IB Chemistry Power Points

Topic 09

Oxidation and Reduction

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REDOXA guide for A level students

KNOCKHARDY PUBLISHING2008

SPECIFICATIONS

Much taken from

OXIDATION

OXIDATION & REDUCTION – Simplified Definitions

OXIDATIONis a GAIN OF OXYGEN

2Mg + O2 ——> 2MgO

magnesium has been oxidised as it has gained oxygen

OXIDATION & REDUCTION – Simplified Definitions

OXIDATIONis a GAIN OF OXYGEN

2Mg + O2 ——> 2MgO

magnesium has been oxidised as it has gained oxygen

is the REMOVAL (LOSS) OF HYDROGEN

C2H5OH ——> CH3CHO + H2

ethanol has been oxidised as it has ‘lost’ hydrogen

OXIDATION & REDUCTION – Simplified Definitions

OXIDATION & REDUCTION – Simplified Definitions

REDUCTION

OXIDATION & REDUCTION – Simplified Definitions

REDUCTIONis a GAIN OF HYDROGEN

C2H4 + H2 ——> C2H6

ethene has been reduced as it has gained hydrogen

OXIDATION & REDUCTION – Simplified Definitions

REDUCTIONis a GAIN OF HYDROGEN

C2H4 + H2 ——> C2H6

ethene has been reduced as it has gained hydrogen

is the REMOVAL (LOSS) OF OXYGEN

CuO + H2 ——> Cu + H2O

copper(II) oxide has been reduced as it has ‘lost’ oxygen

However as chemistry became more sophisticated, it was realised that another definition was required

...

OXIDATION

REDUCTION

REDOX

OXIDATION AND REDUCTION IN TERMS OF ELECTRONS

Oxidation and reduction are not only defined as changes in O

and H

OXIDATION & REDUCTION – Better Definitions

...

OXIDATION Removal (loss) of electrons ‘OIL’

species will get less negative or more positive

REDUCTION

REDOX

OXIDATION AND REDUCTION IN TERMS OF ELECTRONS

Oxidation and reduction are not only defined as changes in O

and H

OXIDATION & REDUCTION – Better Definitions

...

OXIDATION Removal (loss) of electrons ‘OIL’

species will get less negative or more positive

REDUCTION Gain of electrons ‘RIG’species will become more negative or less

positive

REDOX When reduction and oxidation take place

OXIDATION AND REDUCTION IN TERMS OF ELECTRONS

Oxidation and reduction are not only defined as changes in O

and H

OXIDATION & REDUCTION – Better Definitions

OIL - Oxidation Is the Loss of electrons

RIG - Reduction Is the Gain of electrons

Used to... tell if oxidation or reduction has taken place

work out what has been oxidised and/or reduced

construct half equations and balance redox equations

FOR ATOMS AND SIMPLE IONSThe number of electrons which must be added or removed to become neutral

Atoms Na = 0 neutral already ... no need to add any electrons

Cations Na+ = +1 need to add 1 electron to make Na+ neutral

Anions Cl¯ = -1 need to take 1 electron away to make Cl¯ neutral

OXIDATION NUMBERS (STATES)

Q. What are the oxidation states of the elements in the following?

a) C b) Fe3+ c) Fe2+

d) O2- e) He f) Al3+

Q. What are the oxidation states of the elements in the following?

a) C b) Fe3+ c) Fe2+

d) O2- e) He f) Al3+

Q. What are the oxidation states of the elements in the following?

a) C (0) b) Fe3+ (+3) c) Fe2+ (+2)

d) O2- (-2) e) He (0) f) Al3+ (+3)

Q. What are the oxidation states of the elements in the following?

a) C (0) b) Fe3+ (+3) c) Fe2+ (+2)

d) O2- (-2) e) He (0) f) Al3+ (+3)

OXIDATION STATES

• because CO2 is a neutral molecule, the sum of the oxidation states must be ?

• for this, one element must have a positive OS and the other must be ?

Explanation

MOLECULESThe SUM of the oxidation states adds up to ZERO

ELEMENTS H in H2 = 0 both are the same and must add up to Zero

COMPOUNDS C in CO2 = +4O in CO2 = -2 1 x +4 and 2 x -2 = Zero

HOW DO YOU DETERMINE THE VALUE OF AN ELEMENT’S OXIDATION STATE?• from its position in the periodic table and/or• the other element(s) present in the formula (oxygen is almost always -2 etc)

HOW DO YOU DETERMINE WHICH IS THE POSITIVE ONE?• the more electronegative species will have the negative value

OXIDATION STATES

in SO42- the oxidation state of S = +6 there is ONE S

O = -2 there are FOUR O’s +6 + 4(-2) = -2 so the ion has a 2- charge

COMPLEX IONSThe SUM of the oxidation states adds up to THE CHARGE

e.g. NO3- sum of the oxidation states = - 1

SO42- sum of the oxidation states = - 2

NH4+ sum of the oxidation states = +1

Example SO42-

OXIDATION STATES

What is the oxidation number of Mn in MnO4¯ ?

