Introduction to Oxidation-Reduction Reactions

Electron Transfer Reactions

Types of Chemical Reactions

There are four types of chemical reactions:

Acid/Base Precipitation/Solubility Complex Formation/Complex Dissociation Oxidation/Reduction

Any chemical reaction consists of one (or more) of these basic categories.

Oxidation/Reduction Reactions

Acid/Base reactions involve a donation /acceptance of protons

Precipitation/ Solubility reactions involve a donation/ acceptance of negative

charge

what is being donated and accepted in a redox reaction?

Oxidation/Reduction Reactions

Electrons! Consider the reaction taking place in a

disposable battery:

2Zn + 3MnO2 Mn3O4 + 2ZnO

How can you tell that electrons are being donated and accepted? Which species is donating electron( s) and which is accepting electron (s)?

Redox reactions are characterized byRedox reactions are characterized by ELECTRON TRANSFERELECTRON TRANSFER between an between an electron donor and electron acceptor.electron donor and electron acceptor.

REDOX REACTIONSREDOX REACTIONS

Transfer leads to— Transfer leads to—

1.1. increase in oxidation numberincrease in oxidation number of of some element = some element = OXIDATIONOXIDATION

2.2. decrease in oxidation numberdecrease in oxidation number of of some element = some element = REDUCTIONREDUCTION

REDOX REACTIONSREDOX REACTIONS

Electron Transfer in Redox Reactions

Oxidation Loss of electrons Gain in oxygen

Reduction

Gain of electrons Loss of oxygen

“LEO the lion goes Ger”

Example

The reaction of a metal and non-metal All the electrons must be accounted for!

Mg S+ → Mg 2+ + S2-

Oxidation-Reduction

Oxidation means an increase in oxidation state - lose electrons.

Reduction means a decrease in oxidation state - gain electrons.

The substance that is oxidized is called the reducing agent.

The substance that is reduced is called the oxidizing agent.

Assigning Oxidation States

An Oxidation-reduction reaction involves the transfer of electrons.

You should memorize these rules

Rules for Oxidation States

The charge the atom would have in a molecule (or an ionic compound) if electrons were completely transferred.

The oxidation state of elements in their standard states is zero.

Example: Na, Be, K, Pb, H2, O2, P4 = 0

Assigning Oxidation States

Oxidation state for monatomic ions are the same as their charge.

Example: Li+, Li = +1; Fe3+, Fe = +3; O2-, O = -2

Oxygen is assigned an oxidation state of -2 in its covalent compounds except as a peroxide.

Rules for Oxidation States

4. The oxidation number of hydrogen is +1 except when it is bonded to metals in binary compounds. In these cases, its oxidation number is –1.

5. Group IA metals are +1, IIA metals are +2 and fluorine is always –1.

6. The sum of the oxidation numbers of all the atoms in a molecule or ion is equal to the charge on the molecule or ion.

Practice in Oxidation States

Assign the oxidation states to each element in the following.

K2SO4

NO3-

H2SO4 Fe2O3 Fe3O4

Identify the

Oxidizing agent

Reducing agent

Substance oxidized

Substance reduced

On the worksheet

Types of Oxidation-Reduction Reactions

Combination Reaction

A + B C

S + O2 SO2

Decomposition Reaction

2KClO3 2KCl + 3O2

C A + B

0 0 +4 -2

+1 +5 -2 +1 -1 0

Displacement Reaction a.k.a Single Replacement

A + BC AC + B

Sr + 2H2O Sr(OH)2 + H2

TiCl4 + 2Mg Ti + 2MgCl2

Cl2 + 2KBr 2KCl + Br2

Hydrogen Displacement

Metal Displacement

Halogen Displacement

Types of Oxidation-Reduction Reactions

0 +1 +2 0

0+4 0 +2

0 -1 -1 0

The Activity Series for Metals

M + BC AC + B

Hydrogen Displacement Reaction

M is metalBC is acid or H2O

B is H2

Ca + 2H2O Ca(OH)2 + H2

Copper Demonstration

Copper Pennies reacting with nitric acid.

Can you figure out the equation?

Disproportionation Reaction

Cl2 + 2OH- ClO- + Cl- + H2O

Element is simultaneously oxidized and reduced.

Types of Oxidation-Reduction Reactions

Chlorine Chemistry

0 +1 -1

Ca2+ + CO32- CaCO3

NH3 + H+ NH4+

Zn + 2HCl ZnCl2 + H2

Ca + F2 CaF2

Classify the following reactions.

Half-Reactions

All redox reactions can be thought of as happening in two halves.

One produces electrons - Oxidation half.

The other requires electrons - Reduction half.

Half-Reactions

Write the half reactions for the following.

Na + Cl2 → Na+ + Cl-

SO3- + H+ + MnO4

- → SO4- + H2O + Mn+2

Balancing Redox Equations

In aqueous solutions the key is the number of electrons produced must be the same as those required.

For reactions in acidic solution an 8 step procedure.

Balancing Redox Equations

Write separate half reactions

For each half reaction balance all reactants except H and O

Balance O using H2O

Acidic Solution

Balance H using H+

Balance charge using e-

Acidic Solution

Multiply equations to make electrons equal

Add equations and cancel identical species

Check that charges and elements are balanced.

Practice

Balance the following reactions:

Sn 2+ (aq) + 2Fe 3+ → Sn 4+ (aq) + 2Fe 2+

MnO4- (aq) + C2O4-2 (aq) → Mn2+ (aq) + CO2 (g)

Practice

The following reactions occur in aqueous solution. Balance them

Cr(OH)3 + OCl- + OH- CrO4-2 +

Cl- + H2O

MnO4- + Fe+2 Mn+2 + Fe+3

Now for a tough one

Fe(CN)6-4 + MnO4

- Mn+2 + Fe+3 + CO2 + NO3-

Basic Solution

Do everything you would with acid, but add one more step.

Add enough OH- to both sides to neutralize the H+

CrI3 + Cl2 CrO4- + IO4

- + Cl-

CN- + MnO4- → CNO- + MnO2

Redox Titrations

Same as any other titration.

the permanganate ion is used often because it is its own indicator. MnO4

- is purple, Mn+2 is

colorless. When reaction solution remains clear, MnO4

- is gone.

Chromate ion is also useful, but color change, orangish yellow to green, is harder to detect.