Oxidation and Reduction

22 March 2023
Chapter 10. Oxidation and Reduction
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22 March 2023
Chapter 10. Oxidation and Reduction

Slide 1 - Diapositive

Content objectives

  • Define oxidation in terms of: (a) loss of electrons ; (b) an increase in oxidation number
  • Define reduction in terms of: (a) gain of electrons ; (b) a decrease in oxidation number
  • Identify redox reactions as reactions involving gain and loss of electrons
  • Identify redox reactions by changes in oxidation number using: (a) the oxidation number of elements in their uncombined state is zero ; (b) the oxidation number of a monatomic ion is the same as the charge on the ion; (c) the sum of the oxidation numbers in a compound is zero; (d) the sum of the oxidation numbers in anion is equal to the charge on the ion

Slide 2 - Diapositive

Define oxidation in terms of: (a) loss of electrons ; (b) an increase in oxidation number
Define reduction in terms of: (a) gain of electrons ; (b) a decrease in oxidation number
Oxidation
  • loss of electrons
Reduction
  • gain of electrons

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Identify redox reactions as reactions involving the gain and loss of electrons
  • zinc + copper sulfate → zinc sulfate + copper
  • Zn     +  CuSO4               → ZnSO4            + Cu
  • Zn(s) + Cu2+(aq) + SO42-(aq) →Zn2+(aq) + SO42-(aq) + Cu(s)
Remember:
  • Spectator ions (those that do not change) are SO42-(aq). These can be removed, leaving behind the ionic equation.
  • ionic equation:      Zn0(s) + Cu2+(aq) → Zn2+(aq) + Cu0(s)
What are the ionic half equations?

Slide 4 - Diapositive

Identify redox reactions as reactions involving the gain and loss of electrons
  • ionic equation:      Zn0(s) + Cu2+(aq) → Zn2+(aq) + Cu0(s)
What are the ionic half equations?

Slide 5 - Diapositive

Oxidation
  • loss of electrons
  • oxidation number of element is increased
Ag → Ag+ + e-
Reduction
  • gain of electrons
  • oxidation number of element is decreased
O2 + 4e- → 2O2-
Redox reactions can be identified by the changes in the oxidation number when a reactant goes to a product.

Slide 6 - Diapositive

Substance is oxidised when 
  • it has lost electrons
Ag → Ag+ + e-
Substance is reduced when
  • it has gained electrons
O2 + 4e- → 2O2-

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Slide 8 - Diapositive

Practice
  • The equation for the reaction between chlorine and potassium iodide is shown below. 

Cl2 + 2KI → 2KCl + I2

Identify which species has been:

a) oxidised

b) reduced 
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Slide 9 - Diapositive

22 March 2023
Chapter 10. Oxidation and Reduction

Slide 10 - Diapositive

Oxidising agent
  • a substance that oxidises another substance and becomes reduced in the process
  • gains electrons as another substance loses electrons
  • Common examples include hydrogen peroxide, fluorine, and chlorine

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Reducing agent
  • a substance that reduces another substance, and becomes oxidised in the process
  • loses electrons as another substance gains electrons
  • Common examples include carbon and hydrogen

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Identifying Redox Reactions by Colour Changes
  • observation of a colour change in the solution being analysed
  • acidified potassium manganate(VII), KMnO4 - oxidising agent which is often used to test for the presence of reducing agents
  • potassium iodide, KI - a reducing agent which is often used to test for the presence of oxidising agents

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Practice
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Test for reducing agents

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Test for oxidising agents
When added to an acidified solution of an oxidising agent such as aqueous chlorine or hydrogen peroxide (H2O2), the solution turns a red-brown colour due to the formation of iodine, I2
solution
2KI (aq) + H2SO4 (aq) + H2O2 (aq) → I2 (aq) + K2SO4 (aq) + 2H20 (l)

Slide 16 - Diapositive

STEPS
  • Step 1 - Write half equations to work out what has gained/lost electrons
  • Step 2 - Deduce what has been oxidised / reduced (remember OIL RIG)
  • Step 3 - Identify the reducing agent

Slide 17 - Diapositive

Try to apply the steps!
  • When iron reacts with bromine to form iron(II) bromide, a redox reaction occurs:

Fe + Br2 → FeBr2

What is acting as the reducing agent in this reaction?
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Plenary

Slide 19 - Diapositive

What's next?

Slide 20 - Diapositive

Define redox reactions as involving simultaneous oxidation and reduction.
Define oxidation as the gain of oxygen and reduction as the loss of oxygen.
Oxidation
zinc oxide + carbon → zinc  + carbon monoxide
ZnO      +  C         →  Zn   +   CO
  • gain of oxygen
Reduction
  • loss of oxygen

Slide 21 - Diapositive

 Identify redox reactions as reactions involving gain and loss of oxygen
 Identify oxidation and reduction in redox reactions

Slide 22 - Diapositive

 Identify redox reactions as reactions involving gain and loss of oxygen
 Identify oxidation and reduction in redox reactions

Slide 23 - Diapositive

Use a Roman numeral to indicate the oxidation number of an element in a compound
FeO       - Name: Iron (II) oxide
Fe2O3   - Name: Iron (III) oxide
CuCl2    - Name: Copper (II) chloride
CuCl    - Name: Copper (I) chloride

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Slide 25 - Diapositive

Define oxidation in terms of: (a) loss of electrons ; (b) an increase in oxidation number
Define reduction in terms of: (a) gain of electrons ; (b) a decrease in oxidation number
Oxidation Number 
  • (also called oxidation state) is a number assigned to an atom or ion in a compound that indicates the degree of oxidation (or reduction)
  • shows the number of electrons that an atom has lost, gained, or shared in forming a compound
  • helps you to keep track of the movement of electrons in a redox process
  • written as a +/- sign followed by a number (not to be confused with charge, which is written by a number followed by a +/- sign)

Slide 26 - Diapositive