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Oxidation-Reduction Reactions

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by

Raneem Khattab

on 1 May 2013

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Transcript of Oxidation-Reduction Reactions

Oxidation-Reduction
Reactions 20.1
The Meaning of Oxidation and Reduction Redox Reactions Oxidation-Reduction reactions are also known as redox reactions.
Oxidation is the complete or partial loss of electrons or gain of oxygen.
Reduction is the complete or partial gain of electrons or loss of oxygen.
Oxidation and reduction always occur simultaneously. The reducing agent is the substance that loses electrons.
The oxidizing agent is the substance that accepts electrons. Oxidation
Is
Loss
Reduction
Is
Gain Redox with Covalent Compounds 20.2
Oxidation Numbers Corrosion Corrosion is the gradual destruction of metals by interaction with the environment.
Iron corrodes by being oxidized to ions of iron by oxygen.
Water accelerates the rate of corrosion. Resistance to Corrosion Assigning Oxidation Numbers An oxidation number is a positive or negative number assigned to an atom to indicate its degree of oxidation or reduction.
Generally, a bonded atom's oxidation number is the charge that it would have if the electrons in the bond were assigned to the atom of the more electronegative element. 2FeO + 3C 4Fe + 3CO Iron (III) Oxide Carbon Iron Carbon Dioxide Electron Shift Oxygen is the most electronegative element (other than fluorine), so when it bonds with other elements, the electrons shift towards oxygen. So redox reactions do not only involve oxygen, but also any shift of electrons between reactants. In a covalent compound, electrons are not completely transferred within the compound because the electrons are shared between the atoms.
Example:
2H + O 2H O
The hydrogen undergoes a partial loss of electrons because oxygen is more electronegative than hydrogen. So the hydrogen is oxidized and the oxygen is reduced.

Generally, the addition of oxygen or the removal of hydrogen in carbon compounds is oxidation. Rules for assigning
oxidation numbers 1. The oxidation number of a monatomic ion is equal to the sign of its ionic charge.
Br : -1
Fe : +3
2. The oxidation number of hydrogen in a compound is +1, except in metal hydrides, where it is -1.
H S: +1
NaH: -1
3. The oxidation number of oxygen in a compound is -2, except in peroxides where it is -1, and in compounds with fluorine, where it is positive.
Li O: -2
H O : -1
OF : +2
4. A pure element that is uncombined has an oxidation number of 0
K: 0
5. For any neutral compound, the sum of the oxidation numbers of the atoms in the compound must equal 0.
Cf : (+4)+4(-1)=0
6. For a polyatomic ion, the sum of the oxidation numbers must equal the ionic charge of the ion.
NO : O=-2, N=+3 ---> (+3) + 2(-2) = -1 2 3 2 2 2 2 1- 3+ 2 Noble metals (gold and platinum) are resistant to losing electrons by corrosion.
Some metals form an oxide coating on their surface that prevents extensive corrosion.
-Iron forms a coat that is not very strong, and can be penetrated by water and oxygen.
-Aluminum forms a coat that is tightly packed and prevents corrosion. Controlling Corrosion To help prevent corrosion, metals may be coated with oil, paint, plastic, or another metal. These coatings exclude air and water from the surface.
Another way to prevent corrosion is allowing one metal to corrode in order to save a second metal. 2 2 2 2 4 2 1- Examples What is the oxidation number 0f each of the following?
a) SO

b) CO

c) Na SO

d) (NH ) S 2 3 2- 2 4 4 2 An increase in the oxidation number of an atom or ion indicates oxidation.
A decrease in the oxidation number of an atom or ion indicates reduction. The End! Thank you for listening! Mg + S Mg S . . . .. . . . 2+ 2- Reducing
agent Oxidizing
agent
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