Question

Which of these redox reactions is spontaneous as written?

Answer 1

There are a number of ways.

If it's a single displacement reaction, like when a metal displaces another metal from solution, you can use an activity series. If the displacing metal is more active than the metal being displaced, the reaction is spontaneous.

For example, the reaction Mg(s) + CuCl2(aq) --> MgCl2(aq) + Cu(s) is spontaneous because magnesium is a more active metal than copper. The reverse reaction: Cu(s) + MgCl2(aq) --> CuCl2(aq) + Mg(s) would NOT be spontaneous.

You can find a good activity series here:
http://www.chem.vt.edu/RVGS/ACT/notes/ac...

If you break the redox reaction down into oxidation and reduction half-reactions, you can use a standard reduction potential table to determine the voltage of the reaction as written. If the voltage is positive, then the reaction is spontaneous (at least at 25ºC).

Let's re-consider the reaction: Mg(s) + CuCl2(aq) --> MgCl2(aq) + Cu(s)

First of all, we'll strip it down to a net ionic equation:
Mg + Cu(2+) --> Mg(2+) + Cu

Now we'll define half-reactions:
Oxidation: Mg --> Mg(2+) + 2e(-)
Reduction: Cu(2+) + 2e(-) --> Cu

Now we need a standard reduction potential table in order to calculte the potentials of the half-reactions (remember to reverse the sign for the oxidation half-reaction, since the table only gives values for reductions). You can find a good standard reduction potential table here:

http://members.aol.com/logan20/red_tabl....

According to the table, we would have the following potentials:

Oxidation: Mg --> Mg(2+) + 2e(-).......Eox = 2.38 V
Reduction: Cu(2+) + 2e(-) --> Cu.......Ered = 0.34 V

The total reaction potential is 2.38 V + 0.34 V = 2.72 V. Since the potential is positive, the reaction is spontaneous as written. This corresponds nicely to the fact that we already know it should be a spontaneous reaction thanks to the activity series.

Again, if you reverse the reaction it becomes non-spontaneous. The cell potential then would be -2.72 V. Copper does not displace magnesium.

One final way to determine the spontaneity of a reaction is to use thermodynamic data. A reaction is spontaneous if it has a negative free energy change (dG). To calculate dG, you simply need to find a thermodynamics table that has dG values for all of your products and reactants. Then:

dG = (sum of dG values for products) - (sum of dG values for reactants)

Here's an example of a thermodynamics data table:

http://www.chm.davidson.edu/ronutt/che11...

It's not very complete, but you can usually find a much better one in the back of a chemistry textbook. If you calculate the free energy change for the reaction Mg + CuCl2 --> MgCl2 + Cu, you should get a negative value. (Remember, spontaneous redox reactions have positive electric potentials and negative free energy changes) If you calculate dG for the reverse reaction, it will be positive.

Hope this helps!

EDIT: I neglected to mention that all of these methods of determining reaction spontaneity assume that the reaction is occuring under standard conditions.

EDIT 2: Re: the previous answer. While temperature, pressure, concentration, pH, and so on CAN influence reaction spontaneity, catalysis does not. The presence of a catalyst only affects the kinetics of a reaction, not its spontaneity. Those concepts are quite distinct.

Answer 2

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