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Redox reactions 2
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Redox reactions
In oxidation-reduction reactions (redox) there is a transfer of electrons from one chemical species to another.
In redox reactions, two transformation take place simultaneously:
oxidation – a chemical species loses electrons;
reduction – the other species acquires electrons. 3
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4. Redox and the oxidation number
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Redox and the oxidation number
In redox reactions, the oxidation number of the
oxidized atom increases, while that of the atom which is
reduced decreases.
Fe + H2SO4 → FeSO4 + H2
Iron is oxidized
Hydrogen is reduced
The change of the oxidation number corresponds to the number of electrons that each atom acquires or loses.
Fe → Fe + 2e–
H + 2e– → H 4
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5. Balanced redox reactions
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Balanced redox reactions
A redox must always be balanced, considering two criteria:
the total number of electrons transferred from oxidized atoms must equal the number of electrons acquired from reduced atoms;
the total number of atoms and charges must be the same on the left and on the right of the arrow. 5
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6. Balancing redox reactions in acid or basic solutions
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Balancing redox reactions in acid or basic solutions
If the redox takes place in an acid or base, it is possible to use the half-reaction method:
instead of considering individual elements, the entire formula of the species which oxidizes or is reduced is considered;
every semi-reaction becomes balanced based on the respective charges of atoms. 6
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7. Redox and functional groups
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Redox and functional groups
Carbon atoms with different oxidation states may be present in organic molecules.
The oxidation state of carbon atoms depends on the balance between lost electrons (in bonds with oxygen and nitrogen) and acquired electrons (in bonds with hydrogen).
alcohol group
In functional groups, the carbon atom is further reduced the more C–H bonds there are; it is more oxidized the greater the C–N and C–O bonds there are. ox
carbonyl group ox
carboxyl group ox
CO2 7
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8. Biological redox solutions
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Biological redox solutions
In many biological redox solutions, oxidation involves
dehydrogenation, meaning the loss of hydrogen atoms from
one or more carbon atoms.
C6H12O6 + O2 → 6CO2 + 6H2O
Conversely, reduction involves hydrogenation of carbon atoms, meaning the addition of hydrogen atoms.
6CO2 + 6H2O → C6H12O6 + O2 8
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9. Redox couples speciesox / species red
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Redox couples
Two chemical species that can transform into one another through reduction or oxidation form a redox couple.
is reduced
Speciesox has oxidizing ability
Speciesred has reducing ability
speciesox / species red
is oxidized
Each redox reaction involves two redox couples.
The direction of the spontaneous redox reaction depends on the competition for electrons between the two couples. 9
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The galvanic cell /1
If two half-reactions take place separately, but are connected so as to generate an electrical current, this process illustrates the operation principle of a battery (or galvanic cell). 10
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The galvanic cell /2
The Daniell cell is composed of two half-cells connected via metal wire and salt bridge and contains an electrode and redox couple.
The electrode where the oxidation takes place is called an anode.
The electrode where the reduction takes place is called a cathode. 11
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Cell diagram
The Daniell cell can be represented with a cell diagram, which indicates (from left to right): the oxidation of the redox couple at the anode, the salt bridge, the reduction of the redox couple at the cathode.
salt bridge
cathode
anode
Zn(s) | Zn2+(w) || Cu2+(w) | Cu(s)
electrode
electrolyte
electrolyte
electrode 12
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Electromotive force
The electromotive force measures the battery capacity required to accomplish work.
Under standard conditions, the electromotive force depends on the ability of the oxidized species at the cathode to acquire electrons and the ability of the reduced species at the anode to lose electrons. 13
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Reduction potential
Each redox couple has a standard reduction potential (E°red) which measures the tendency to take in electrons in the couple’s oxidized form.
Ered is measured in comparison to a hydrogen half-cell, it is expressed in volts and can have a positive or negative value.
The oxidation potential Eox has the same value of Ered, but an opposite sign. 14
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15. electromotive force = Ered(cathode) – Ered(anode)
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Reduction potential and electromotive force
The values of the reduction potential allow us to establish the oxidation and reduction power of a chemical species, the spontaneity of redox and the electromotive force of a battery.
electromotive force = Ered(cathode) – Ered(anode) 15
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Electrolysis
Electrolysis uses electricity to conduct a non spontaneous
redox reaction in which positive ions are reduced at the cathode and negative ions are oxidized at the anode.
In takes place in the electrolytic cell. 16
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