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9.6 Coupled Reactions

3 min readjanuary 8, 2023

Jillian Holbrook

Jillian Holbrook

Jillian Holbrook

Jillian Holbrook

Thermodynamically Unfavorable Reactions

For most of this unit, we have discussed thermodynamically favorable reactions, reactions for which ΔG° < 0 and K > 1. However, this section looks at thermodynamically unfavorable reactions. do not occur spontaneously and, therefore, will not happen without any sort of external energy source.

can help nonspontaneous processes run. A common source of energy for spurring nonspontaneous processes is . By using electrical energy, can take place. For example, a can be connected to an (a topic we will discuss in Unit 9.7) to “push” electrons from a negatively charged ion to a positively charged ion in a reaction of the form Y+ + Z- → Y + Z (assuming Y + Z → Y+ + Z- is spontaneous). These cells are also similar to the way a is charged! 🔋

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-4yz6yhKnpQrG.png?alt=media&token=e88a84be-a8aa-4569-b810-1bfedeb0ffea

Image From LibreTexts

Coupled Reactions Explained

Another way to make nonspontaneous reactions spontaneous is through the use of . are a combination of a nonspontaneous reaction and a spontaneous reaction that have a . Recall from kinetics that an intermediate is a product of one part of a and a reactant in the next. For example, in the following , O is an intermediate:

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-VlBCUOQtZwxh.jpg?alt=media&token=be632ada-7ea7-4cec-b326-02c757299682

By using reactions with common intermediates, mechanisms for new reactions can form. By adding together these , a new reaction results, which is spontaneous. An example of the following is clear in the subsequent reaction:

Cu2S → 2Cu + S (ΔG° = 86.2 kJ)

We see that this reaction is nonspontaneous and requires some external energy to occur. However, we can also find a spontaneous reaction with either Cu or S as a reactant that we can add to the reaction to create a spontaneous process overall (ΔG° < 0).

For example, let’s use the reaction:

S + O2 → SO2 (ΔG° = -300.1 kJ)

By adding these reactions together we find the following:

Cu2S → 2Cu + S (ΔG° = 86.2 kJ)

S + O2 → SO2 (ΔG° = -300.1 kJ)


Cu2S + O2 → 2Cu + SO2 (ΔG° = 86.2 + (-300.1) = -213.9)

As we see, by adding these reactions together, we can use the spontaneous process of sulfur and oxygen forming sulfur dioxide to couple the reactions and form a process that is spontaneous overall.

This concept finds many applications in biology, including the conversion of ATP to ADP in biological systems. The example below comes from organic chemistry but is another example of . Do not worry if you are unfamiliar with the terms ATP and ADP or what all the hexagons and pentagons mean:

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-p4POE7kltp8h.png?alt=media&token=42e5019a-6a2d-48cb-b63e-c77044c9387a

Image From Khan Academy

Practice Problem

Given the following reactions and thermodynamic data, calculate the ΔG° value for the reaction Fe2O3 + 3CO → 2Fe + 3CO2:

Fe2O3 → 2Fe + 3/2O2 (ΔG° = 742.2 kJ)

CO + 1/2O2 → CO2 (ΔG° = -283.5 kJ)

We can use to solve this problem because the two reactions have a of O2. However, we first want to multiply the bottom reaction by three to make the O2s cancel out. When we do so, we also multiply ΔG° by the same factor.

Fe2O3 → 2Fe + 3/2O2 (ΔG° = 742.2 kJ)

3CO + 3/2O2 → 3CO2 (ΔG° = -283.5 * 3 kJ = -850.5 kJ)


Fe2O3 + 3CO + 3/2O2 → 2Fe + 3CO2 + 3/2O2 (ΔG° = 742.2 kJ + -850.5 kJ = -108.3 kJ)

Fe2O3 + 3CO+ → 2Fe + 3CO2 (ΔG° = -108.3 kJ)

Key Terms to Review (12)

ATP to ADP Conversion

: This process involves breaking down adenosine triphosphate (ATP) into adenosine diphosphate (ADP), releasing energy that cells can use for various functions.

Battery

: A battery is a device consisting of one or more electrochemical cells with external connections for powering electrical devices such as flashlights, smartphones, and electric cars.

Common Intermediate

: A common intermediate is a species produced in one step of a reaction mechanism and consumed in another. It does not appear in the overall balanced equation for the reaction.

Coupled Reactions

: Coupled reactions are a pair of chemical reactions in which one provides the energy that drives the other.

Electricity

: A form of energy resulting from charged particles such as electrons and protons, either static or moving.

Electrolytic Cell

: An electrolytic cell uses electrical energy to drive a nonspontaneous redox reaction.

Elementary Steps

: Elementary steps are individual processes within complex reactions that occur in one single event or stage.

External Energy Sources

: These are sources outside of a system that provide the necessary energy for certain processes or reactions to take place.

Mechanism

: In chemistry, mechanism refers to step-by-step sequence of elementary reactions by which an overall chemical change occurs.

Nonspontaneous Redox Reactions

: Nonspontaneous redox reactions are chemical reactions that do not occur naturally without the input of energy. They involve the transfer of electrons from one species to another.

Reaction Coupling

: A phenomenon where an energetically unfavorable reaction (endergonic) is driven by a favorable one (exergonic), allowing the overall process to occur.

Thermodynamically Unfavorable Reactions

: These are chemical reactions that do not occur spontaneously because they require an input of energy to proceed. They have a positive change in Gibbs free energy.

