Oxidation reduction processes
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Oxidation – Reduction Processes. Redox Reaction. Any chemical reaction that involves the transfer of one or more electrons between atoms. One reactant gains electrons and the other reactant loses electrons. Examples of Redox Reactions. Combustion of gasoline Burning of wood

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Oxidation – Reduction Processes

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Oxidation – ReductionProcesses


Redox Reaction

Any chemical reaction that involves the transfer of one or more electrons between atoms. One reactant gains electrons and the other reactant loses electrons.


Examples of Redox Reactions

  • Combustion of gasoline

  • Burning of wood

  • Energy from food

  • Bleaching stains

  • Iron rusting


Reduction

  • Electrons are fully or partially gained

S0(s) + 2 e- S2-

electrons gained:

written as a reactant


Oxidation

  • Electrons are fully or partially lost

Mg0 (s) Mg 2+ + 2e-

Electrons lost:

written as a product


Oxidation and Reduction

  • Opposing reactions

  • Must occur together

    • Redox (Reduction – Oxidation)


Mnemonic Device

  • LEO the lion goes GER

LEO: Loss of Electrons is Oxidation

GER: Gain of Electrons is Reduction


Mnemonic Device

  • OIL RIG

    • Oxidation Is Loss of electrons

    • Reduction Is Gain of electrons


Complete Redox Reaction

  • Add together the reduction half-reaction with the oxidation half-reaction to get the complete redox reaction.


Complete Redox Reaction

Add together the 2 half-reactions

Mg0 (s) Mg 2+ + 2e-

Oxidation

+

S0(s) + 2 e- S2-

Reduction

Complete Redox Reaction

Mg0 (s) + S0(s) Mg 2+ + S2-

Mg

+

S

Mg+2

+

S

-2


Assigning Oxidation Numbers


Oxidation Numbers

  • Numbers assigned to all of the elements involved in the reaction to determine if electrons have been transferred between atoms in a reaction.


Oxidation Numbers

  • The oxidation number is usually equal to the charge on the ion if it was formed.


Rule #1

  • Free elements are assigned an oxidation state of 0.

    • Al = 0

    • Na = 0

    • H2 = 0


Rule #2

  • The oxidation state for any simple one-atom ion is equal to its charge.

    • Na+ = +1

    • Be2+= +2

    • F- = -1


Rule #3

  • Hydrogen in compounds is assigned an oxidation state of +1. 

    Exception - Hydrides, ex. LiH (H=-1).

    • H2SO4: H=+1

    • HCl: H=+1


Rule #4

  • Oxygen in compounds is assigned an oxidation state of -2.

    Exception - Peroxide, ex. H2O2 (O = -1).

    • H3PO4: O=-2

    • H2O: O=-2


Rule #5

  • The sum of the oxidation numbers of all atoms in a neutral compound is 0.

    • H2SO4: add up to 0

    • CO2: add up to 0


Rule #6

  • The sum of the oxidation numbers of all atoms in a polyatomic ionequals the charge on the ion.

    • SO42-: add up to -2

    • NH4+: add up to +1


Examples

  • Al(s):

    • Al = ____

Rule #1

0


Examples

  • CaCl2:

    • Ca = ____

    • Cl=____

Rule #2

Rule #5 (or rule #2)

Ca+2

1 (+2) + 2 (Cl) = 0

-1

Cl = -1


Examples

  • HNO3:

    • H= ____

    • N= ____

    • O= ____

Rule # 3 (Hydrogen)

Rule # 4 (Oxygen)

+1

Rule # 5 (solve for N)

+5

1(1) + 1(N) + 3(-2) = 0

-2

N = +5


Examples

  • SO42-:

    • S= ____

    • O= ____

Applied to:

Rule #4 (oxygen)

Rule #6 (polyatomics)

Solve for S

+6

1(S) + 4(-2) = -2

-2

S= +6


Examples

  • H2O:

    • H= ____

    • O= ____

Rule #3 (hydrogen)

Rule #4 (oxygen)

+1

-2


Examples

1(N)+4(+1)=+1

  • (NH4)2CO3

N= -3

NH4+

CO32-

1(C)+3(-2)=-2

Rule 7

Rule 6

C=+4

Rule 9

-3

+4

  • N=____ C=___

  • H=____ O=___

+1

-2


Oxidized Element

  • The substance that loses electrons is the oxidized element (it undergoes oxidation.

  • Atoms that are oxidized will have an increase in their oxidation number


Oxidized Element

Increase in oxidation number

Mg0 (s) + S0(s) Mg 2+ + S2-

Mg is oxidized


Reduced Element

  • The substance that gains electrons is the reduced element (it undergoes reduction).

  • Atoms that are reduced will have an decrease in their oxidation number


Reduced Element

Decrease in oxidation number

Mg0 (s) + S0(s) Mg 2+ + S2-

S is reduced


Oxidizing Agent/Reducing Agent

  • The element or compound that is reduced is the oxidizer(or oxidizing agent).

  • The element or compound that is oxidized is the reducer(or reducing agent).


Examples

  • Are the following redox reactions?

  • If yes:

    • What element is oxidized?

    • What element is reduced?

    • What is the oxidizing agent?

    • What is the reducing agent?


Examples

+2

-2

0

0

Mg(s) + S(s) MgS

Mg

  • What element is oxidized?

  • What element is reduced?

  • What is the oxidizing agent?

  • What is the reducing agent?

Step # 2: Do the oxidation numbers change?

- if no  it is not a redox reaction

- if yes  it is a redox reaction

Step #1: Assign Oxidation Numbers

Yes – It is a redox

S

S

Mg


Examples

0

0

+1

+2

2AgNO3(aq) + Cu(s) Cu(NO3)2(aq) + 2Ag(s)

NO3-

NO3-

Step # 2: Do the oxidation numbers change?

- if no  it is not a redox reaction

- if yes  it is a redox reaction

  • What element is oxidized?

  • What element is reduced?

  • What is the oxidizing agent?

  • What is the reducing agent?

Cu

Ag

Yes – It is a redox

Step #1: Assign Oxidation Numbers

AgNO3(aq)

Cu(s)


Check for Understanding

0

0

+3

-2

4Fe(s) + 3O2(g)2Fe2O3(s)

  • What element is oxidized?

  • What element is reduced?

  • What is the oxidizing agent?

  • What is the reducing agent?

Fe

Yes

O

O2

Fe


Check for Understanding

+1

-2

+1

+1

-1

+1

-1

+1

-2

HCl + NaOH NaCl + H2O

No – it is not a redox


Check for Understanding

+1

-1

+1

+1

+1

-1

  • NaCl + AgNO3 NaNO3 + AgCl

NO3-

NO3-

No – it is not a redox


Check for Understanding

0

+3

-2

+2

-2

+4

-2

  • Fe2O3(s) + 3CO(g) 2Fe(s) + 3CO2(g)

  • What element is oxidized?

  • What element is reduced?

  • What is the oxidizing agent?

  • What is the reducing agent?

C

Yes

Fe

Fe2O3

CO


Any General Rules

  • Double displacement is NOT typically redox

  • Single Displacement is ALWAYS redox

  • Synthesis (from elements) is ALWAYS redox

  • Decomposition (to elements) is ALWAYS redox


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