Electrode Potential Comedy

Electrode 1: Oh no! I think those electrons are coming my way.

Electrode 2: Are you sure?

Electrode 1: Yes, I'm positive!

Explanation (if you need one)

Which way do electrons flow in a cell?

Towards the positive electrode. 

Slowly, Slowly, Catchee Monkey (or Rate Determining Step)

So, you have written your rate equation and the next question asks you to suggest a rate determining step.

The key phrase here is suggest, when ever you see this the examiner is telling you that there are lots of possible answers, you just have to come up with one of them. There is an implication in the word "suggest" that there is some educated guess work involved.

So what do you suggest?

Let's look at an example...

Here is an overall reaction



Here is the rate equation



Question - Suggest a rate determining step.

In this relatively simple example, the rate equation  tells me that the OH- isn't involved in the slow step. It is only the (CH3)3CBr. The implication is that the OH- comes in to play in some unknown later fast step, I guess theoretically it could be involved in an earlier fast step but realistcally what can OH- do on its own in a first step?

So, all we know is that the rate determining step has (CH3)3CBr as a reactant and nothing else, giving us...

(CH3)3CBr -->

What about the products?

Well you would get away with "intermediates", i.e.

(CH3)3CBr --> intermediates

 but in this question you can do better than that

because if all you have is one molecule of (CH3)3Br on its own it must fall apart and logically what must fall off is the Br-. As it is a product in the full equation. This gives us

(CH3)3CBr --> (CH3)3C+  and  Br-                  as the slow (rate determining) step

and then theoretically

(CH3)3C+ and OH- -->   (CH3)3COH              as the the fast step.

Will that get you the marks in the exams? Yep
Is that what really happens? Doesn't matter

Which way do you flow?

OK let's talk cells.

EMF to be specific. How do you know which way things are going to go?

It all comes down to EMF. The EMF, just to remind you, is that thing you work out where you draw the cell put in the the Eo values, go against one and with the other and eventually you get a voltage. Remember? If not, go revise.

EMF = positive --> electrons flow left to right --> right electrode is positive (because electrons flow towards the positive)

EMF = negative --> electrons flow right to left  --> left electrode is positive (because electrons flow towards the positive)

Mine's a double

How do you know which values to double when doing Born-Haber Cycles?

Simple really.

In a compound like CaCl2, you always have to deal with 2 chlorines so any value which refers to chlorine will need to be doubled to make it refer to 2 chlorines.

In this case that will be the enthalpy of  atomisation

1/2 Cl2 (g) --> Cl (g) needs to be doubled

and

enthalpy of  electron affinity

Cl + e-  --> Cl- needs to be doubled.

The possibility is in something like Na2O, now you are dealing with 2 sodiums so you would need to double the sodium values, i.e.

enthalpy of  atomisation

Na (s) --> Na(g) needs to be doubled

and

enthalpy of  ionisation (energy)

Na   --> Na+ + e- needs to be doubled

It is worth being careful though as every now and again they pre-double some things for you in the table, so have a read and double only if it refers to one Na (or whatever) and you need it to be 2.

It's at time like this that I wish blogspot did superscript and subscript.

What colour is a giraffe's tongue and spit?

Blue?

Black?

Blue/Black?

Depends on who you ask. The same is true of the iodine-starch complex. Depending on who you ask, will depend on what colour they say it is.

So if in an exam question they ask what colour is  Iodine Starch complex. Go Blue, black, blue/black. It's all the same to the average giraffe (or examiner)

What colour is Chromium 3+?

Have you ever wondered what colour Chromium 3+ is?
If you haven't then fine, move on.
If not take a look at this

This is Chromium 3+

That look about right doesn't it.
Green.
The colour of the positive test for alcohols, orange to green
All nice and straight forward.

but look at this...

This is Chromium 3+

Confused.

Let me explain.

Chromium 3+ with only water ligands is in fact this violet colour seen above but if there are any other possible ligands present then it will turn green.

So, what about in the dichromate to chromium 3+ test for alcohols? In that case, there is the presence of Cl- or SO4 2-  ions (from sulphuric acid or hydrochloric acid used to acidify the solution) which replace the water and make it look green.

So, what colour is chromium 3+? purple, if it is on its own, or green if some nice looking anions swing by.