• the oxidation state of oxygen in most compounds is - 2• there are 4 O’s so the sum of its oxidation states - 8• overall charge on the ion is - 1• therefore the sum of all the oxidation states must add up to - 1• the oxidation states of Mn four O’s must therefore equal - 1• therefore the oxidation state of Mn in MnO4¯is +7

+7 + 4(-2) = - 1

COMPLEX IONSThe SUM of the oxidation states adds up to THE CHARGE

e.g. NO3- sum of the oxidation states = - 1

SO42- sum of the oxidation states = - 2

NH4+ sum of the oxidation states = +1

Example

HYDROGEN +1 except 0 atom (H) and molecule (H2)

-1 hydride ion, H¯ in sodium hydride NaH

OXYGEN -2 except 0 atom (O) and molecule (O2)-1 in hydrogen peroxide, H2O2

+2 in F2O

HALOGENS -1 except 0 atom (X) and molecule (X2)

OXIDATION STATES

CALCULATING OXIDATION STATE - 1

Many elements can exist in more than one oxidation state In compounds, certain elements are used as benchmarks to work out other values

Q. Give the oxidation state of the element other than O, H or F in...SO2 NH3 NO2 NH4

+ IF7 Cl2O7

NO3¯ NO2¯ SO32- S2O3

2- S4O62- MnO4

2-

What is odd about the value of the oxidation state of S in S4O62- ?

Q. Give the oxidation state of the element other than O, H or F in...SO2 NH3 NO2 NH4

+ IF7 Cl2O7

NO3¯ NO2¯ SO32- S2O3

2- S4O62- MnO4

2-

What is odd about the value of the oxidation state of S in S4O62- ?

OXIDATION STATES

A. The oxidation states of the elements other than O, H or F are

SO2 O = -2 2 x -2 = - 4 overall neutral S = +4

NH3 H = +1 3 x +1 = +3 overall neutral N = - 3

NO2 O = -2 2 x -2 = - 4 overall neutral N = +4

NH4+ H = +1 4 x +1 = +4 overall +1 N = - 3

IF7 F = -1 7 x -1 = - 7 overall neutral I = +7

Cl2O7 O = -2 7 x -2 = -14 overall neutral Cl = +7

NO3¯ O = -2 3 x -2 = - 6 overall -1 N = +5

NO2¯ O = -2 2 x -2 = - 4 overall -1 N = +3

SO32- O = -2 3 x -2 = - 6 overall -2 S = +4

S2O32- O = -2 3 x -2 = - 6 overall -2 S = +2

MnO42- O = -2 4 x -2 = - 8 overall -2 Mn = +6

METALS • have positive values in compounds

• value is usually that of the Group Number Al is +3

• where there are several possibilities the values go no higher than the Group No. Sn can be +2 or +4

Mn can be +2,+4,+6,+7

NON-METALS • mostly negative based on their usual ion Cl usually -1 • can have values up to their Group No. Cl +1 +3 +5 or +7

OXIDATION STATES

CALCULATING OXIDATION STATE - 2

The position of an element in the periodic table can act as a guide

OXIDATION STATES

Q. What is the theoretical maximum oxidation state of the following elements?

Na P Ba Pb S Mn Cr

What will be the usual and the maximum oxidation state in compounds of?

Li Br Sr O B N +1

Q. What is the theoretical maximum oxidation state of the following elements?

Na P Ba Pb S Mn Cr

What will be the usual and the maximum oxidation state in compounds of?

Li Br Sr O B N +1

METALS • have positive values in compounds

• value is usually that of the Group Number Al is +3

• where there are several possibilities the values go no higher than the Group No. Sn can be +2 or +4

Mn can be +2,+4,+6,+7

NON-METALS • mostly negative based on their usual ion Cl usually -1 • can have values up to their Group No. Cl +1 +3 +5 or +7

CALCULATING OXIDATION STATE - 2

The position of an element in the periodic table can act as a guide

OXIDATION STATES

CALCULATING OXIDATION STATE - 2

The position of an element in the periodic table can act as a guide

A. What is the theoretical maximum oxidation state of the following elements?

Na P Ba Pb S Mn Cr+1 +5 +2 +4 +6 +7 +6

What will be the usual and the maximum oxidation state in compounds of?

