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Oxidation-Reduction Reactions (Redox Reactions)

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Oxidation – reduction reactions (REDOX) - The chemical changes that occur when electrons are transferred between reactants.

Assigning Oxidation Numbers

1. Ox# of element is zero.

2. Sum of ox#’s is zero.

3. If covalent, more EN element is negative.

4. Oxygen is usually a -2, except peroxides where it is -1 or when it is electropositive and a +2

Ex: Fe0 Cu0 O20

Ex: CaBr2

+2 -1(2) = 0

Ex: NH3 (N closer to F)

-3 +1(3) = 0

Ex: O2- H2O2

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

OF2

+2 -1(2)=03

Assigning Oxidation Numbers

5. Hydrogen is usually a +1 except when it comes second after a metal, then it’s -1. (H is attracting e-!)

6. The sum of a polyatomic is equal to its charge.

7. Sandwich elements get assigned last.

Ex: HCl AlH3

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

Ex: NH4+

-3 +1(4) = +1

Ex: H2 S O4

+1(2) + x -2(4) = 0

x = 6

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Working out oxidation numbers

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Examples - assigning oxidation numbers

Assign oxidation states to all elements:

H2 SO3 SO42-

K+ NH3 MnO4-

Cr2O72- CH3OH PO4

3-

ClO3- HSO3

- Cu

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-    -principal source of energy on earth-    -combustion of gasoline-    -burning of wood- burning food in your body

oxidation reactions

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Oxidation Originally meant combining with oxygen

Ex:   iron rusting (iron + oxygen)

Today it means a LOSS of e-

Mg(s) Mg2+ + 2e-Magnesium lost 2 electrons

 

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 Reduction Originally meant the loss of oxygen from a compound.

Ex: iron ore ( iron II oxide) reduces to make iron

Today, REDUCTION means a GAIN of e-

Fe2+ + 2e- Fe(s)

Iron gained 2 electrons 

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Balancing half equations

Oxidation reactions always happen with a reduction

reaction

Fe0(s) + Cu2+ 2Fe2+

+ Cu0

The electrons from the iron are transferred to the copper (II) ions.

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e- e-

Oxidation reactions always happen with a reduction

reaction

Fe0(s) + Cu2+ 2Fe2+

+ Cu0

The electrons from the iron are transferred to the copper (II) ions.

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Iron lost e- = oxidized

Copper gained e- = reduced

Oxidation reactions always happen with a reduction

reactionFe(s) + Cu2+ 2Fe2+

+ Cu

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iron is OXIDIZED (lost e-)copper is REDUCED (gained e-)

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Redox Reactions - What’s Happening?

Zinc is added to a blue solution of copper(II) sulfate

The blue color disappears, the zinc metal reacts, and solid copper metal precipitates on the zinc strip

The zinc is oxidized (loses electrons)

The copper ions are reduced (gain electrons)

Zn (s) + CuSO4 (aq) ZnSO4 (aq) + Cu (s)

Zn (s) + Cu2+ (aq) Zn2+ (aq) + Cu (s)

e- e-

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Copper ions (Cu2+) collide with the zinc metal surface. A zinc atom (Zn) gives up two of its electrons to the copper ion. The result is a neutral atom of Cu deposited on the zinc strip, and a Zn2+ ion released into the solution.

Oxidizing & Reducing Agents

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The substance that donates electrons in a redox reaction is the REDUCING AGENT. 

The substance that takes electrons in a redox reaction is the OXIDIZING AGENT.

Fe(s) + Cu2+ 2Fe2+ + Cu

Fe(s) + Cu2+ 2Fe2+ + Cu

Reducing agent

Oxidizingagent

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Oxidation or reduction?

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In each reaction, look for changes in oxidation state.

If changes occur, identify the substance being reduced, and the substance being oxidized.

Identify the oxidizing agent and the reducing agent.

Examples - labeling redox reactions

H2 + CuO Cu + H2O0 -2+2 0 -2+1

= +1 (H is oxidized)

= -2 (Cu is reduced)

(reducing agent)

(oxidizing agent)

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5 Fe2+ + MnO4- + 8 H+ 5 Fe3+ + Mn2+ + 4 H2O

Zn + 2 HCl ZnCl2 + H2

Try These!!

+1 = Fe 2+ is oxidized (reducing agent)

- 5 = Mn 7+ is reduced (oxidizing agent)

+2 = Zn 0 is oxidized (reducing agent)

- 1 = H 1+ is reduced (oxidizing agent)

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SnCl2 + PbCl4 SnCl4 + PbCl2

CuS + H+ + NO3- Cu+2 + S + NO + H2O

-1+2 -1+4 -1+4 -1+2

+2 = LEO

-2 = GER

-2+2 +1 -2+5 +2 0 -2+2 -2+1

-3 = GER

+2 = LEO

RA

OA

RA

OA

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Redox or not?

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Voltaic Cell

Electrolysis

SIMULATIONS

Redox Titration