Unit 3, Topic 1:

Electron Transfer Reactions

Lesson Instructions

Review the learning goals and success criteria for Topic 1: Electron Transfer Reactions.
- Review the minds on, action, and consolidation sections of the lesson, completing any tasks as outlined.
- Complete the fill-in-the-blanks handouts that have been provided to you. They have been linked below.
- Practice and apply what you've learned by completing the practice tasks outlined at the end of the lesson.

Learning Goals

I am learning to ...

- Identify the reactant being oxidized and reduced in a redox reaction.
- Assign oxidation numbers to determine if a chemical reaction is a redox reaction.
- Identify the reducing and oxidizing agents in a redox reaction.

Minds On

The interconnection between electricity and chemical reactions is called electrochemistry. Specifically, electrochemistry deals with the transfer of electrons from one substance to another. These reactions can produce electricity in a spontaneous and a non-spontaneous manner. In this unit you will learn about two important chemical reactions involving the transfer of electrons: oxidation and reduction. You will also learn about how the energy is harvested from oxidation and reduction reactions using special cells called galvanic and electrolytic cells.

Electron transfer occurs by two important reactions called oxidation and reduction reactions. These reactions often occur in conjunction with one another and hence are termed REDOX reactions. In this lesson you will learn how to identify oxidation and reduction reactions.

Action

OXIDATION

- the process by which one or more electrons is lost by a chemical entity

REDUCTION

- the process by which one or more electrons is gained by a chemical entity

OXIDATION-REDUCTION (REDOX) REACTION

- the reaction in which one or more electrons are transferred between chemical entities

The following acronym, OIL RIG, can be used to help us remember where electrons are transferred to and from with respect to oxidation and reduction.

Example 1.

When copper wire is placed in a solution of silver nitrate, the copper atoms in the wire are displaced by silver ions in solution. The blue tint of the solution is due to copper ions, and the fuzzy coating on the wire is the silver metal.

Step 1.

Write a balanced chemical equation for the reaction between copper solid and aqueous silver nitrate.

Cu_(s) + 2 AgNO_3(aq) → 2 Ag_(s) + Cu(NO₃)_2(aq)

Step 2.

Write a total ionic equation for the reaction between copper solid and aqueous silver nitrate.

Cu_(s) + 2 Ag⁺_(aq) + 2 NO₃^-_(aq) → 2 Ag_(s) + Cu²⁺_(aq) + 2 NO₃^-_(aq)

Step 3.

Write a net ionic equation for the reaction between copper solid and aqueous silver nitrate. To go from a total ionic equation to a net ionic equation, you need to cancel out like terms (identical entities that are reactants and products).

Cu_(s) + 2 Ag⁺_(aq) → 2 Ag_(s) + Cu²⁺_(aq)

Step 4.

Redox reactions can be broken into two half-reactions. This will allow us to identify the total number of electrons lost and gained. In a chemical equation, the total number of electrons gained in a reaction, must equal the total number of electrons lost.

Oxidation Half-Reaction

Cu_(s) → Cu²⁺_(aq) + 2e^-

Reduction Half-Reaction

Ag⁺_(aq) + e^- → Ag_(s)

Example 2.

Write the half-reaction equations for the redox reaction that occurs when aluminum is placed in a silver nitrate solution.

Step 1: Write a balanced chemical equation.
Step 2: Write a total ionic equation.
Step 3: Write a net ionic equation.
Step 4: Write the oxidation and reduction half reactions.

OXIDATION NUMBERS

- a number used to keep track of electrons in redox reactions
- also known as oxidation state

Definition:

- the apparent net electric charge that an atom would have if the electron pairs in covalent bonds belonged entirely to the more electronegative atom

RULES FOR ASSIGNING OXIDATION NUMBERS

APPLYING OXIDATION NUMBERS TO REDOX REACTIONS

OXIDATION - increase in oxidation number

- Oxidation is the LOSS of electrons. Each electron has a negative charge associated with it. As an entity loses electrons, its net charge becomes more positive, and thus, the oxidation number increases.

