AP Chemistry 9.7 Coupled Reactions Study Notes - New Syllabus Effective fall 2024
AP Chemistry 9.7 Coupled Reactions Study Notes- New syllabus
AP Chemistry 9.7 Coupled Reactions Study Notes – AP Chemistry – per latest AP Chemistry Syllabus.
LEARNING OBJECTIVE
Explain the relationship between external sources of energy or coupled reactions and their ability to drive thermodynamically unfavorable processes.
Key Concepts:
- Using External Energy to Drive Thermodynamically Unfavorable Processes
- Coupling Favorable and Unfavorable Reactions
Using External Energy to Drive Thermodynamically Unfavorable Processes
Some chemical or physical processes are thermodynamically unfavorable because they have a positive Gibbs free energy change (\( \mathrm{\Delta G^\circ > 0} \)). Such processes do not occur spontaneously, but they can be made to occur by supplying energy from an external source.
The external energy source provides the required energy to overcome the thermodynamic barrier, effectively making the overall process possible.
Key Equation
\( \mathrm{\Delta G^\circ = \Delta H^\circ – T\Delta S^\circ} \)
- When \( \mathrm{\Delta G^\circ > 0} \): process is nonspontaneous — requires energy input.
- When \( \mathrm{\Delta G^\circ < 0} \): process is spontaneous — can occur naturally.
External energy sources (electrical, light, mechanical, etc.) supply the needed energy to make \( \mathrm{\Delta G^\circ_{total}} \) negative.
Common Examples of Energy-Driven Processes
| Energy Source | Process Driven | Example |
|---|---|---|
| Electrical energy | Drives nonspontaneous redox reactions | Electrolysis of water; charging a battery |
| Light energy (photons) | Drives endergonic biochemical reactions | Photosynthesis — conversion of CO₂ to glucose |
Example :
Explain how water can be decomposed into hydrogen and oxygen gases using an external energy source.
▶️ Answer / Explanation
Step 1: The spontaneous direction for water is formation (combustion): \( \mathrm{2H_2(g) + O_2(g) \rightarrow 2H_2O(l)} \) \( \mathrm{\Delta G^\circ = -474\ kJ/mol} \).
Step 2: The reverse process (electrolysis): \( \mathrm{2H_2O(l) \rightarrow 2H_2(g) + O_2(g)} \) has \( \mathrm{\Delta G^\circ = +474\ kJ/mol} \) — nonspontaneous.
Step 3: Electrical energy from an external power source supplies the +474 kJ/mol needed to drive the reaction.
Step 4: The overall system (chemical + electrical) has an effective \( \mathrm{\Delta G^\circ_{total} < 0} \), making the process occur when powered.
Final Answer: Water electrolysis is a thermodynamically unfavorable process (\( \mathrm{\Delta G^\circ > 0} \)) that proceeds when electrical energy is supplied from an external circuit.
Example :
How does light energy make the conversion of carbon dioxide and water to glucose possible in photosynthesis?
▶️ Answer / Explanation
Step 1: Photosynthesis overall reaction: \( \mathrm{6CO_2(g) + 6H_2O(l) \rightarrow C_6H_{12}O_6(s) + 6O_2(g)} \)
Step 2: \( \mathrm{\Delta G^\circ = +2870\ kJ/mol} \): strongly endergonic (nonspontaneous).
Step 3: Absorption of light energy by chlorophyll converts photon energy into chemical energy (ATP and NADPH).
Step 4: This energy drives carbon fixation reactions that synthesize glucose from CO₂ and H₂O.
Final Answer: Photosynthesis is thermodynamically unfavorable (\( \mathrm{\Delta G^\circ > 0} \)) but occurs because light provides the energy required to make \( \mathrm{\Delta G^\circ_{total} < 0} \).
Coupling Favorable and Unfavorable Reactions
A thermodynamically unfavorable reaction (\( \mathrm{\Delta G^\circ > 0} \)) can occur if it is coupled to a thermodynamically favorable reaction (\( \mathrm{\Delta G^\circ < 0} \)) such that the overall combined process has a negative total free energy change (\( \mathrm{\Delta G^\circ_{net} < 0} \)).
The two reactions are connected by a common intermediate — often an ion, molecule, or energy carrier that transfers energy between them.
Key Relationship
\( \mathrm{\Delta G^\circ_{net} = \Delta G^\circ_1 + \Delta G^\circ_2} \)
- If \( \mathrm{\Delta G^\circ_{net} < 0} \), the overall process is thermodynamically favorable.
- Coupling allows an endergonic (unfavorable) process to proceed by using the energy released from an exergonic process.
Examples of Coupled Systems
| Favorable Reaction | Unfavorable Reaction | Overall Result |
|---|---|---|
| ATP → ADP + Pi (\( \mathrm{\Delta G^\circ = -30.5\ kJ/mol} \)) | Glucose + Pi → Glucose-6-phosphate (\( \mathrm{\Delta G^\circ = +13.8\ kJ/mol} \)) | Glucose phosphorylation proceeds spontaneously |
| Electron flow in cell respiration (exergonic) | ADP + Pi → ATP (endergonic) | ATP synthesis (chemiosmosis) |
Example:
Explain how the phosphorylation of glucose to glucose-6-phosphate, an unfavorable process, becomes spontaneous when coupled with ATP hydrolysis.
▶️ Answer / Explanation
Step 1: Unfavorable process: \( \mathrm{Glucose + Pi \rightarrow Glucose\text{-}6\text{-}phosphate} \), \( \mathrm{\Delta G^\circ = +13.8\ kJ/mol} \).
Step 2: Favorable process: \( \mathrm{ATP \rightarrow ADP + Pi} \), \( \mathrm{\Delta G^\circ = -30.5\ kJ/mol} \).
Step 3: Combine the reactions by canceling the common intermediate (\( \mathrm{Pi} \)):
\( \mathrm{Glucose + ATP \rightarrow Glucose\text{-}6\text{-}phosphate + ADP} \)
Step 4: Add the free energy changes:
\( \mathrm{\Delta G^\circ_{net} = +13.8 + (-30.5) = -16.7\ kJ/mol} \)
Step 5: Interpretation:
- The unfavorable glucose phosphorylation is driven by the energy released from ATP hydrolysis.
- Since \( \mathrm{\Delta G^\circ_{net} < 0} \), the overall coupled process is spontaneous.
Final Answer: The coupling of an endergonic process (\( \mathrm{+13.8\ kJ/mol} \)) with an exergonic process (\( \mathrm{-30.5\ kJ/mol} \)) makes the overall reaction spontaneous (\( \mathrm{\Delta G^\circ_{net} = -16.7\ kJ/mol} \)).