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Either way, the universe is destined for thermodynamic equilibrium—maximum entropy. This is often called the heat death of the universe , and will mean the end of all activity. However, whether the universe contracts and heats up, or continues to expand and cools down, the end is not near. Calculations of black holes suggest that entropy can easily continue for at least 10 100 size 12{"10" rSup { size 8{"100"} } } {}  years.

Order to disorder

Entropy is related not only to the unavailability of energy to do work—it is also a measure of disorder. This notion was initially postulated by Ludwig Boltzmann in the 1800s. For example, melting a block of ice means taking a highly structured and orderly system of water molecules and converting it into a disorderly liquid in which molecules have no fixed positions. (See [link] .) There is a large increase in entropy in the process, as seen in the following example.

Entropy associated with disorder

Find the increase in entropy of 1.00 kg of ice originally at 0º C size 12{0°C} {} that is melted to form water at 0º C size 12{0°C} {} .

Strategy

As before, the change in entropy can be calculated from the definition of Δ S size 12{ΔS} {} once we find the energy Q size 12{Q} {} needed to melt the ice.

Solution

The change in entropy is defined as:

ΔS = Q T . size 12{ΔS= { {Q} over {T} } } {}

Here Q size 12{Q} {} is the heat transfer necessary to melt 1.00 kg of ice and is given by

Q = mL f , size 12{Q= ital "mL" rSub { size 8{f} } } {}

where m size 12{m} {} is the mass and L f size 12{L rSub { size 8{f} } } {} is the latent heat of fusion. L f = 334 kJ/kg size 12{L rSub { size 8{f} } ="334"" kJ/kg"} {} for water, so that

Q = ( 1.00 kg ) ( 334 kJ/kg ) = 3 . 34 × 10 5 J. size 12{Q= \( 1 "." "00"" kg" \) \( 3 "." "34"" kJ/kg" \) =3 "." "34" times "10" rSup { size 8{5} } " J"} {}

Now the change in entropy is positive, since heat transfer occurs into the ice to cause the phase change; thus,

Δ S = Q T = 3 . 34 × 10 5 J T . size 12{ΔS= { {Q} over {T} } = { {3 "." "34" times "10" rSup { size 8{5} } " J"} over {T} } } {}

T size 12{T} {} is the melting temperature of ice. That is, T = C=273 K size 12{T=0°"C=273 K"} {} . So the change in entropy is

Δ S = 3 . 34 × 10 5 J 273 K = 1.22 × 10 3 J/K. alignl { stack { size 12{DS= { {3 "." "34"´"10" rSup { size 8{5} } " J"} over {"273 K"} } } {} #" "=1 "." "22"´"10" rSup { size 8{3} } " J/K" "." {} } } {}

Discussion

This is a significant increase in entropy accompanying an increase in disorder.

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The diagram has two images. The first image shows molecules of ice. They are represented as tiny spheres joined to form a floral pattern. The system is shown as ordered. The second image shows what happens when ice melts. The change in entropy delta S is marked between the two images shown by an arrow pointing from first image toward the second image with change in entropy delta S shown greater than zero. The second image represents water shown as tiny spheres moving in a random state. The system is marked as disordered.
When ice melts, it becomes more disordered and less structured. The systematic arrangement of molecules in a crystal structure is replaced by a more random and less orderly movement of molecules without fixed locations or orientations. Its entropy increases because heat transfer occurs into it. Entropy is a measure of disorder.

In another easily imagined example, suppose we mix equal masses of water originally at two different temperatures, say 20.0º C size 12{"20" "." 0°C} {} and 40.0º C size 12{"40" "." 0°C} {} . The result is water at an intermediate temperature of 30.0º C size 12{"30" "." 0°C} {} . Three outcomes have resulted: entropy has increased, some energy has become unavailable to do work, and the system has become less orderly. Let us think about each of these results.

First, entropy has increased for the same reason that it did in the example above. Mixing the two bodies of water has the same effect as heat transfer from the hot one and the same heat transfer into the cold one. The mixing decreases the entropy of the hot water but increases the entropy of the cold water by a greater amount, producing an overall increase in entropy.

Second, once the two masses of water are mixed, there is only one temperature—you cannot run a heat engine with them. The energy that could have been used to run a heat engine is now unavailable to do work.

Practice Key Terms 3

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Source:  OpenStax, College physics for ap® courses. OpenStax CNX. Nov 04, 2016 Download for free at https://legacy.cnx.org/content/col11844/1.14
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