Voltage and Spontaneity

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Voltage and Spontaneity

A redox reaction will occur spontaneously if its potential has a positive value. We also know from thermodynatmics that a reaction that occurs spontaneously has a negative value for free energy change. The relationship between reaction potential and free energy for a redox reaction is given by the equation below, which serves as a bridge between thermodynamics and electrochemistry.

Voltage and equilibrium

The standard reaction potential is related to the equilibrium constant by the following expression

(standard condition)

Galvanic Cells

In a galvanic cell (also called a voltaic cell), a spontaneous redox reaction is used to generate a flow of current.


In galvanic cell, the two half reaction take place in separate chamber, and the electrons that are released by the oxidation reaction pass through a wire to the chamber where they are consumed in the reduction reaction. That's how the current is created. Current is defined as the flow of positive charge, so current is always in the opposite direction from the flow of electrons.


In any electric cell, oxidation takes place at hte electrode called the anode. Reduction takes place at the electrode called the cathode.


The salt bridge maintains electrical neutrality in the system by providing enough negative ions to equal the positive ions being created at hte anode and providing positive ions to replace the coppor ions being used up at the cathode. The slat bridge can be an actual slat or it can be a slim passage that allows ions to move between the two chambers.

Electrolytic cells

IN an electrolytic cells, an outside source of voltage is used to force a non-spontaneous redox reaction to take place.  That is, E <0