REDUCTION - decrease in oxidation number

- Reduction is the GAIN of electrons. Each electron has a negative charge associated with it. As an entity gains electrons, its net charge becomes more negative, and thus, the oxidation number decreases.

*** If there is no change in oxidation number, then a REDOX reaction does not take place.

*** In other words, the reaction can only be considered REDOX if there is a change in oxidation numbers.

EXAMPLE 3.

Determine whether the following reactions are redox reactions. If so, identify what is being oxidized and reduced.

a) CH₄ + Cl₂ → CH₃Cl + HCl

b) CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂

OXIDIZING AND REDUCING AGENTS

OXIDIZING AGENT

- Is the REACTANT that is REDUCED.
- Associated with a GAIN in electrons.
- An oxidizing agent gains electrons and is reduced in a chemical reaction. Also known as the electron acceptor, the oxidizing agent is normally in one of its higher possible oxidation states because it will gain electrons and be reduced.
- An oxidizing agent oxidizes other substances and gains electrons; therefore, its oxidation state decreases.

REDUCING AGENT

- Is the REACTANT that is OXIDIZED
- Associated with a LOSS in electrons.
- A reducing agent loses electrons and is oxidized in a chemical reaction. A reducing agent is typically in one of its lower possible oxidation states, and is known as the electron donor. A reducing agent is oxidized, because it loses electrons in the redox reaction.
- A reducing agent reduces other substances and loses electrons; therefore, its oxidation state increases.

EXAMPLE 4.

Identify the oxidizing and reducing agents in each of the following.

a) 3 H₂S + 2 NO₃^- + 2 H⁺ → 3 S + 2 NO + 4 H₂O

b) MnO₂ + 4 HBr → MnBr₂ + Br₂ + 2 H₂O

Consolidation

Try out some of these extra practice questions on redox reactions:

(1) Identify the reactant oxidized and the reactant reduced in the following reaction. (Show Answer)

Pb_(s)+ Cu(NO₃)_2(aq)→ Cu_(s)+ Pb(NO₃)_2(aq)

(2) Identify the reactant oxidized and the reactant reduced in the following reactions. (Show Answer)

a) CuCl_2(aq)+ Zn_(s)→Cu_(s)+ ZnCl_2(aq)

b) 2Ag_(s)+ S_(s)→ Ag₂S_(s)

c) CuSO_4(aq)+Mg_(s)→ Cu_(s)+MgSO_4(aq)

(3) Determine the oxidation number of the nitrogen atom in lithium nitrate, LiNO₃. (Show Answer)

(4) Assign oxidation numbers to each atom in the following compounds. (Show Answer)

NiCl₂, Mg_(s),TiO₄, K₂Cr₂O₇, SO₃^2-

(5) Assign oxidation numbers to the bolded atoms. (Show Answer)

a) ClO₄^-

b) CrCl₃

c) SnS₂

d) Au(NO₃)₃

(6) Assign oxidation numbers to the elements in the following compounds. (Show Answer)

a) SnCl₄

b) MnO₄^-

c) MnO₂

(7) Answer the following question. (Show Answer)

- Molybdenum disulphide, MoS₂, has a structure that allows it to behave as a dry lubricant, much like graphite. What are the oxidation numbers of the atoms in MoS₂?

(8) Determine if a REDOX reaction has occurred. (Show Answer)

a) Zn _(s)+ S _(s)→ ZnS _(s)

b) SO₃ _(g)+ H₂O _(l)→ H₂SO_{4 (aq)}

(9) Determine if a REDOX reaction has occurred. (Show Answer)

This lesson is now complete. Consolidate your learning by evaluating the self-check below and the assigned practice questions.

Self-Check

(This is a self-assessment of your learning – you do not need to submit this).

How prepared am I to start my homework? Can I ...

- identify the reactant being oxidized and reduced in a redox reaction?
- assign oxidation numbers to determine if a chemical reaction is a redox reaction?
- identify the reducing and oxidizing agents in a redox reaction?

Practice

Using your textbook, complete the following questions:

- p601 #1a, 4
- p604 #1-4
- p606 #1b, 3
- p607 #1, 5, 6ab, 7-10

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