Oxidation - Reduction

RedOx

Oxidation States

. . . of an element are determined from the number of electrons that are ______ other atoms Gained from Lost toAND Shared with

Rules of assigning oxidation states

Atoms have a negative oxidation state if they have the higher e-neg in the bond NH3

N=-3 H=+1 Oxidation states on

all the atoms of a molecule & compoundmust add upto equal zero

Uncombined elementsfree state

Elements that are not bonded to a different element have oxidation states of zero.

Al(s) Cu(s) Ag(s) Cl2 O2 O3 Ne

H2S sulfur has a -2 oxidation state

H2SO3

sulfur has a +4 oxidation state

H2SO4

sulfur has a +6 oxidation state

NOW More complicated . . .

Assigning oxidation states to atoms in a polyatomic ion

Oxidation states on individual atoms must equal the charge on the ion

(SO4)(S+6 O4-2

)

-8+6 = -2

(S O4) = -2

(S O4-2

)

-8 = -2

Try these . . .

hydroxide

dichromate

ammonium

Assign oxidation states to each atom

NaOH

MnCrO4

(NH4)2SO4

K2O2

Balancing RedOx reactionsusing oxidation states

Zn + HNO3 Zn(NO3)2 + NO2 + H2O

Go through the steps on paper…

1. Assign oxidation states to all atoms determine the

number of e- lost and gained

+4

+3

+2

+1

0

-1

-2

-3

-4L

osin

g e- o

xida

tion

Gain e

- Reduction

2. Select product coefficients that balance the e- HINT: switch #’s balance major elements with reactants

3. Balance leftovers with inventory

*coefficients must be lowest possible whole #s

Balance - use oxidation states

Cu + HNO3 Cu(NO3)2 + H2O + NO

3Cu + 8HNO3 3Cu(NO3)2 + 4H2O + 2NO

K2Cr2O7 + H2O + S SO2 + KOH + Cr2O3

2K2Cr2O7 + 2H2O + 3S 3SO2 + 4KOH + 2Cr2O3

Balancing net ionic rxns

Mg + Al3+ Mg2+ + Al

Al3+ and Mg+2 are ionsif they give you the charge use it! Its part of a net ionic rxn

they have removed the spectator ions.

Balance for mass and charge

Balance . . . for mass and charge

Cu2+ + K K+ + Cu

RedOx reactions

Chemical reactions involving an

electron transfer between

reactants

Recognizing RedOx

assign oxidation states to the individual elements in reactants and the products

if the oxidation state changes for some of the particles it is considered a RedOx reaction

Single Replacement Reactions are ALWAYS RedOXDouble Replacement Reactions are NEVER RedOX

In a RedOx reaction

If electrons are lost by one species in a reaction they

all MUST be gained by another!

If one atom is being oxidized another must be reduced.

In other words . . . oxidation and reduction always occur

together

Oxidation

Oxidation results in an increase in oxidation state

Atoms that Lose e-lectrons undergo

Oxidation

Metals tend to undergo oxidation

+3 +2 +1 0 -1 -2 -3

ReductionReduction results in a decrease

in oxidation state

Atoms which Gain e-lectrons

undergo Reduction

Non-metals tend to undergo reduction

+3 +2 +1 0 -1 -2 -3

GeR

RedOx: Yes or NO?

HCl + NaOH HOH + NaCl

Mg + 2HCl MgCl2 + H2

MnO2 + 4HBr MnBr2 + Br2 + 2H2O

Al + CuCl2 ? Write a balanced equation based

on these reactants. Is this a RedOx reaction?What species is oxidized?

Reduced?

Predicting Products of Single Replacement Reactions

Roaring Animals Often Attack

Elements that undergo oxidation cause reduction Reducing agent

Elements that undergo reduction cause oxidation Oxidizing agent

LEO GER

RA OA

2Al + 3CuCl2 2AlCl3 + 3Cu

What is the Oxidizing Agent? Cu+2 because it gets reduced

What is the Reducing Agent? Al because it gets oxidized

These answers always come from the reactant

side

MnO2 + 4HCl MnCl2 + Cl2 + 2H2O

Assign oxidation states to all atomsWhat species is oxidized?

How do you know?What species is reduced?

How do you know?

What species is the oxidizing agent? Mn+4 because it gets reduced

What species is the reducing agent? Cl- because it gets oxidized

RedOx Practice

Remember . . .+4

+3

+2

+1

0

-1

-2

-3

-4L

osin

g e- o

xida

tion

Gain e

- Reduction

Writing half-reactions

A half reaction shows either oxidation or reduction of a RedOx reaction.

The electrons being lost (oxidation) or gained (reduction) are also shown.

Oxidation half-reaction

Fe(s) Fe+3(aq) + 3e-

oxidation number increaseselectrons are lost

productsconservation of mass AND CHARGE

charges on both sides equal each other

Reduction half-reaction

Sn+4 + 2e- Sn+2

oxidation number decreaseselectrons are gained

reactantsconservation of mass AND CHARGE

charges on both sides equal each other

Balancing RedOx using 1/2 reactions

1. ASSIGN OXIDATION STATES . . .2. Write the oxidation 1/2 reaction3. Write the reduction 1/2 reaction4. Balance the two half reactions so that

the number of electrons transferred is equal

5. Use these coefficients to balance the RedOx atoms

6. Balance leftover atoms by inventory

. . . use 1/2 reactions

KMnO4 + HCl MnCl2 + KCl + Cl2 + H2O

ZnS + O2 SO2 + ZnO

The End

I came . . . I saw . . . I RedOxed