Hi everybody I've begun studying entropy at college and something is bugging me. I know that for any reaction:

DeltaG=DeltaH-TDeltaS

and for a reaction to be feasible the change in "free energy" or G has to be negative. Does this apply for photosynthesis too?

As far as I am aware it would be endothermic, producing a positive deltaH value and it produces glucose and oxygen[gas and solid] from water and carbon dioxide[gas and liquid]. This would cause a drop in entropy, therefore making the reaction impossible. Have I missed something critical? Please help me resolve this!

there can be local decreases in entropy but a lot of energy has to be put in. to create this energy entropy increases(in the case of photosynthesis in the sun) usually by a good deal more than the decrease and never less.

As far as I am aware it would be endothermic, producing a positive deltaH value and it produces glucose and oxygen[gas and solid] from water and carbon dioxide[gas and liquid]. This would cause a drop in entropy, therefore making the reaction impossible.

Have I missed something critical?

 

I wouldnt say impossible, just unlikely. the assimilation of co2 is indeed were unlikely and very slow. that is the reason why RUBISCO is the most abundant protein in the world. Each plant needs tons of it to get enough co2. under uncatalized circumstances, this reaction wouldnt take place.

it all depends on the photons hitting the leaves. this provides the energy for the reaction to procede.

 

edit: w00t 900th post

This would cause a drop in entropy, therefore making the reaction impossible.

 

This is a mistake, a positive enthalpy does not mean a drop in entropy.

 

And yes the equation applies to photosynthesis. Okay you're forgetting about the increase in entropy that is caused by dispersal of solar radiation as heat/kentic movement. So while creation of glucose from water and CO2 has a decrease in entropy as well as an increase in enthalpy there is a greater increase in entropy that results from dispersal of solar radiation as heat, and this is what drives the process forward.

 

Hope that clears it up.

 

basically when a photo hits the photsynthesis apparatus it causes a conformational change, alot of this change is dispersed as heat when the molecules relax, satisfying entropy. And most of the remainder of it is funneled into energy for the plant; which is uses to produce glucose etc.

The second law of thermodynamics does not state that [math]\Delta S_{sys} > 0[/math], it states that [math]\Delta S_{tot} = \Delta S_{sys} + \Delta S_{surr} > 0[/math]. So, for photosynthesis while the entropy of the system decreases, the corresponding increase in entropy of the surroundings (e.g. from heat created during photosynthesis and the breakdown of water to oxygen gas and protons) will make the overall change in entropy positive.