS restricted to just metal ions? UV-Vis can be used for all coloured compounds/solutions and can also be used for colourless organic compounds which absorb UV light.

I suppose that's another thing that gets me. Why do we say that uv is used for colored compounds when we're not observing colour, but rather, uv/vis absorption?

[zvezda]

Hey,
How many hydrogen environments would a double bonded carbon on the end of a molecule have? I think it's 1, but I came across a structure elucidation q which considers it as having 2?

[zvezda]

Hey,
When heinemann gives the maxwell-boltzmann distribution for explaining that for an increase in the rate of reaction (due to increase in temperature) the kinetic energy should be greater than Ea, shouldnt they be referring to chemical energy?
Also, why must the energy be greater than the Ea and why not equal to it?
Help is much appreciated

[psyxwar]

Why would it be chemical energy? Collision theory states that in order for a reaction to occur, the particles need to collide with sufficient energy (among other requirements). This is kinetic energy of the particle, and not the chemical energy holding it together.

I'm pretty sure that if energy = Ea then the reaction can proceed (assuming orientation is right and all that). Where did it say Ea must be exceeded?

[zvezda]

Why would it be chemical energy? Collision theory states that in order for a reaction to occur, the particles need to collide with sufficient energy (among other requirements). This is kinetic energy of the particle, and not the chemical energy holding it together.

I'm pretty sure that if energy = Ea then the reaction can proceed (assuming orientation is right and all that). Where did it say Ea must be exceeded?

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

[brightsky]

yes i think it should be chemical energy (or enthalpy to be more precise). enthalpy takes into account kinetic energy, potential energy, and energy required to make room for the system. so, in order to 'activate' a reaction, you need to input enough energy into the system from the surroundings so that the ENTHALPY of the system is above or equal to the activation energy for the reaction. i think the diagram in the textbook, depicting the probability density function of the maxwell-boltzmann distribution is a bit misleading, although it is certainly true that if kinetic energy = activation energy, then the reaction will take place (the enthalpy will then be greater than or equal to the activation energy).

and to answer your second question explicitly: if the enthalpy of a system equals to the activation energy of the reaction that is to occur, then the reaction will take place. the enthalpy does not need to be greater than the activation energy. this is kind of implied in the definition of activation energy: "the MINIMUM energy required to break bonds in reactants so as to allow reaction to commence". if you hit the minimum, then the reaction will occur.

[zvezda]

When you say that when kinetic energy = activation energy, the enthalpy will be greater than or equal to activation energy, are you implying that its possiible to have 0 enthalpy?

[brightsky]

When you say that when kinetic energy = activation energy, the enthalpy will be greater than or equal to activation energy, are you implying that its possiible to have 0 enthalpy?

as i have said, enthalpy takes into account kinetic energy, potential energy and the energy required to make room for the system. the actual formula for enthalpy is H = U + pV, where U is the internal energy of the system (sum of kinetic energy and potential energy), that is the energy required to CREATE the system, and pV is the energy required to make room for the system by displacing the surroundings. you can see that if the kinetic energy equals to the activation energy, then H must be equal to or greater than the activation energy, since neither potential energy nor pV can be negative (or so i think...although i wouldn't be surprised if negative energy were invented, given that negative mass and so on have been conceived).

[scribble]

∆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.

[zvezda]

as i have said, enthalpy takes into account kinetic energy, potential energy and the energy required to make room for the system. the actual formula for enthalpy is H = U + pV, where U is the internal energy of the system (sum of kinetic energy and potential energy), that is the energy required to CREATE the system, and pV is the energy required to make room for the system by displacing the surroundings. you can see that if the kinetic energy equals to the activation energy, then H must be equal to or greater than the activation energy, since neither potential energy nor pV can be negative (or so i think...although i wouldn't be surprised if negative energy were invented, given that negative mass and so on have been conceived).

Wow, sorry. What i was meant to ask was whether you implied that potential energy and pv would equal zero, but given that its possible to have them as negative.

Cheers brighstky and scribble

[brightsky]

Wow, sorry. What i was meant to ask was whether you implied that potential energy and pv would equal zero, but given that its possible to have them as negative.

Cheers brighstky and scribble

oohh i see what you mean now...and scribble, so negative energy is definitely possible? i'm not too sure why pV even corresponds to an energy value...can you provide us with any insight?

[zvezda]

Yeah apologies haha.

Also, when we're calculating the K constant, what does the M signify, whether itd be squared or powered by -2 etc.?
There's a little bit in heinemann that gives something really brief (the chemfact on pg 267), but it doesnt really tell me much.

[brightsky]

technically Kc does not have any units. but vcaa randomly decided that Kc should have units, so we play their game. the units are random and reaction-specific.

[zvezda]

technically Kc does not have any units. but vcaa randomly decided that Kc should have units, so we play their game. the units are random and reaction-specific.

Thats ridiculous.
Thanks anyway lol