Book cover for Objective Chemistry for Engineering and Medical Entrance Examinations

Objective Chemistry for Engineering and Medical Entrance Examinations

K Rama Rao

ISBN #9789332541771

1st Edition

2,463 Questions

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12,716 Students Helped

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Summary
Learning Objectives
Key Concepts
Example Problems
Explanations
Common Mistakes

Summary

Redox reactions are characterized by the simultaneous occurrence of oxidation and reduction, involving the transfer of electrons between chemical species. Key tools for understanding these reactions include oxidation numbers and the electrochemical series. Mastery of the differences between oxidation state, oxidation number, and valency is crucial to accurately analyze and predict electron flow in chemical processes.

Learning Objectives

1

Understand the definition and fundamental principles of redox reactions.

2

Explain the processes of oxidation and reduction, including electron transfer.

3

Utilize oxidation numbers to track electron movement in chemical reactions.

4

Apply the electrochemical series to predict the reactivity of different elements.

5

Differentiate between terms such as oxidation state, oxidation number, and valency.

Key Concepts

CONCEPT

DEFINITION

Redox Reaction

A chemical process that involves simultaneous oxidation (loss of electrons) and reduction (gain of electrons).

Oxidation

The process in which a chemical species loses electrons.

Reduction

The process in which a chemical species gains electrons.

Oxidation Number

A numerical value assigned to an element in a compound that represents its degree of oxidation, useful for keeping track of electron transfer.

Electrochemical Series

A ranking list of elements based on their propensity to lose electrons, which aids in predicting the reactivity of substances in redox reactions.

Oxidation State

Often used interchangeably with oxidation number, though sometimes it refers to the actual charge on an ion in a compound.

Valency

The combining capacity of an element, indicating how many electrons an atom can gain, lose, or share in a reaction.

Example Problems

Example 1

Which of the following is a redox reaction? (1) $\mathrm{AlH}_{3}(\mathrm{~g})+\mathrm{H}^{-}(\mathrm{g}) \longrightarrow \mathrm{A} \mathrm{lH}_{4}^{-}(\mathrm{g})$ (2) $\mathrm{Al}_{2} \mathrm{Cl}_{6}(\mathrm{~g}) \longrightarrow \mathrm{AlCl}_{3}(\mathrm{~g})$ (3) $\mathrm{Al}^{3+}(\mathrm{aq})+\mathrm{OH}^{-}(\mathrm{aq}) \longrightarrow \mathrm{Al}(\mathrm{OH})_{3}(\mathrm{~s})$ (4) $2 \mathrm{Al}_{(\mathrm{s})}+3 \mathrm{Cl}_{2}(\mathrm{~g}) \longrightarrow 2 \mathrm{~A} 1 \mathrm{Cl}_{3}(\mathrm{~g})$

Example 2

In which of the following changes there is a transfer of five electrons? (1) $\mathrm{MnO}_{4}^{-} \longrightarrow \mathrm{Mn}^{3+}$ (2) $\mathrm{CrO}_{4}^{2} \longrightarrow \mathrm{Cr}^{2}$ (3) $\mathrm{MnO}_{4} \longrightarrow \mathrm{MnO}_{2}$ (4) $\mathrm{Cr}_{7} \mathrm{O}_{7}^{2-} \longrightarrow 2 \mathrm{Cr}^{3+}$

Example 3

In which of the following reaction there is change in the oxidation state of chromium? (1) $2 \mathrm{~K}_{2} \mathrm{CrO}_{4}+\mathrm{II}_{2} \mathrm{SO}_{4} \longrightarrow \mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}+\mathrm{K}_{2} \mathrm{SO}_{4}+$ $\mathrm{II}_{2} \mathrm{O}$ (2) $\mathrm{K}_{2} \mathrm{CrO}_{4}+\mathrm{II}_{2} \mathrm{SO}_{4}+\mathrm{H}_{2} \mathrm{O}_{2} \longrightarrow \mathrm{K}_{2} \mathrm{SO}_{4}+\mathrm{CrO}_{5}+$ $3 \mathrm{II}_{2} \mathrm{O}$ (3) $\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}+6 \mathrm{II}_{2} \mathrm{SO}_{4}+4 \mathrm{NaCl} \longrightarrow 2 \mathrm{KIISO}_{4}+$ $4 \mathrm{NaIISO}_{4}+2 \mathrm{CrO}_{2} \mathrm{Cl}_{2}+3 \mathrm{II}_{2} \mathrm{O}$ (4) Nonc

Example 4

Oxidation number of carbon in $\mathrm{CHI}_{2} \mathrm{Cl}_{2}$ is (1) $-4$ (2) $+4$ (3) 3 (4) 0

Example 5

Oxidation number of carbon in $\mathrm{CH}_{2} \mathrm{Cl}_{2}$ is (1) $-4$ (2) $+4$ (3) 3 (4) 0
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Step-by-Step Explanations

QUESTION

How do you determine the electron transfer process in a redox reaction?

STEP-BY-STEP ANSWER:

Step 1: Identify the reactants involved in the reaction.
Step 2: Assign oxidation numbers to each element in the reactants and products.
Step 3: Determine which elements have an increase in oxidation number (oxidation) and which have a decrease (reduction).
Step 4: Confirm that the number of electrons lost in the oxidation process equals the number of electrons gained in the reduction process.
Final Answer: A redox reaction is validated by verifying that oxidation numbers change appropriately, indicating the transfer of electrons.

Redox Reaction

QUESTION

How can the electrochemical series be used to predict the reactivity of elements in redox reactions?

STEP-BY-STEP ANSWER:

Step 1: Consult the electrochemical series to find the relative positions of the elements involved.
Step 2: Identify the element that appears higher in the series, indicating a higher tendency to lose electrons.
Step 3: Recognize that the element with the lower position is more likely to gain electrons.
Step 4: Use this information to predict the direction of electron flow in the reaction.
Final Answer: The electrochemical series provides a framework for predicting which element will oxidize and which will reduce during a redox reaction.

Electrochemical Series

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Common Mistakes

  • Confusing oxidation state with oxidation number or valency.
  • Failing to account for both the oxidation and reduction processes, leading to incomplete reaction analysis.
  • Overlooking the balance of electrons transferred, resulting in unbalanced half-reactions.
  • Misinterpreting the electrochemical series by not recognizing the relative reactivity of elements.