[wacki]

2nd law of thermo:  For a spontaneous process, the entropy of the universe increases. ΔS > 0

Combustion is a spontaneous process.  So why isn't Delta S positive?


One example where moles of gas decreases (more order)
Combustion #1
2CO(g) + O2(g) → 2CO2(g)
ΔS (298 K) = –173 J K–1 mol–1.

Another example where moles of gas stays the same
Combustion #2
Methane + 2* O2 gas -> 2* H2O(g) + CO2
∆S = 283.6 – 596 = -5.2


BOTH of these have ∆S < 0
Both of these are spontaneous or ∆G is negative.

I know the system is not the universe but still.  Why is the RXN ΔS < 0 when the 2nd law implies ΔS > 0.

[Yggdrasil]

Transfer of heat from the system to the surroundings increases the entropy of the surroundings.  The increase in entropy of the surroundings is equal to -ΔH/T (for reactions carried out at constant pressure).  Based on this information, you should be able to see that the entropy of the universe is increasing in both of these reactions.

Bonus:  You can use this information to prove that ΔG < 0 is equivalent to the second law.

[Borek]

I know the system is not the universe but still.

That's the answer to your question. 2nd law doesn't tell you can't have locally decreasing entropy, it tells total entropy grows.

What you did is you have thrown a bottle through the window and stated "mass conservation doesn't hold, I had three bottles, I have only two now".

[Corribus]

What you did is you have thrown a bottle through the window and stated "mass conservation doesn't hold, I had three bottles, I have only two now".

LOL, I just had to give you a snack for this.

[wacki]

The increase in entropy of the surroundings is equal to -ΔH/T (for reactions carried out at constant pressure). 

this is what I was looking for.   So in an exothermic reaction ΔS of the universe is

-ΔS = -ΔH/T

or

ΔH = ΔS * DegreesKelvin

correct?

[Yggdrasil]

ΔS_universe = ΔS_system + ΔS_surroundings

Usually, the ΔS reported for a reaction refers to ΔS_system, so I'll write ΔS_system as ΔS and ΔH_system as ΔH.  Recall from my previous post that the change in entropy of the surroundings is given by the amount of heat transferred to the surroundings divided by the temperature of the surroundings.  Therefore, we can write the change in entropy of the universe as:

ΔS_universe = ΔS - ΔH/T