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  1. AP Chemistry
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Coupled Reactions

Ethan Taylor

Ethan Taylor

7 min read

Next Topic - Galvanic (Voltaic) and Electrolytic Cells
Study Guide Overview

This study guide covers thermodynamically favorable and unfavorable reactions, including spontaneity based on ΔG° and K. It explains how electrolytic cells drive nonspontaneous reactions with electricity. It also details coupled reactions, using a spontaneous reaction to drive a nonspontaneous one via a common intermediate, with examples and calculations using Hess's Law. Finally, it provides practice problems and exam tips focusing on these concepts.

#Thermodynamically Unfavorable Reactions & Coupled Reactions

#Introduction: Spontaneity and Energy

Most of the time, we talk about reactions that happen on their own—thermodynamically favorable reactions, where ΔG° < 0 and K > 1. But what about reactions that don't want to happen? These are thermodynamically unfavorable reactions (ΔG° > 0, K < 1), and they won't go without a little push. Let's explore how we can make these reactions happen.

Understanding thermodynamically favorable and unfavorable reactions is crucial, as it ties into spontaneity, equilibrium, and energy changes. Expect to see this concept in both multiple-choice and free-response questions.

#External Energy Sources

Nonspontaneous reactions need an external energy source to get going. A common source is electricity. Think of it like jump-starting a car. By using electrical energy, we can force nonspontaneous redox reactions to occur in electrolytic cells. These cells are like the opposite of batteries, using electricity to drive a reaction that wouldn't happen on its own. Imagine charging your phone—that's an electrolytic cell in action! 🔋

Key Concept

Electrolytic cells use electrical energy to drive nonspontaneous redox reactions. This is the reverse of what happens in a voltaic cell (battery).

Electrolytic Cell

Caption: An electrolytic cell uses an external power source to drive a non-spontaneous redox reaction.

#Coupled Reactions: Making the Unfavorable Favorable

Another way to make nonspontaneous reactions happen is through coupled reactions. This is like using a strong reaction to pull a weaker reaction along. Coupled reactions involve a nonspontaneous reaction and a spontaneous reaction that share a common intermediate. Remember from kinetics, an intermediate is a substance pro...

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Previous Topic - Free Energy and EquilibriumNext Topic - Galvanic (Voltaic) and Electrolytic Cells

Question 1 of 10

🎉 A reaction is considered thermodynamically unfavorable when:

It occurs spontaneously

It has a ΔG° < 0

It requires an external energy source to proceed

It has an equilibrium constant K > 1