Photosynthesis is the process by which plants effectively capture energy from the sun and store it for later use by organisms. It's the fundamental process that enables the entire food chain. Actual photosynthesis is a highly complex process involving many steps and many different chemicals, but we can get an idea of what's happening to thermodynamic quantities (enthalpy, Gibbs free energy, and entropy) using simpler reactions.
Let's consider a toy model in which an input of energy (for example from the sun) is be used to combine water and carbon dioxide to create an organic molecule (formaldehyde) and oxygen, effectively "storing" that energy for use later. The simplest possible such process is given by:
CO2 + H2O â O2 + CH2O
The table lists the enthalpy of formation, ÎHf and the entropy S for 1 mol of each of the substances involved in this reaction at 298 K and 105 Pa.
ÎHf (kJ) S (kJ/K) CH2O -115.9 0.2189 H2O -241.82 0.1888 CO2 -393.51 0.2138 O2 0 0.2051
Answer the following questions assuming that 1 mol of formaldehyde molecules are formed.
1. What is the change in enthalpy of this reaction?
ÎH = ____ kJ
Does this change in enthalpy represent energy that must be added to the system, or energy that is released from the system?
-Energy must be added or Energy is released?
2. What is the change in entropy for this reaction?
ÎS = ____ kJ/K
Does the entropy increase or decrease?
-Entropy increases or Entropy decreases?
At 298 K, what is the heat transfer that corresponds to this change in entropy?
Q = ____ kJ
Does this heat transfer represent energy that is added to the system, or energy that is released from the system?
--Energy is added Energy is released ?
3. What is the change in Gibbs free energy for this reaction?
ÎG = _____ kJ
The Gibbs free energies of formation, ÎGf, are: -228.57 kJ for water; -394.36 kJ for CO2; and 0 kJ for O2. What is the Gibbs free energy of formation for formaldehyde?
ÎGf (formaldehyde) = _______ kJ
4. Using the formation of formaldehyde as a model, comment on photosynthesis and the formation of sugars using what we know about entropy and free energy.
Photosynthesis is the process by which plants effectively capture energy from the sun and store it for later use by organisms. It's the fundamental process that enables the entire food chain. Actual photosynthesis is a highly complex process involving many steps and many different chemicals, but we can get an idea of what's happening to thermodynamic quantities (enthalpy, Gibbs free energy, and entropy) using simpler reactions.
Let's consider a toy model in which an input of energy (for example from the sun) is be used to combine water and carbon dioxide to create an organic molecule (formaldehyde) and oxygen, effectively "storing" that energy for use later. The simplest possible such process is given by:
CO2 + H2O â O2 + CH2O
The table lists the enthalpy of formation, ÎHf and the entropy S for 1 mol of each of the substances involved in this reaction at 298 K and 105 Pa.
| ÎHf (kJ) | S (kJ/K) | |
| CH2O | -115.9 | 0.2189 |
| H2O | -241.82 | 0.1888 |
| CO2 | -393.51 | 0.2138 |
| O2 | 0 | 0.2051 |
Answer the following questions assuming that 1 mol of formaldehyde molecules are formed.
1. What is the change in enthalpy of this reaction?
ÎH = ____ kJ
Does this change in enthalpy represent energy that must be added to the system, or energy that is released from the system?
-Energy must be added or Energy is released?
2. What is the change in entropy for this reaction?
ÎS = ____ kJ/K
Does the entropy increase or decrease?
-Entropy increases or Entropy decreases?
At 298 K, what is the heat transfer that corresponds to this change in entropy?
Q = ____ kJ
Does this heat transfer represent energy that is added to the system, or energy that is released from the system?
--Energy is added Energy is released ?
3. What is the change in Gibbs free energy for this reaction?
ÎG = _____ kJ
The Gibbs free energies of formation, ÎGf, are: -228.57 kJ for water; -394.36 kJ for CO2; and 0 kJ for O2. What is the Gibbs free energy of formation for formaldehyde?
ÎGf (formaldehyde) = _______ kJ
4. Using the formation of formaldehyde as a model, comment on photosynthesis and the formation of sugars using what we know about entropy and free energy.
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