9.6 Coupled Reactions

3 min readjanuary 8, 2023

Jillian Holbrook

Jillian Holbrook

Jillian Holbrook

Jillian Holbrook

Thermodynamically Unfavorable Reactions

For most of this unit, we have discussed thermodynamically favorable reactions, reactions for which ΔG° < 0 and K > 1. However, this section looks at thermodynamically unfavorable reactions. do not occur spontaneously and, therefore, will not happen without any sort of external energy source.

can help nonspontaneous processes run. A common source of energy for spurring nonspontaneous processes is . By using electrical energy, can take place. For example, a can be connected to an (a topic we will discuss in Unit 9.7) to “push” electrons from a negatively charged ion to a positively charged ion in a reaction of the form Y+ + Z- → Y + Z (assuming Y + Z → Y+ + Z- is spontaneous). These cells are also similar to the way a is charged! 🔋

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-4yz6yhKnpQrG.png?alt=media&token=e88a84be-a8aa-4569-b810-1bfedeb0ffea

Image From LibreTexts

Coupled Reactions Explained

Another way to make nonspontaneous reactions spontaneous is through the use of . are a combination of a nonspontaneous reaction and a spontaneous reaction that have a . Recall from kinetics that an intermediate is a product of one part of a and a reactant in the next. For example, in the following , O is an intermediate:

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-VlBCUOQtZwxh.jpg?alt=media&token=be632ada-7ea7-4cec-b326-02c757299682

By using reactions with common intermediates, mechanisms for new reactions can form. By adding together these , a new reaction results, which is spontaneous. An example of the following is clear in the subsequent reaction:

Cu2S → 2Cu + S (ΔG° = 86.2 kJ)

We see that this reaction is nonspontaneous and requires some external energy to occur. However, we can also find a spontaneous reaction with either Cu or S as a reactant that we can add to the reaction to create a spontaneous process overall (ΔG° < 0).

For example, let’s use the reaction:

S + O2 → SO2 (ΔG° = -300.1 kJ)

By adding these reactions together we find the following:

Cu2S → 2Cu + S (ΔG° = 86.2 kJ)

S + O2 → SO2 (ΔG° = -300.1 kJ)


Cu2S + O2 → 2Cu + SO2 (ΔG° = 86.2 + (-300.1) = -213.9)

As we see, by adding these reactions together, we can use the spontaneous process of sulfur and oxygen forming sulfur dioxide to couple the reactions and form a process that is spontaneous overall.

This concept finds many applications in biology, including the conversion of ATP to ADP in biological systems. The example below comes from organic chemistry but is another example of . Do not worry if you are unfamiliar with the terms ATP and ADP or what all the hexagons and pentagons mean:

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-p4POE7kltp8h.png?alt=media&token=42e5019a-6a2d-48cb-b63e-c77044c9387a

Image From Khan Academy

Practice Problem

Given the following reactions and thermodynamic data, calculate the ΔG° value for the reaction Fe2O3 + 3CO → 2Fe + 3CO2:

Fe2O3 → 2Fe + 3/2O2 (ΔG° = 742.2 kJ)

CO + 1/2O2 → CO2 (ΔG° = -283.5 kJ)

We can use to solve this problem because the two reactions have a of O2. However, we first want to multiply the bottom reaction by three to make the O2s cancel out. When we do so, we also multiply ΔG° by the same factor.

Fe2O3 → 2Fe + 3/2O2 (ΔG° = 742.2 kJ)

3CO + 3/2O2 → 3CO2 (ΔG° = -283.5 * 3 kJ = -850.5 kJ)


Fe2O3 + 3CO + 3/2O2 → 2Fe + 3CO2 + 3/2O2 (ΔG° = 742.2 kJ + -850.5 kJ = -108.3 kJ)

Fe2O3 + 3CO+ → 2Fe + 3CO2 (ΔG° = -108.3 kJ)

Key Terms to Review (12)

ATP to ADP Conversion

: This process involves breaking down adenosine triphosphate (ATP) into adenosine diphosphate (ADP), releasing energy that cells can use for various functions.

Battery

: A battery is a device consisting of one or more electrochemical cells with external connections for powering electrical devices such as flashlights, smartphones, and electric cars.

Common Intermediate

: A common intermediate is a species produced in one step of a reaction mechanism and consumed in another. It does not appear in the overall balanced equation for the reaction.

Coupled Reactions

: Coupled reactions are a pair of chemical reactions in which one provides the energy that drives the other.

Electricity

: A form of energy resulting from charged particles such as electrons and protons, either static or moving.

Electrolytic Cell

: An electrolytic cell uses electrical energy to drive a nonspontaneous redox reaction.

Elementary Steps

: Elementary steps are individual processes within complex reactions that occur in one single event or stage.

External Energy Sources

: These are sources outside of a system that provide the necessary energy for certain processes or reactions to take place.

Mechanism

: In chemistry, mechanism refers to step-by-step sequence of elementary reactions by which an overall chemical change occurs.

Nonspontaneous Redox Reactions

: Nonspontaneous redox reactions are chemical reactions that do not occur naturally without the input of energy. They involve the transfer of electrons from one species to another.

Reaction Coupling

: A phenomenon where an energetically unfavorable reaction (endergonic) is driven by a favorable one (exergonic), allowing the overall process to occur.

Thermodynamically Unfavorable Reactions

: These are chemical reactions that do not occur spontaneously because they require an input of energy to proceed. They have a positive change in Gibbs free energy.


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© 2024 Fiveable Inc. All rights reserved.

AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.