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Electrochemistry. Define oxidation and reduction. Determine oxidation numbers for atoms. Identify the oxidizing agent, the reducing agent. Distinguish between redox and non-redox reactions. Burning and corrosion needs oxygen – oxidation. Oxidation-reduction reactions – ( redox )

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Presentation Transcript
slide2

Define oxidation and reduction.

  • Determine oxidation numbers for atoms.
  • Identify the oxidizing agent, the reducing agent.
  • Distinguish between redox and non-redox reactions.
slide3

Burning and corrosion needs oxygen – oxidation.

Oxidation-reduction reactions – (redox)

Chemical changes when electrons are transferred from one reactant to another.

Oxidation - an atom loses one or more electrons.

Reduction- an atom gains one or more electrons.

"LEO says GER” 
Losing Electrons is Oxidation, Gaining Electrons is Reduction

slide4

2 Mg (s) + O2 (g) → 2 MgO (s)

Mg – neutral Mg2+ ion

O – neutral O2– ion

Magnesium is oxidized. Oxygen is reduced.

O + 2e- → O2-

Mg → Mg2+ 2e-

slide5

Mg (s) + Cl2 (g) → MgCl2 (s)

Written difference between ion and oxidation:

Chlorine ion – Cl1- ion charge = 1-
 oxidation number = -1

Cl + 1e- → Cl1-

Mg → Mg2+ 2e-

Sometimes these numbers are the same (like above) sometimes they are very different – which is why we write them differently.

slide6

Oxidation number represents the chargethe atom would haveifevery bond were ionic.

**You assign them to EACH atom**

1. Assign known numbers first (below).

Then calculate the others.

  • All uncombined elements (diatomic) – zero.
  • Monatomic ions in ionic bond -equals ion charge.
slide7

In compounds:

  • Alkali metals - always +1.
  • Earth metals - always +2
  • Al: +3, F: -1, H: +1*, O: –2*

Neutral compound:

Sum of ox.numbers for each atom must be zero.

Polyatomic ion:

Sum of ox.numbers must be the charge of that ion.

Assign oxidation numbers to each atom in SO2.

S = +4 O = –2

slide8

Assign ox.numbers for each atom in K2Cr2O7

Step 1: Start with atoms which are known.

O: –2
 K: +1

Step 2: Solve for other atoms.

All O atoms: –2 × 7 = –14

All K atoms: +1 × 2 = +2

The total for the compound must be zero.

All Cr atoms: -14 + 2 + ?? = 0

Two Cr atoms - (+12) ÷ 2 = +6 each

K = +1 Cr = +6 O = –2.

slide9

Assign ox. numbers for each atom in Fe(NO3)3

Ionic bond between Fe3+ and NO3–

Fe: +3

Sum of ox.numbers for the compound must be 0.

All O atoms: -2 × 9 = –18 +3 (-18) + ?? = 0

All N atoms: 15 ÷ 3 =

Fe = +3 N = +5 O = –2.

slide10

Use ox.numbers to determine if reaction is a redox reaction.

-2

+1

-2

+4

+4

+1

-2

SO2 + H2O → H2SO3

+4

-2

+2

-4

+2

-6

+4

Ox.numbers do not change – no e- transferred – NOT a redox reaction.

slide11

Is the following reaction a redox reaction?

-2

+5

0

+1

-2

+1

+5

0

Cu(s) + 2 AgNO3(aq) → CuNO3(aq) + 2 Ag(s)

0

+5

+1

-6

+5

+1

0

-6

• Cu – oxidized (loss of electrons). 


• Ag – reduced (gain of electrons).

Oxidation cannot occur without reduction.

slide12

Oxidizing agent - causes the oxidation of another substance.

AgNO3 is the oxidizing agent

Reducing agent - causes the reduction of another substance.

Cu is the reducing agent

Oxidizing agent becomes reduced and the reducing agent becomes oxidized.

+1

0

Cu(s) + 2 AgNO3(aq) → CuNO3(aq) + 2 Ag(s)

+1

0

slide13

Identify the substance oxidized, the substance reduced, the oxidizing agent and the reducing agent.

+1

0

-2

+2

-2

-2

+1

+1

+5

-2

2 HNO3(aq) + 3 H2S(g) → 2 NO(g) + 3 S(s) + 4 H2O(l)

+2

0

+5

+1

-2

+2

-2

+2

-6

-2

S – oxidized

N – reduced

H2S – reducing agent

HNO3 – oxidizing agent

slide14

How many electrons are transferred in the reaction below:

gains 3e-

+1

0

-2

+2

-2

-2

+1

+1

+5

-2

3

HNO3(aq) + H2S(g) → 2 NO(g) + 3 S(s) + 4 H2O(l)

2

loses 2e-

S: (3 atoms) x (2e- lost) = 6 electrons lost

N: (2 atoms) x (3e- gained) = 6 electrons gained

Stoichiometry used to determined total electrons transferred.

slide15

Strong oxidizing agents:

Are very reactive – will take from anything

Oxidizing Agents Reaction Products

O2 O2–, H2O, CO2

F2, Cl2, Br2, I2 F–, Cl–, Br–, I–

MnO4– Mn2+

Cr2O72– Cr3+

HNO3 NO, NO2

H2O2 O2, H2O

Strong reducing agents:

Are very reactive – will give to anything.

Metals, substances that burn easily – H2, CxHy