Oxidation States

Review:Redox reactions involve one substance being

oxidised and another reduced.What is oxidation?What is reduction?

Review:

• Oxidation always involves loss of e-; it can also involve the gain of oxygen and/or loss of hydrogen

• Reduction always involves gain of e-; it can also involve loss of oxygen and/or gain of hydrogen.

• The oxidation state describes the extent to which an element has been oxidised or reduced.

• The oxidation state can be identified by oxidation numbers.

• Oxidation numbers can be seen as a method to account for electrons taking part in a reaction.

• Oxidation numbers can be used to establish what element has been oxidised and what element has been reduced in a reaction.

• There is an increase in oxidation number with oxidation

• A decrease in oxidation number with reduction

• Oxidation numbers are assigned to elements.

www.youtube.com/watch?v=EHe8-AFMsMA

Short Video clip on the Oxidation Number trends in the Periodic Table.

Oxidation number rulesRules Examples

1.When atoms exist as elements they have an O.N of 0.

Na, Cl2, Ne, C & H2 all have an O.N of zero.

2. The oxidation number of a monotomic ion is the same as the charge on the ion.

Cu2+ has O.N +2, Cl- has O.N -1.

3. Hydrogen in compounds has an O.N of +1 (except in metal hydrides, where its O.N is -1)

H in H2O, CH4, NH3 has O.N +1 (In NaH the O.N of H is -1)

4. Oxygen in compounds has an O.N of -2 (except in H2O2, where its O.N is -1)

Oxygen in MgO, H2SO4, H2O & KMnO4 has O.N -2, in H2O2, O.N of O is -1

5. For polyatomic ions the sum of the O.N’s equals the charge of the ion.

For NH4+, the sum of

the oxidation numbers is +1, for SO4

2- the sum is -2.

6. The sum of the O.N’s in a molecule is zero.

The sum of the O.N’s for the elements in H2SO4, C4H10 & H2O is zero.

Examples:• Calculate the ON of nitrogen in the following

compounds

O = -2 x 2 = -4

Charge = -1

N = +3

NO2-

NH3

N2O

NO

HNO3

Review:

• An increase in oxidation number is…

• A decrease in oxidation number is…

Recognising oxidation & reduction

PbO2(s) + C(s) -> Pb(s) + CO2(g)

?,2(-2) 0 0 ?,2(-2)4,2(-2) 0 0 4,2(-2)Carbon oxidised(ON increased from 0 to 4)Lead oxide reduced(ON reduced from 4 to 0)

• Mg(s) + 2H+ -> Mg2+(aq) + H2(g)

0 2(+1) +2 0

Magnesium oxidised (ON, increased 0 to 2)Hydrogen ions reduced (ON, decreased +1 to 0)

H+(aq) + OH-(aq) -> H2O(l)Assign oxidation numbers+1 (-2+1) (2+1)(-2)

no change in oxidation number .. Not redox

Oxidation number quick check. Calculate the oxidation number of the

bold element

• Mn2+

• MnO2

• MnO4-

• S2Cl2• SCl2• SO2

• SO3

• S2O32-

• Cr2O72-

+2+4+7+1+2+4+6+2+6

Work out the Oxidation state of each atom

• MnO4- Mn2+

• Cr2072- Cr3+

• Cl2 Cl-

• H2O2 H2O

• MnO2 Mn2+

• NO3- NO2

• NO3- NO

Oxidants and Reductants

• An oxidant is defined as an electron acceptor.• An oxidant can also be called an oxidising agent.• An oxidant is therefore reduced.

• A reductant is defined as an electron donor.• A reductant can also be called a reducing agent.• A reductant is therefore oxidised.

Review:• What is the oxidation number of:

a.) Manganese in manganese dioxide, MnO2.

b.) Nitrogen in nitrate ions, NO3-

• Give the oxidation number of each type of atom present in the following:a.) Mg b.) SO2 c.) MgO d.) S8 e.) Ag+ f.)NaHCO3 g.)H2SiO3

• Element M has O.N +3. The most likely formula for the oxide of M is:a.) MO b.) M2O c.) M2O3 d.) MO2

Recall…

• The reactant which is oxidised is called the reductant (or reducing agent) since it reduces the other reactant.

• The reactant which is reduced is called the oxidant (or oxidising agent) since it oxidises the other reactant.

e.g. 2Mg(s) + O2(g) 2MgO(s)Mg is the because .Oxygen is the because .

Oxidation in terms of oxygen transfer

Oxidation is the gain of oxygen, reduction is the loss of oxygen

Example:2Fe2O3(s) + 3C(s) 4Fe(s) + 3CO2(g)

oxygen transfer• Fe2O3 is reduced since it loses oxygen• C is oxidised since it gains oxygen to form CO2• Fe2O3 is the oxidant and C is the reductant.

Oxidation in terms of hydrogen transfer

Oxidation is the loss of hydrogen, reduction is the gain of hydrogen

Example:2H2S(g) + O2(g) 2S(s) + 2H2O(l)

Hydrogen transferH2S is oxidised because H is lost

O2 is reduced because H is gained to form H2O

H2S is the reductant and O2 is the oxidant.

Oxidation in terms of electron transfer

Oxidation is loss of electrons, reduction is gain of electrons

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

2e-Zn metal loses electrons and is oxidised.Copper ions gain electrons to form copper metalZinc is the reductant and copper ions are the

oxidant.

Questions

1. For the reaction 2PbO + C 2Pb + CO2a.) Use the transfer of oxygen definition of redox to identify the oxidant and reductant.b.) Use oxidation numbers to show the same oxidant and reductant would be identified.

Explain your answers.

Questions

2. For the reaction Fe + Cu2+ Fe2+ + CO2a.) Use transfer of electrons to identify the oxidant and reductantb.) Show that, using oxidation numbers, the same oxidant and reductant would be identified.