Electrochemistry

[plzNOCarribbean]

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Ok, so I have to admit I am pretty well seasoned i'm G.Chem and Ochem, but for some reason I never understood electrochemistry!! I hate it! even after just going over it and trying to learn everything, I feel like I don't get SH**!!.

I understand oxidation/reduction and what an oxidizing agent is. I have a basic sense of galvanic cells (spon) and electrolytic cells (non spon) But as I go through EK and TPR (haven't even tried TBR yet bc im sure it will kill me) I keep seeing questions about small detailed stuff. What are we supposed to know about electrochem??? Like, I'm expected to know what causes a reaction to explode?? and that Na metal will explode when reacted w X,Y,Z but not with an inert gas?? and that alkali metals are reducing agents because?? WTFF...please someone help because I am supposed to be moving on to other topics and this has REALLLY slowed down my progress. Thanks guys!



I keep hearing its an MCAT favorite so I wanna make sure i cover it adequately.

 

[loveoforganic]

This is too big a topic to just explain everything. Basic rundown:

Different atoms have different affinities for electrons, due to things like the number of protons in their nucleus, the distance of the valence electrons from the nucleus, etc. Generally, you have low affinity to the left of the periodic table and high affinity to the right. Those with low affinity will tend to give their electrons away to other atoms (reducing agents) and those with high affinity will tend to accept electrons from other atoms (oxidizing agents).

The relative tendency to be reduced has been standardized in something known as redox potential. Protons accepting electrons to become hydrogen gas has arbitrarily been set as 0. Those atoms that accept electrons better than hydrogen will have a positive redox potential. Those atoms that accept electrons worse than hydrogen will have a negative redox potential.

To make batteries, you couple the oxidation of one species with the reduction of another, typically holding each species in separate containers connected through a wire. This is so that the species are forced to react (pass electrons) through that wire, making the energy easy to direct to your own purposes.

The exploding reactions you're talking about are just violent redox reactions.

 

[plzNOCarribbean]

Ok thanks! That helped. Now if i know that stuff would you say I am ok for the MCAT. Or should I also know the specifics of like concentration cells and know the Nerst equation?? is that also necessary??

Also, for the equation

Delta G not=-RT ln(K)

if K>1 at equilibrium, it is saying that the products are favored at equilibrium, the reaction is spontaneous??
and if K=1 , or at equilibrium, there is no free energy to do work??

is this right?

 

[loveoforganic]

1) No, what I typed in two paragraphs isn't sufficient 😛

2) The MCAT prep books have types of cells and the nernst equation and electrolytic cells and whatnot, so I'd say yes.

3) Sounds good to me. The only thing I'd be careful about is the use of the word spontaneous. In electrochem, there isn't generally a high activation energy to overcome, so favorable reactions are in fact spontaneous. However, if a reaction has a high activation energy, it may not happen spontaneously (under normal conditions), due to not being able to overcome that energy barrier. An example woudl be the reaction of oxygen and hydrogen to form water.

 

[boaz]

1) No, what I typed in two paragraphs isn't sufficient 😛

2) The MCAT prep books have types of cells and the nernst equation and electrolytic cells and whatnot, so I'd say yes.

3) Sounds good to me. The only thing I'd be careful about is the use of the word spontaneous. In electrochem, there isn't generally a high activation energy to overcome, so favorable reactions are in fact spontaneous. However, if a reaction has a high activation energy, it may not happen spontaneously (under normal conditions), due to not being able to overcome that energy barrier. An example woudl be the reaction of oxygen and hydrogen to form water.

The word "spontaneous" in gen chem refers to thermodynamic spontaneity, not kinetic spontaneity. Thermodynamic spontaneity is determined by equilibria. Kinetic spontaneity is determined by activation energy. Therefore, "spontaneity" is unaffected by activation energy.

 

[loveoforganic]

Gotcha, thanks for the correction 🙂