Oxidation and Reduction

• Physics
• Secondary Education
• Age 12,13

22 March 2023

Chapter 10. Oxidation and Reduction

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

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

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?

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?

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^- → 2O^2-

Redox reactions can be identified by the changes in the oxidation number when a reactant goes to a product.

Substance is oxidised when 

• it has lost electrons

Ag → Ag⁺ + e^-

Substance is reduced when

• it has gained electrons

O₂ + 4e^- → 2O^2-

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 

22 March 2023

Chapter 10. Oxidation and Reduction

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

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

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

Practice

Test for reducing agents

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)

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

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?

Plenary

What's next?

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

 Identify redox reactions as reactions involving gain and loss of oxygen

 Identify oxidation and reduction in redox reactions

 Identify redox reactions as reactions involving gain and loss of oxygen

 Identify oxidation and reduction in redox reactions

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

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)