Entropy of universe always increases
Which of following describe a case where the entropy of a system increases?
a) Water freezes and turns into ice.
b) Mercury oxide decomposes into mercury and oxygen.
c) Hydrogen is combusted, forming water.
d) A smell diffuses in a room.
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1 solution
Calculate the entropy changes ΔS for the combustion of hydrogen to form liquid water.
H2(g) + 1/2O2(g) ==> H2O(l) ΔHsys = –286 kJ mol–1
ΔSθsys–298K = Sθ298K(H2O(l)) – Sθ298K(H2(g)) – Sθ298K(O2(g))/2
ΔSθsys = 69.9 –130.6 –205/2 = –163.2 J mol–1 K–1 notice the much smaller entropy for liquid water
The entropy decrease in this case is much larger in forming liquid water compared to forming gaseous water. The liquid state offers less ways of distributing the particles and there energy.
ΔSθtot = ΔSθsys + ΔSθsurr
ΔSθtot = ΔSθsys + (–ΔHθsys/T)
ΔSθtot = –163.2 + (––286 x 1000/298) don't forget ΔH values come along in kJ
ΔSθtot = –163.2 +959.7 = +796.5 J mol–1 K–1
So despite, the smaller entropy of the product liquid water, the overall entropy change is even more positive! because reaction (ii) involves condensation which is an exothermic process and more energy is transferred to increase the entropy of the surroundings.
Answer: b ) a n d d )