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REDOX reactions

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REDOX reactions involve the transfer of electrons between two chemical substances: one willing to accept them (oxidant) and the other willing to donate them (reductor). The oxidant will get reduced while the reductor gets oxidised.

What are REDOX reactions?

Many reactions occurring naturally are REDOX reactions, for example combustions and enzymatic reactions. Many reactions of interest in ForensicSciences are also REDOX reactions (explosions, drug metabolism, fire, ... )In REDOX reactions the oxidation number of the oxidant is reduced while the oxidation number of the reductor is increased

reductor

oxidant +e

-e

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Acidic media

Balancing redox equations

+2 +5 +7 +3

Basic media

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Cu(s) + HNO3(aq) → Cu(NO3)2(aq) + NO(g) + H2O(l)

3Cu(s) + 8HNO3(aq) → 3Cu(NO3)2(aq) + 2NO(g) + 4H2O(l)

Oxidation: Cu (0) to Cu (+2)Reduction: N (+5) to N (+2)

Examples

NO2(g) + H2(g)  →  NH3(g) + H2O(l)

Oxidation: H (0) to H (+1)Reduction: N (+4) to N (+3)

2NO2(g) + 7H2(g)  -->  2NH3(g) + 4H2(l)

Standard conditions (represented for the A symbol) are pressure of 1 bar (≅1 atm), temperature of 298° K (or 25° C, or room temperature) and concentration of 1.0 M

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Nernst equationThe transfer of electrons occurs at a specific point of energy, known as redox potential (E˚), in which electrons are transferred. This point is characteristic for each metal or substance and for the number of electrons exchanged. This potential is measured in volts (V).

Fe2+ (aq) + 2 e- ------> Fe (s), E° = -0.44 VCd2+ (aq) +2 e- ------> Cd (s), E° = -0.40 V

Nernst equation is the equation we use in cases in which we do not have standard conditions. E˚ values can be found in tables

E= E˚ - (RT/nF) ln Q

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Free energy and the standard potential can also be related through the following equation:

We can therefore predict whether an electrochemical process will be spontaneousor not depending on ∆G < O

An application of the spontaneous electrochemical reactions can be found in the energy cells.

By introducing two metal rods in solutions containing ions of the same metal, we can produce potential and a flow of electrons between the two metals that we can use to power instruments. The intensity of the electric current producedWill depend on the concentration of the different solutions in the electrochemicalcells

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Electrochemical cells

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Acknowledgements

• JISC• HEA• Centre for Educational Research and Developmen

t• School of natural and applied sciences• School of Journalism• SirenFM• http://tango.freedesktop.org