Chem – Entropy Definition and Relationship to Energy

What is entropy?

It is one of the most confusing scientific terms, but it is also one of the concepts that people are most familiar with. Entropy is the measure of disorder. As time goes on the entropy of all things will increase. All things will become more disorderly. Something with more entropy (more disorderly) has a higher value (number) associated with it. Something with less entropy (more orderly) has a lower value (number) associated with it. The easiest example to think of how entropy works is a measure of disorder of your bedroom from Monday to Friday. On Monday all your clothes are neatly folded in your dresser or hanging in your closet (very orderly). By Friday cloths are on your floor, on your bed, and all over (very disorderly). So as time passes a bedroom moves from low entropy (orderly) to high entropy (disorderly). The same thing happens to all chemicals. Paint starts to become flaky and break off over time. Tires deflate over time. These are all examples of how entropy affects our daily lives. We can also think about how entropy affects very small things like molecules. Entropy causes larger molecules to break down into smaller molecules over time. The symbol for the change in entropy is Δ S.


What is the relationship between entropy and energy?

This is one of the most fundamental and misunderstood concepts in science today and is one of the arguments that come up between religious believers and non-religious believers. If you want to know more about these arguments then I suggest you use this link for an expanded explanation after you read the rest of this section. As you put energy into anything (including a molecule) you can decrease its entropy. This allows you to make a bigger molecule or a bigger anything. Just like taking multiple eggs and scrabbling them together and then cooking them. The energy from the cooking process fuses them together into a larger mass and decreases the entropy of the egg. We can also think about the bedroom analogy that we were discussing about before. From Monday to Friday the entropy of the bedroom increases (becomes more messy) but over the weekend what happens to the bedroom? The bedroom gets cleaned by a person putting energy into it. The energy is expended by the person when they lift the clothes up from the floor or bed and fold them into the closet or dresser.




How can entropy be asked about or displayed in chemistry?


The first way is to think about each molecule as having a specific entropy and looking up the value (number) associated that molecule on an entropy table. This will later be important to do in the next section spontaneous.




The second way is to look at the different states of matter between the products and reactants of a chemical equation. Entropy increases as we go from solid —> liquid —-> aqueous —> gas. In other words, solid has the lowest entropy and gas had the highest entropy. Lets look at some examples below.








The third way is to look at the relative entropy between the reactants and the products of a chemical equation. You do this by counting up the moles of the products versus the moles of the reactants. If the products have more moles than the reactants the entropy increased as the chemical reaction proceeds. If the reactants had more moles than the products the entropy decreased as the chemical reaction proceeds. Lets look at some examples below.





What happens when you are asked to access the state and the number of moles in an equation in terms of increasing or decreasing entropy? In almost all cases, the state of the chemicals is more important than the number of moles. The reason for this is when you change states you go through very dramatic changes in volume (the amount of space you take up). The spacing of the chemicals in total is the primary way to measure entropy, so you should analyze the state change in a chemical equation and then analyze the amount of moles if there is no state change.


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