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Redox & Electrochemistry. C 3 H 8 O + CrO 3 + H 2 SO 4  Cr 2 (SO 4 ) 3 + C 3 H 6 O + H 2 O. What’s the point ?. REDOX reactions are important in …. Electrical production (batteries, fuel cells). Purifying metals (e.g. Al, Na, Li). Producing gases (e.g. Cl 2 , O 2 , H 2 ).

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Redox & Electrochemistry


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what s the point

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

What’s the point ?

REDOX reactions are important in …

  • Electrical production (batteries, fuel cells)
  • 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)
what s the point3
What’s the point ?

REDOX reactions are important in …

  • Manufacture of ammonia for fertilizers
  • Haber Bosch Process
  • N2 + H2 2NH3
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 OILRIG
  • Oxidation Is Loss
  • Reduction Is Gain
what is redox5
What is Redox?
  • Loss Electrons = Oxidation
  • Gain Electrons = Reduction
  • Another mnemonic you may like
  • remember LEO says GER
testing concepts
Testing concepts

Q- 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–

testing concepts7
Testing concepts

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)

testing concepts8
Testing concepts

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

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

testing concepts9
Testing concepts

Q- Define oxidizing agent, reducing agent.

A- An oxidizing agent causes oxidation by being reduced itself

A reducing agent causes reduction by being oxidized itself

testing concepts10
Testing concepts

Q- Identify oxidizing and reducing agents in reactions.

A- CaCl2 is an ionic compound with a positive calcium ion and negative chlorine ions

Ca + 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 numbers
Oxidation 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
oxidation numbers12
Oxidation numbers
  • E.g. in HCl, Cl has a higher electronegativity (EN pg. 426). 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
rules and rationale
Rules (and rationale)

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 CO32– the oxidation number of C is +4)

rules and rationale 12 2
Rules (and rationale - 12.2)

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

rule

total

Ox.#

or rule 5

6

3

6

6

3

6

4

3

6

3

6

6

3

5

+2

+5

-8

+1

+5

-6

+2

+12

-14

-4

+6

-2

+1

-1

+1

+5

-2

+1

+5

-2

+1

+6

-2

-2

+1

-2

+1

-1

H2PO4–

HNO3

K2Cr2O7

C2H6O

AgI

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

more practice
More practice
  • Section A9 page 429 in Chem Com text.

For more lessons, visit www.chalkbored.com