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Wize University Physics Textbook (Master) > Thermodynamics

Second Law of Thermodynamics

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2nd Law of Thermodynamics

Entropy (S) is a measure of disorder or randomness in a system.




The 2nd Law of Thermodynamics states that disorder of the universe increases in a spontaneous process.

In other words, things naturally tend to go towards disorder and the universe is constantly getting more and more disordered!


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Entropy Equations

Entropy is defined as:

ΔSsurr=qsurrT\boxed{\Delta S_{surr}=\frac{q_{surr}}{T}}

ΔSsurr=entropy of the surroundings (in J/molK)
qsurr=heat in the surroundings (in J)
T=Temperature (in K)
Recall:
qsurr=−qsysq_{surr}=-q_{sys}
At constant pressure and temperature:
ΔHsys=qsys\Delta H_{sys}=q_{sys}


ΔSsurr=−ΔHsysT\boxed{\Delta S_{surr}=-\frac{\Delta H_{sys}}{T}}

The 2nd law of thermodynamics states that whenever a spontaneous event takes place in the universe, the total entropy of the universe increases:

∆Suniverse=∆Ssystem+∆Ssurroundings>0\boxed{∆S_{universe}=∆S_{system}+∆S_{surroundings}>0}

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Chemical Reactions

In the same way we can use standard heats of formation to calculate a reaction enthalpy at room temperature and pressure, we can use So (values will be provided to you in a table)

ΔSo=[(∑nSoproducts)−(∑nSoreactants)]\boxed{\Delta S^o=[(\sum nS^oproducts)-(\sum nS^oreactants)]}
A gas absorbs 10 J of heat at a constant temperature of 300 K. Determine the change in entropy.

entropy change is:
ΔS = Qsurr / T
= 10 J/300 K = 0.033 J/K
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An ice tray contains 500 g of water and it freezes slowly at 0°C. What is the change in entropy of the water?