Oxidation-Reduction Reaction

Oxidation-Reduction Reaction

Oxidation–reduction (redox) reaction – involves a transfer of electrons from the reducing agent to the

oxidizing agent

Electrochemical processes are oxidation-reduction reactions in which: the energy released by a

spontaneous reaction (galvanic cell) is converted

to electricity or electrical energy is used to cause

a nonspontaneous reaction (electrolysis) to occur

 Oxidation–reduction (redox) reaction – involves a transfer of electrons from the reducing agent to the oxidizing agent

 Oxidation – loss of electrons

 Reduction – gain of electrons

Review of Terms

Electrodes are classified according to whether oxidation or reduction takes place there.

If oxidation takes place, the electrode is called the anode.

If reduction takes place, the electrode is called the cathode.

 Oxidizing Agent- a substance that accepts electrons from another substance, causing the other substance to be oxidized.

 Reducing Agent- a substance that donates electrons to another substance, causing it to become reduced.

Zn (s) + CuSO

(aq) ZnSO

(aq) + Cu (s)

Zn Zn²⁺ + 2e^- Zn is oxidized

Zn is the reducing agent Cu²⁺ + 2e^- Cu Cu²⁺ is reduced

Cu²⁺ is the oxidizing agent

In Galvanic Cell:

Oxidation occurs at the anode.

Reduction occurs at the cathode.

Salt bridge or porous disk allows ions to flow without extensive mixing of the solutions.

Salt bridge – contains a strong electrolyte held in a gel–like matrix.

Porous disk – contains tiny passages that allow

hindered flow of ions.

Cell Potential

• An electrochemical cell consists of two half-cells with electrodes joined by a wire and solutions joined by a salt bridge.

• Electrons flows through a wire from the anode half- cell to the cathode half-cell.

• The driving force that allows electrons to flow is

called the electromotive force (emf) or the cell

potential (E

).

Standard Electrode Potentials

Potentials of individual electrodes, however, cannot be precisely established. If we could make such measurements, cell voltages could be obtained just by subtracting one electrode potential from another.

The same result can be achieved by arbitrarily choosing a particular half-cell that is assigned an electrode potential of zero. Other half cells can then be compared with this reference.

The commonly accepted reference is the standard

hydrogen electrode.

By international agreement, a standard electrode potential, E°, measures the tendency for a reduction process to occur at an electrode.

In all cases, the ionic species are present in

 aqueous solution at unit activity (approximately 1M), and

gases are at 1 bar pressure (approximately 1 atm).

Where no metallic substance is indicated, the

potential is established on an inert metallic

electrode, such as platinum.

A standard cell potential, is the potential difference, or voltage, of a cell formed from two standard electrodes.

The difference is always taken in the following way:

The Effect of Concentration on Cell Emf

Not all reactions in galvanic cells can occur under standard state conditions.

•Nernst Equation- equation used to

calculate cell potential under nonstandard state conditions.

- 0.0592 V

n log Q

E⁰ E =