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# Entropy

## Defines entropy and gives examples of systems with higher or lower relative entropy.

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Order and Chaos

### Order and Chaos

Credit: Breber
Source: https://commons.wikimedia.org/wiki/File:BOA_2007_%28247%29.JPG

Earlier in the day, the members of this marching band were not this well-organized. Some were walking around, some were sitting, or practicing, or talking to friends. They did not normally line up in tight rows. Outside energy (from the drum major and band director) was required to bring about this new structure. Information was needed for directions so that each member knew where they needed to be. After the performance, the structure dissipated once again, producing a random collection of players.

#### Why It Matters

• Disorder and decay are very much a part of our daily lives. Some disorder comes quickly and unexpectedly. A car runs a stoplight and collides with another vehicle. In addition to the emotional (people in the cars get frightened) and economic (cost of repairs, traffic fines, increase in insurance premiums) disorder, the two cars have increased physical disorder. Bumpers may be knocked loose, car bodies are bent, glass may be broken – the entropy of the system has increased.
• Other disorder is more gradual. Salt is used to melt the ice on a street in the winter. Over time, the street surface begins to deteriorate. Surface cracking occurs and potholes may form. Increased fragmentation is the order of the day.
• Credit: Steven Depolo
Source: http://www.flickr.com/photos/stevendepolo/4310847670/

What used to be a flat road has decayed over time [Figure2]

• Entropy changes in reactions can be calculated to give a quantitative estimate of energy released or required. It is a good idea to the qualitative assessment at the same time to predict whether entropy will increase or decrease. Sometimes we lose a sign in our calculations, and the qualitative prediction can help keep us on the right track.

#### Show What You Know

1. What does entropy have to do with energy?
2. What is the relationship between order and entropy?
3. Describe the entropy changes seen in burning wood.
4. Why do gases have more entropy than solids?
5. The reaction for rust formation is 4Fe + 302 → 2Fe203. Does entropy increase or decrease in this reaction?

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