THC_The 2nd Law of Thermodynamics Lesson

The 2nd Law of Thermodynamics

A hot frying pan cools down when taken off the stove. Iron rusts in the air. Ice cubes melt in a warm room. Other than just being common occurrences, these events actually have something fundamental in common. They all o ccur for the same reason. The reason is that energy spontaneously tends to flow from being concentrated in one place to becoming spread out if it is not hindered in some way. This is the concept known as the 2nd Law of Thermodynamics.  

This law has been described as "time's arrow" because the natural phenomena we observe all around us follow the direction explained by this law. A swimmer doesn't shoot up out of a pool onto a diving board. A rock in a valley doesn't suddenly roll up the hill. These events would be the result of energy becoming concentrated rather than spreading out. They would be a violation of the 2nd Law of Thermodynamics. Our experiences in the natural progression of processes give us great insight into this law. Stated a more formal way, this law says that whenever a spontaneous event takes place in our universe, the total entropy of the universe increases.    

Entropy

Flat Tire imageSo, what is entropy? If you took a first year chemistry course, you probably learned that entropy is the measure of the disorder of system. The greater the disorder of a system, the greater its entropy. You probably saw pictures of a messy desk or a pile of bricks as the example for entropy. This is not just over-simplistic way to view entropy; it is wrong! Entropy is really the measure of energy dispersal of a system, as a function of temperature. It measures how much energy is spread out in a process, or how widely that energy is spread. A better picture to represent entropy would be air escaping from a high pressure tire that has just been punctured by a nail.

Here are some factors that affect entropy:

  • Entropy of a gas increases with increasing volume.

Entropy Volume diagram showing increasing entropy

Entropy increases with increasing temperature.

At absolute zero, the atoms (spheres), rest at their equilibrium lattice positions. there is perfect order and minimum entropy. At higher temperatures, the particles vibrate more. We can see that there is greater disorder than at absolute zero. Increasing Entropy and Temperature At a still higher temperature, vibration is more violent, and at any instant the particles are found in even more disordered arrangements.

  • Entropy increases with freedom of molecular movement. Gases have greater entropy than liquids, liquids greater than solids. In fact,

diagram of solid, liquid, and gas showing increasing entropy

  • When all other things are equal, entropy increases in chemical reactions that increase the number of particles in the system.

Calculating Entropy Change

Changes in entropy are calculated the same way as changes in enthalpy.

CHANGE IN ENTROPY ΔS° = ΣS(products) - ΣS° (reactants)

You will look up these So values in the appendix of your book just as you did for ∆Hof. Notice that the unit is different for So. It is J/molK. You may also notice that you don't see any zero values for So like you did for ∆Hof. This is because the only substances with zero entropy are pure crystals at 0 Kelvin. This is the 3rd Law of Thermodynamics. Most thermodynamic data tables are written at 25°C (298 K).  

Remember to work on the module practice problems as you complete each section of content.  

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