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 form a organic molecule (formaldehyde) and oxygen, effectively "storing" that energy for use later. The simplest possible such process is given by: CO₂ + H₂O O₂ + CH₂O The table lists the enthalpy of formation, H_f and the entropy S for 1 mol of each of the substances involved in this reaction at 298 K and 10⁵ Pa.

	Hf (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? H = ________ kJ

Does this change in enthalpy represent energy that must be added to the system, or energy that is released from the system?

Select - Energy must be added or Energy must be released

2. What is the change in entropy for this reaction? S = ______ kJ/K

Does the entropy increase or decrease?

At 298 K, what is the heat transfer that corresponds to this change in entropy? Q = ____________ kJ

3. What is the change in Gibbs free energy for this reaction? G = ___________ kJ

The Gibbs free energies of formation, G_f, 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? G_f (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.