# Entropy- second law of thermodynamics

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#1
ΔStotal = ΔSsystem + ΔSsurroundings
ΔStotal = ΔSsystem - (ΔHsystem/T)

Photosynthesis is endothermic therefore
ΔSsurroundings is negative. Also, there is a decrease in the entropy of the system therefore that is negative too which means that the total entropy is decreasing which is not possible as it goes against the second law of thermodynamics.

My book does give an explanation saying that the increase in entropy of the Sun compensates for this but I don't see how the reaction taking place on earth affects the Sun. The Sun provides light and then the reaction takes place. The reaction doesn't cause the increase in entropy of the Sun.
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7 years ago
#2
ΔStotal = ΔSsystem + ΔSsurroundings
ΔStotal = ΔSsystem - (ΔHsystem/T)

Photosynthesis is endothermic therefore
ΔSsurroundings is negative. Also, there is a decrease in the entropy of the system therefore that is negative too which means that the total entropy is decreasing which is not possible as it goes against the second law of thermodynamics.

My book does give an explanation saying that the increase in entropy of the Sun compensates for this but I don't see how the reaction taking place on earth affects the Sun. The Sun provides light and then the reaction takes place. The reaction doesn't cause the increase in entropy of the Sun.
It's all part of the universe you are considering. It's the entropy of the universe that mustn't decrease.
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#3
(Original post by JMaydom)
It's all part of the universe you are considering. It's the entropy of the universe that mustn't decrease.
OK.
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7 years ago
#4
ΔStotal = ΔSsystem + ΔSsurroundings
ΔStotal = ΔSsystem - (ΔHsystem/T)

Photosynthesis is endothermic therefore
ΔSsurroundings is negative. Also, there is a decrease in the entropy of the system therefore that is negative too which means that the total entropy is decreasing which is not possible as it goes against the second law of thermodynamics.

My book does give an explanation saying that the increase in entropy of the Sun compensates for this but I don't see how the reaction taking place on earth affects the Sun. The Sun provides light and then the reaction takes place. The reaction doesn't cause the increase in entropy of the Sun.
You need to define what is in your system here. For example if you are just taking the plant as your system then the second law does not apply in this context. Since you are adding energy from the sun then this is not an isolated system and therefore the entropy change can be negative
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#5
(Original post by langlitz)
You need to define what is in your system here. For example if you are just taking the plant as your system then the second law does not apply in this context. Since you are adding energy from the sun then this is not an isolated system and therefore the entropy change can be negative
Oh right... so the entropy of the universe increases though the entropy of the isolated system may decrease.
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7 years ago
#6
Oh right... so the entropy of the universe increases though the entropy of the isolated system may decrease.
Take a look at these interactives:

Entropy

Gibbs Free Energy
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7 years ago
#7
Oh right... so the entropy of the universe increases though the entropy of the isolated system may decrease.
No the universe is the isolated system so the entropy of the universe must increase. The plant is not an isolated system so therefore its entropy may decrease
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7 years ago
#8
Can I suggest you buy An Introduction to Thermodynamics by Van Ness (it's a Dover book for about £10)? Just finished it - short and it explains entropy very well, using Maxwell's Demon and a few other examples.

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#9
(Original post by langlitz)
No the universe is the isolated system so the entropy of the universe must increase. The plant is not an isolated system so therefore its entropy may decrease
What?
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#10
Can I suggest you buy An Introduction to Thermodynamics by Van Ness (it's a Dover book for about £10)? Just finished it - short and it explains entropy very well, using Maxwell's Demon and a few other examples.

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I have my exams in 15 days so I don't really have that much time. Thanks for the suggestion anyway.
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7 years ago
#11
I have my exams in 15 days so I don't really have that much time. Thanks for the suggestion anyway.
I will try and find the relevant portion for you later.

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7 years ago
#12
What?

Oh right... so the entropy of the universe increases though the entropy of the isolated system may decrease.
What's in bold is the problem.

The second law of thermodynamics states that the entropy of an isolated system never decreases in the course of every spontaneous (natural) change.
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#13
(Original post by langlitz)
What's in bold is the problem.

The second law of thermodynamics states that the entropy of an isolated system never decreases in the course of every spontaneous (natural) change.
Oh, I see, OK thanks... I think I'm more or less fine with this concept for now. Thanks for helping.
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#14
I will try and find the relevant portion for you later.

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Don't, it's ok... Thanks
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