20.1. Entropy http://www.ck12.org
20.1 Entropy
Lesson Objectives
- Describe what it means to call a process spontaneous, and give examples.
- Define entropy.
- Be able to predict whether the entropy of a system is increasing or decreasing.
- Calculate∆S°rxnfrom appropriate data.
Lesson Vocabulary
- spontaneous: Describes a reaction in which the given conditions favor the formation of the products.
- entropy (S): A measure of probability or the degree of order.
- standard entropy: A measure of the entropy of a substance at 25°C and 1 atm of pressure.
- second law of thermodynamics: States that the entropy of the universe will increase for any spontaneous
process. - third law of thermodynamics: States that a perfectly regular crystal at a temperature of 0 K (absolute zero)
would have an entropy value of 0.
Check Your Understanding
- How can we tell whether a reaction is exothermic or endothermic?
- What does the value of an equilibrium constant tell us about a reversible reaction?
- What information is needed to calculate the enthalpy change for a given reaction?
Introduction
Have you ever wondered why its easy to let your bedroom get messy, but much more difficult to keep it neat? When
your bedroom is neat and orderly, everything is in a place where it belongs. When it’s messy, everything seems to
be out of order. It’s as if you’re bedroom naturally becomes messy, but you have to make an effort to keep it neat.
Physical and chemical processes can be thought of in a similar way. There is a natural tendency for a physical or
chemical process to progress in a certain direction. In this lesson, you will learn about the quantity used describe
orderliness and how to predict the reaction direction in physical and chemical processes.