Li Br Sr O B NUSUAL +1 -1 +2 -2 +3 -3 or +5MAXIMUM +1 +7 +2 +6 +3 +5

manganese(IV) oxide shows that Mn is in the +4 oxidation state in MnO2

sulphur(VI) oxide for SO3 S is in the +6 oxidation state

dichromate(VI) for Cr2O72- Cr is in the +6 oxidation state

phosphorus(V) chloride for PCl5 P is in the +5 oxidation state

phosphorus(III) chloride for PCl3 P is in the +3 oxidation state

OXIDATION STATES

THE ROLE OF OXIDATION STATE IN NAMING SPECIES

To avoid ambiguity, the oxidation state is often included in the name of a species

Q. Name the following... PbO2

SnCl2

SbCl3

TiCl4

BrF5

OXIDATION STATES

Q. Name the following... PbO2 lead(IV) oxide

SnCl2 tin(II) chloride

SbCl3 antimony(III) chloride

TiCl4 titanium(IV) chloride

BrF5 bromine(V) fluoride

manganese(IV) oxide shows that Mn is in the +4 oxidation state in MnO2

sulphur(VI) oxide for SO3 S is in the +6 oxidation state

dichromate(VI) for Cr2O72- Cr is in the +6 oxidation state

phosphorus(V) chloride for PCl5 P is in the +5 oxidation state

phosphorus(III) chloride for PCl3 P is in the +3 oxidation state

THE ROLE OF OXIDATION STATE IN NAMING SPECIES

To avoid ambiguity, the oxidation state is often included in the name of a species

REDOX When reduction and oxidation take place

OXIDATION Removal (loss) of electrons ‘OIL’species will get less negative or more positive

REDUCTION Gain of electrons ‘RIG’species will become more negative or less positive

REDOX REACTIONS

OXIDATION AND REDUCTION IN TERMS OF ELECTRONS

Oxidation and reduction are not only defined as changes in O and H

+7+6+5+4+3+2+1 0-1-2-3-4

REDUCTION

OXIDATION

REDOX When reduction and oxidation take place

OXIDATION Removal (loss) of electrons ‘OIL’species will get less negative or more positive

REDUCTION Gain of electrons ‘RIG’species will become more negative or less positive

REDUCTION in O.N. Species has been REDUCED

e.g. Cl is reduced to Cl¯ (0 to -1)

INCREASE in O.N. Species has been OXIDISED

e.g. Na is oxidised to Na+ (0 to +1)

REDOX REACTIONS

OXIDATION AND REDUCTION IN TERMS OF ELECTRONS

Oxidation and reduction are not only defined as changes in O and H

+7+6+5+4+3+2+1 0-1-2-3-4

REDUCTION

OXIDATION

REDUCTION in O.S. INCREASE in O.S.Species has been REDUCED Species has been OXIDISED

REDOX REACTIONS

OXIDATION AND REDUCTION IN TERMS OF ELECTRONS

Q. State if the changes involve oxidation (O) or reduction (R) or neither (N)

Fe2+ —> Fe3+

I2 —> I¯

F2 —> F2O

C2O42- —> CO2

H2O2 —> O2

H2O2 —> H2O

Cr2O72- —> Cr3+

Cr2O72- —> CrO4

2-

SO42- —> SO2

REDOX REACTIONS

REDUCTION in O.S. INCREASE in O.S.Species has been REDUCED Species has been OXIDISED

OXIDATION AND REDUCTION IN TERMS OF ELECTRONS

Q. State if the changes involve oxidation (O) or reduction (R) or neither (N)

Fe2+ —> Fe3+ O +2 to +3 I2 —> I¯ R 0 to -1

F2 —> F2O R 0 to -1

C2O42- —> CO2 O +3 to +4

H2O2 —> O2 O -1 to 0

H2O2 —> H2O R -1 to -2

Cr2O72- —> Cr3+ R +6 to +3

Cr2O72- —> CrO4

2- N +6 to +6

SO42- —> SO2 R +6 to +4

OXIDATION STATES - Review

CALCULATING OXIDATION STATE – MOST IMPORTANT

Q. What is the oxidation state of each element in the following compounds/ions ?

CH4

PCl3

NCl3

CS2

ICl5

BrF3

PCl4+

H3PO4

NH4Cl

H2SO4

MgCO3

SOCl2

OXIDATION STATES

CALCULATING OXIDATION STATE - 2

Q. What is the oxidation state of each element in the following compounds/ions ?

CH4 C = - 4 H = +1

PCl3 P = +3 Cl = -1

NCl3 N = +3 Cl = -1

CS2 C = +4 S = -2

ICl5 I = +5 Cl = -1

BrF3 Br = +3 F = -1

PCl4+ P = +5 Cl = -1

H3PO4 P = +5 H = +1 O = -2

NH4Cl N = -3 H = +1 Cl = -1

H2SO4 S = +6 H = +1 O = -2

MgCO3 Mg = +2 C = +4 O = -2

SOCl2 S = +4 Cl = -1 O = -2