Redox ReactionsRedox Reactions

Redox ReactionsRedox Reactions

Reactants: Zn + IReactants: Zn + I22 Product: Zn IProduct: Zn I22

Redox Equations:Redox Equations:At the conclusion of our time together, At the conclusion of our time together,

you should be able to:you should be able to:

1.1. Define redoxDefine redox2.2. Figure out oxidation numbers for any Figure out oxidation numbers for any

elementelement3.3. Show the change in oxidation numbers in a Show the change in oxidation numbers in a

reactionreaction

What’s the Point ?What’s the Point ?

• Electrical production (batteries, fuel cells)

REDOX reactions are important in …

• Purifying metals (e.g. Al, Na, Li)

• Producing gases (e.g. Cl2, O2, H2)

• Electroplating metals

• Protecting metals from corrosion• Balancing complex chemical equations

• Sensors and machines (e.g. pH meter)

C3H8O + CrO3 + H2SO4 Cr2(SO4)3 + C3H6O + H2O

What is Redox?What is Redox?

REDOX stands for REDuction/OXidation Oxidation is often thought of as a combination of

a substance with oxygen (rusting, burning) Oxidation refers to a loss of electrons Reduction refers to a gain of electrons As a mnemonic remember LEO says GER

• Lose Electrons = Oxidation• Gain Electrons = Reduction

Let’s See How You’re Doing??Let’s See How You’re Doing??

Q- What is oxidation? What is reduction? Q- What is oxidation? What is reduction? Represent each as a chemical equation.Represent each as a chemical equation.

oxidation oxidation = loss of e= loss of e–– … … X X X X+ + + e+ e– –

reduction reduction = gain of e= gain of e–– … … X + eX + e– – X X––

Q- Why are 2Na + ClQ- Why are 2Na + Cl22 2NaCl & 2H 2NaCl & 2H22 + O + O22 H H22O O considered redox reactions?considered redox reactions?

Both involve the transfer of electrons. (Na has no Both involve the transfer of electrons. (Na has no charge, the atoms in diatomic molecules have no charge, the atoms in diatomic molecules have no partial charge. After reaction the atoms have partial charge. After reaction the atoms have different shares of the electrons because of different shares of the electrons because of different EN values)different EN values)

Let’s See How You’re Doing??Let’s See How You’re Doing??

Q- Is it possible to oxidize a material without reducing Q- Is it possible to oxidize a material without reducing something else?something else?

No. A lost eNo. A lost e–– is taken up by something else. is taken up by something else.

Determination of Oxidation and ReductionDetermination of Oxidation and Reduction

If oxidation # decreased; If oxidation # decreased; substance reducedsubstance reduced

If oxidation # increased; If oxidation # increased; substance oxidized substance oxidized

Review of Oxidation NumbersReview of Oxidation Numbers

We will see that there is a simple way to keep track We will see that there is a simple way to keep track of oxidation and reductionof oxidation and reduction

This is done via “oxidation numbers”This is done via “oxidation numbers” An oxidation number is the charge an atom would An oxidation number is the charge an atom would

have if electrons in its bonds belonged completely to have if electrons in its bonds belonged completely to the more electronegative atomthe more electronegative atom

e.g. in HCl, Cl has a higher EN. Thus, oxidation e.g. in HCl, Cl has a higher EN. Thus, oxidation numbers are Cl = -1, H = +1numbers are Cl = -1, H = +1

1.1. Any element, when not combined with atoms of a Any element, when not combined with atoms of a different element, has an oxidation # of zero. different element, has an oxidation # of zero. (O in O(O in O22 is zero, Na by itself is zero) is zero, Na by itself is zero)

2.2. Any simple monatomic ion (one-atom ion) has an Any simple monatomic ion (one-atom ion) has an oxidation number equal to its charge oxidation number equal to its charge (Na(Na++ is +1, O is +1, O2–2– is –2) is –2)

3.3. The sum of the oxidation numbers of all of the The sum of the oxidation numbers of all of the atoms in a formula must equal the charge written atoms in a formula must equal the charge written for the formula. (if the oxidation number of O is –2, for the formula. (if the oxidation number of O is –2, then in COthen in CO33

2–2– the oxidation number of C is +4) the oxidation number of C is +4)

Review of Oxidation NumbersReview of Oxidation Numbers

4.4. In compounds, the oxidation # of IA metals is +1, In compounds, the oxidation # of IA metals is +1, IIA is +2, IIIA is +3, Zn & Cd is +2, Ag is +1.IIA is +2, IIIA is +3, Zn & Cd is +2, Ag is +1.

5.5. In ionic compounds, the oxidation # of a In ionic compounds, the oxidation # of a nonmetal or polyatomic ion is equal to the charge nonmetal or polyatomic ion is equal to the charge of its associated ion. of its associated ion. (MgCl(MgCl22, , Mg is +2, therefore Cl is –1)Mg is +2, therefore Cl is –1)

6.6. F is always –1, O is always –2 F is always –1, O is always –2 (unless combined with F), (unless combined with F),

H is usually +1, H is usually +1, exceptexcept when it is when it is bonded to metals in binary bonded to metals in binary

compounds. compounds. (ex. NaH, H oxidation # is (ex. NaH, H oxidation # is ––11 oror when it’s in when it’s in elemental form elemental form HH22, oxidation # is 0)., oxidation # is 0).

Review of Oxidation NumbersReview of Oxidation Numbers

NaIO3

Na = +1 O = -2

3x(-2) + 1 + ? = 0

Therefore, I = +5

IF7

F = -1

7x(-1) + ? = 0

Therefore, I = +7

Oxidation numbers of all the elements in the following ?

Redox Equations:Redox Equations:Let’s see if you can:Let’s see if you can:

1.1. Define redoxDefine redox2.2. Figure out oxidation numbers for any Figure out oxidation numbers for any

elementelement3.3. Show the change in oxidation numbers in a Show the change in oxidation numbers in a

reactionreaction

RuleRule(from earlier slide)(from earlier slide)

TotalTotalOx.#Ox.# +1

6+1

-2

6-6+5

3

+5 +1

4+2

-2

6-14+12

3

+6

or rule 5

+1

6+6

-2

6-2-4

3

-2 -1

5-1+1

4

+1H N O 3 C2H6OK2Cr2O7

AgI+1

6+2

-2

6-8+5

3

+5H2PO4

–

Let’s Apply the Oxidation RulesLet’s Apply the Oxidation Rules

Corrosion – Deterioration of Metals by Corrosion – Deterioration of Metals by Electrochemical ProcessElectrochemical Process

Determine and balance the reaction, determine what is oxidized and what is reduced.

Corrosion of SilverCorrosion of Silver

Ag + O2 Ag2O

4 Ag + O2 2 Ag2O

Each Ag loses 1e--

Each O gains 2e--

Silver = oxidizedOxygen = reducedSilver = reducing agentOxygen = oxidizing agent