Transcript REDOX reactions This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License.

REDOX reactions
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What are REDOX reactions?
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.
Many reactions occurring naturally are REDOX reactions, for example
combustions and enzymatic reactions. Many reactions of interest in Forensic
Sciences 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
oxidant
+e
reductor
-e
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Balancing redox equations
Acidic media
+2
+5
+7
+3
Basic media
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Examples
Cu(s) + HNO3(aq) → Cu(NO3)2(aq) + NO(g) + H2O(l)
Oxidation:
Reduction:
Cu (0) to Cu (+2)
N (+5) to N (+2)
3Cu(s) + 8HNO3(aq) → 3Cu(NO3)2(aq) + 2NO(g) + 4H2O(l)
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)
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Nernst equation
The 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 V
Cd2+ (aq) +2 e- ------> Cd (s), E° = -0.40 V
Standard conditions (represented for the ̊ symbol) are pressure of 1 bar (≅1 atm),
temperature of 298° K (or 25° C, or room temperature) and concentration of 1.0 M
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 spontaneous
or 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 produced
Will depend on the concentration of the different solutions in the electrochemical
cells
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Electrochemical cells
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Acknowledgements
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JISC
HEA
Centre for Educational Research and Development
School of natural and applied sciences
School of Journalism
SirenFM
http://tango.freedesktop.org
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