Ms StephensSaturday, 22 April 2023

Redox Introduction

Oxidation NumbersReview of KeywordsIdentifying Redox ReactionsCommon OxidantsCommon Reductants

Oxidation Numbers1. Any element, oxidation # of zero. 2. An Ion = its charge3.The sum O # = charge for the formula.4. Group 1 metals is +1, group 2 is +2, and aluminum (in group III) is +35. nonmetal or polyatomic ion = charge6. F =–1, O = –2 , H = +1 (almost always)

+1

6

+1

-2

6

-6+5

3

+5 +1

4

+2

-2

6

-14+12

3

+6 +1

6

+6

-2

6

-2-4

3

-2 -1

5

-1+1

3

+1

H N O 3 C2H6OK2Cr2O7 AgI+1

6

+2

-2

6

-8+5

3

+5

H2PO4–

ruletotalOx.#

Why bother with Oxidation #s?

Sign of oxidation numberPositive has less control over it’s electrons in a bond (less electronegative)Negative has more control over it’s electrons in a bond (more electronegative)

Value of oxidation NumberIndicates the number of electrons involved in bondingOxidation is regarded as an increase in oxidation number of an atom in an element ion or compound

Says…

Some Keywords

Oxidation – a chemical reaction which results in the loss of electrons from a chemical speciesReduction – a chemical reaction which results in a chemical species gaining electrons

Oxidant – a chemical species that has the ability to take electrons from other chemicals – i.e. it causes oxidationReductant – a chemical species that has the ability to give electrons to another species – i.e. it causes reduction

Electrolysis – the process by which ionic compounds are split into their atoms using electric currents

Identifying Redox Reactions

A redox reaction must result in the movement of electrons from one element to another.To identify whether this is happening we must prove that one element is changing it’s oxidation number For example:

SO2 → SO3

Oxidation Number = +4 Oxidation Number = +6

Therefore, an oxidation number change has occurredIncrease means it is Oxidation

Common Oxidants

Oxygen: O2 + 4e- → 2O2-

Permanganate: MnO4- + 8H+ + 5e- → Mn2+ + 4H2O

Dichromate: Cr2O72-

+ 14H+ +6e- → 2Cr3+ + 7H2O

Iron(III) ion: FeFe3+3+ + e- → FeFe2+2+

Halogens: I2 + 2e- 2I-

Cl2 + 2e- 2Cl-

Hydrogen peroxide: H2O2 + 2H+ + 2e- → 2H2O

Common Reductants

Hydrogen: H2 → 2H+ + 2e-

Thiosulfate: 2S2O32- → S4O6

2- + 2e-

Iron(II) ion: FeFe2+ 2+ → FeFe3+3+ + e-

Halides (Halogen ions): 2X- → X2 + 2e-

Zinc: Zn → Zn2+ + 2e-

Sulfur dioxide: SO2 + 2H2O→ SO42- + 4H+ + 2e-

Practice Questions

1.Find the oxidation numbers of the N atoms in each of the molecules.NO2, HNO3, NO, N2, N2O,

2.(a) Calculate the oxidation number of S in each of these ions:

I) SO42, II)SO3

2, III) S2O32, IV) S4O6

2

2.(b) By assigning oxidation numbers show which of the following reactions is not a redox reaction.

(i) CuCO3 CuO + CO2

(ii) Cu + 2AgNO3 Cu(NO3)2 + 2Ag

(iii) Cr2O72 + 6Fe2+ 6Fe3++ 2Cr3+ + 7 H2O

(iv) Cr2O72 + 2OH 2 CrO4

2 + H2O

Answers

1. +4 +5 +2 0 +1

2. a) I. +6 II. +4 III. +2 IV.

+2.5 b) I no II. yes III. yes IV. no