How can one tell if a redox reaction will be nonspontaneous?

Answer 1

The reduction potential plus oxidation potential is negative.

Answer 2

The spontaneity of a redox reaction is a function of the reduction potentials of the two species involved in the reaction: the reducing agent and the oxidizing agent.To determine if a reaction is spontaneous or not, you must first isolate the two half-reactions. To do this, see the Related Questions link to the left "How do you balance a redox reaction?"

Once you have the two half-reactions, you must look at a table of standard potentials to find the values for the half-reactions occurring in the overall reaction in question. For a table of standard reduction potentials, see the Web Links to the left of this answer.

Standard potentials are almost always listed as reductions -- in other words the energy either require or released when an element gains an electron. However, a redox reaction always involves one species (an atom or molecule) gaining one or more electrons and another species losing one or more electrons. To convert a reduction potential into the opposite reaction, an oxidation reaction, just flip the direction of the arrow in the reaction and reverse the sign of the potential.

Now that you have the standard potentials of the two two half-reactions, correctly written as one reduction and one oxidation with the correct signs, then you have to simply add up the two half-cell potentials. If the sum of the potentials is positive, then the reaction is spontaneous. If the sum is negative, than the reverse reaction is spontaneous. The larger the magnitude of the sum, the more the reaction is spontaneous (or that the reverse reaction is spontaneous is negative). If you can't remember whether positive is spontaneous or not, look at the table of potentials -- remember that elements like potassium are very easily oxidized (strong reducing agent) and react violently to give up an electron, whereas an element like fluorine is a very strong oxidizing agent (and is easily reduced).

This calculation however only applies to standard conditions (pure substances and 1 M solutions), and when away from these conditions (a more dilute solution for instance), then the Nernst equation must be used to correct for this difference. The Nernst equation is:

E = E0 - RT/nF ln(Q)

where E0 is the standard potential of the reaction, R is the Universal Gas constant (8.314510 J K-1 mol-1), T is the temperature, n is the number of electrons being transferred in the reaction, and F is Faraday's constant (96,484.56 C/mole), and Q is the reaction quotient (concentration of products over reactants raised to the power of their respective coefficients in the balanced reaction).

For more about how to use the Nernst equation, see the other links under the Web Links to the left of this answer.

Answer 3

The reduction potential plus oxidation potential is negative