Redox Reactions

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  • Topic: Redox, Electrochemistry, Hydrogen
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  • Published : March 18, 2013
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Redox reactions
(Redox terminology, galvanic cells, standard electrode potentials, strength of oxidizing/reducing agents, corrosion and protection against it)
Oxidation is defined as the loss of electrons and reduction is defined as the gain of electrons. Oxidizing agents, such as oxygen are defined as substances which accept electrons; reducing agents are substances which donate electrons.

Redox reactions are:
a) The reactions of metals with non-metals2Na+Cl2→2Na+Cl- b) The reactions of metals with water and steamCa+2H2O→Ca2+(OH)2- c) The reactions of metals with acidsMg+2H+→Mg2++H2
d) The reactions at the electrodes during electrolysis
e) Disproportionation reactions – the ones where one element undergoes both, oxidation and reduction Electromotive force – gives the quantitative measure of likelihood of reaction taking place in the cell. Only reactions with positive overall values of e.m.f will take place.

Standard hydrogen half-cell – serves to measure e.m.f of other cells since it was given value of 0.

Standard electrode potentials – also called standard reduction potentials is an e.m.f value of a given half-cell in relative to hydrogen cell. Galvanic cell - when zinc metal placed in CuSO4 solution, following reaction takes place:  Oxidation:   Zn(s) Zn+2 + 2e-1 

Reduction:  Cu+2 + 2e-1 Cu 
Overall:      Zn(s) + Cu+2 Zn+2 + Cu(s) 
 

The salt bridge is necessary to complete the circuit and maintain charge neutrality.  Zn+2  cathode 
SO4-2  anode 
 

Measuring the strength of oxidizing and reducing agents – standard electrode potentials provide direct measure of the relative oxidizing and reducing agents. It is important to remember that the more positive the value of e.m.f, the more energetically favorable is the reaction. Oxidizing and reduction agents – every agent has its own reciprocal or conjugate which is created when the reduction or oxidation takes place. The stronger the oxidizing agent, the weaker its...
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