A2 Chemistry

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Just look at this flower...it mimics a bee beautifully!

Isn't biology wonderful?

electrochem interactive

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more about redox...

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Quantitative redox chemistry

The understanding and application of electrode potentials allows us to do one of two things: Build cells or predict the likelihood of a redox equation.

In building cells two couples are required with different values of redox potential. One couple will become the negative pole of the cell, and the other the positive. When we start to put values on the redox potentials it will not come as a shock to learn that the more negative couple will form the negative pole of the cell.
The resulting reaction may then be used to construct a current-delivery device called a CELL although the result may not be of much practical use because the cell reaction rate is too slow. It can't keep up with the demand for current, in other words. And there in lies the drawback of these data. They cannot be used to predict the RATE of the reaction. All they will do for you is to tell you whether it is spontaneous or not.
Spontaneous in chemistry does not always mean fast... all it means is that when mixed together a certain collection of reagents with speciifed concentrations are not at equlibrium, and will move forwards until that condition is reacahed. The reaction will then stop..

Absolute electrode potentials

Although it is not difficult to measure relative half-cell potentials, it is impossible to determine absolute half-cell potentials because all voltage measuring devices measure only differences in potential. To measure the potential of an electrode, one contact of a voltmeter is connected to the electrode in question. The other contact from the meter must then be brought into electrical contact with the solution in the electrode compartmenet via another conductor. This second contact, however, inevitably involves a solid/solution interface that acts a a second half-cell at which chemical change must take place if charge is to flow and a potential difference is to be measured. A potential is associated with this second reaction. Thus an absolute half-cell potential is not obtained but rather the difference between the half-cell potential of interest and a half-cell made up of the second contact with the solution.
Our inability to measure absolute half-cell potentials presents no real obstacle because relative half-cell potentials are just as useful provided they are all measured against the same half-cell. Relative half-cell potentials can be combined to generate cell potentials. The standard reference half-cell is the standard hydrogen electrode.

Building cells....

Using redox chemistry as a source of mobile electrical energy is inspired! I mean..without this you'd NEVER be able to say "I'm on the train...!".

Two half equations with different E^o values is all that it takes. Thes can be [++] or [+-] or [--].Whether this cell turns out to be "practical" or not depends on many things...not least the states of the reagents used!

Inspect the two E^o vlaues: one will be more negative, even if they are both positive.

The couple which is more negative becomes the ....negative [!] pole of the cell. Well it would do , wouldn't it.

The process which makes this electrode negative is the one in which [negative] electrons are lost...oxidation.

The other couple must therefore be the POSITIVE pole, where Reduction happens.

The REDUCTION couple is placed on the RIGHT and the Oxidation [LOSS] of electrons on the LEFT

Ecell = E red - E ox

Please remember the SIGN and UNITS [V] for your answer.

The cell emf will be positive. This means that the cell reaction is spontaneous in the forward direction, and when the circuit is made current will flow until equilibrium is reached, at which point the cell emf becomes zero.

There is no connection between the magnitude of Ecell and the RATE of the reaction...so even a cell with a very hign voltage may not be practical in allowing current to be drawn from it.

Predicting Redox Reactions

This is a further use to which a table of redox potentials may be put. To do this you need to adopt a specific protocol, which once mastered is easy to follow.

Start by reading the question carefully and combine the two couples in the way instructed by the question. DO NOT PAY ATTENTION TO THE E VALUES AT THIS STAGE! Remember, as usual, that you must ensure the same number of electrons in each half equation before they can be added. Once the redox equation is constructed, look at the E values and calculate the EMF for the reaction you have written. If it is positive, the cell reaction is spontaneous IN THE FORWARD sense, but of course you do NOT know the rate. The reaction may just be too slow to be useful as a means of extracting a current. On the other hand if it turns out to be negative, the reaction will not proceed forward at all.

Remember that the values are determined at some very precise conditions of T and P and concentration. Thus, the prediction is also only valid under these conditions. This means that a slow reaction, or one for which E<0 can be encouraged to move forward by changing the conditions; heating it up or using concentrated solutions. You are a resourceful chemist...so don't take NO for an answer!

Building cells...

Two couples with different electrode potentials may be combined to build a cell, a device to physically separate the oxidation from the reduction so that when the electrons move from one process to the other they pass through the external circuitry to do work. In this way, electrical energy may be extracted from a redox reaction. WOW!
The couple which is most negative will be the negative pole of the cell...and will be driven into oxidation. The other will, therefore, be reduction and the positive pole. Properly combined, remembering to balance the electron count in each half equation, the redox equation represents the cell reaction too.
With the emf being positive [ E cell = E rhe - E lhe] means that the reaction, when first built, is NOT at equilibrium. What is holding it up is the fact that no current is flowing. Once the circuit is completed, the cell reaction gets under way, and moves towards equilbrium. Once the system reaches equilibrium, the cell voltage falls to ZERO. The cell reaction is over, and a new cell must be built.

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..... as AS/A2 students would you find revision notes set out like this of any use? Please be honest; don't spare my blushes..

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Acid sand base s

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gc slides...

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tlc slides...

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nmr slides

probably more background than is needed, but it is helpful to have some understanding of the theory...

infra red slides

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Mass spectrometry

slide sshow for your review...
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