Unit 7: Redox & Unit 7: Redox &

ElectrochemistryElectrochemistry

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)• Just like with acid/base definitions the

definition of oxidation is expanded• Oxidation refers to a loss of electrons• Reduction refers to a gain of electrons• As a mnemonic remember LEO says GER• Loss Electrons = Oxidation• Gain Electrons = Reduction• Read 12.1 (pg. 443 - 445)

Testing conceptsTesting conceptsQ- What is oxidation? What is reduction? Represent

each as a chemical equation.A- oxidation = loss of e– … X X+ + e–

reduction = gain of e– … X + e– X– Q- Why are 2Na + Cl2 2NaCl & 2H2 + O2 H2O

considered redox reactions?A- Both involve the transfer of electron density (Na

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

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

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

Testing conceptsTesting conceptsQ- Define oxidizing agent, reducing agent.A- An oxidizing agent causes oxidation by being

reduced itselfA reducing agent causes reduction by being oxidized itself

Q- PE 1A- CaCl2 is an ionic compound with a positive

calcium ion and negative chlorine ionsCa + Cl2 CaCl2 Ca Ca2+ + 2e–, Cl2 + 2e– 2Cl–. Thus Ca is losing electrons (oxidation) and Cl is gaining electrons (reduction).

These are called “half reactions”

Oxidation numbersOxidation numbers• We will see that there is a simple way to keep

track of oxidation and reduction• This is done via “oxidation numbers”• An oxidation number is the charge an atom

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

• E.g. in HCl, Cl has a higher EN (pg. 255). Thus, oxidation numbers are Cl = -1, H = +1

• Notice that oxidation numbers are written as +1 vs. 1+ to distinguish them from charges.

• Instead of referring to EN chart, a few rules are followed to assign oxidation numbers

• Refer also to study note

Rules (and rationale - 12.2)Rules (and rationale - 12.2)1.Any element, when not combined with atoms

of a different element, has an oxidation # of zero. (O in O2 is zero)

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

3.The sum of the oxidation numbers of all of the atoms in a formula must equal the charge written for the formula. (if the oxidation number of O is –2, then in CO3

2– the oxidation number of C is +4)

4. In compounds, the oxidation # of IA metals is +1, IIA is +2, and aluminum (in IIIA) is +3

5. In ionic compounds, the oxidation # of a nonmetal or polyatomic ion is equal to the charge of its associated ion. (CuCl2, Cl is –1)

6.F is always –1, O is always –2 (unless combined with F), H is usually +1

ruletotalOx.#

Rules (and rationale - 12.2)Rules (and rationale - 12.2)

+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

3

+1

PE 2 (450), 12.9, 12.12 (484), 12.10, 12.11, 12.13 (484)

H N O 3 C2H6OK2Cr2O7 AgI+1

6+2

-2

6-8+5

3

+5H2PO4

–

More practiceMore practicePE 2 pg. 450: answers in back of book

12.9, 12.12 (pg. 484): answers in back of book

12.10: a) Cl=+7, b) Cr=+3, c) Sn=+4, d) Au=+3

12.11: a) Na= +1, H = +1, P= +5, O= -2

b) Ba= +2, Mn= +6, O= -2

c) Na= +1, S= +2.5, O= -2

d) Cl= +3, F= -1

12.13: a) +2, b) +4, c) +3, d) +5, e) -2, f) 0,

g) –1, h) -3 I) –1/3 For more lessons, visit www.chalkbored.com