Supplemental Instruction
Iowa State University / Leader: / Danielle
Course: / Chem 167
Instructor: / Schewe-Miller
Date: / 11-6-12
What you need to know (Study Sheet)
The idea of spontaneity blossoms from the concept of a “directionality” in nature. A way of expressing this is in noticing that some things “just happen”, while others do not. Nature tends in the direction of ______.
A spontaneous reaction is one that ______. While this may seem like an obvious distinction, sometimes it is not so!
Think of gasoline. It can sit in a can in your garage indefinitely, always being exposed to air, but for the most part it stays the same. But, when you mix this same gasoline with oxygen in your car’s engine and ignite it with a spark plug, it burns until it’s gone! Is this a spontaneous reaction? Y/N
Why?
It is important to keep this concept in mind.
Entropy is a measure of ______. I also think of it as craziness! Where gases are the most crazy of all forms. They just move around and bump into things like there’s no tomorrow. Solids, on the other hand, are pretty docile—not much in the way of excitement there.
So it follows that going from, say, solid to liquid, you expect spontaneity to
decrease / increase.
Practice
1. Based on your experiences, which of these reactions would you predict are spontaneous at 25 oC?
a. CO2(s) à CO2(g)
b. NaCl(s) à NaCl(l)
c. 2 NaCl(s) à 2Na(s) + Cl2(g)
d. CO2(g) à C(s) + O2(g)
2. If the combustion of butane is spontaneous, how is it that you can carry a butane lighter safely in your pocket?
3. Identify each of the processes listed as spontaneous or nonspontaneous. For each nonspontaneous process, describe the corresponding spontaneous process in the opposite direction.
a. A group of cheerleaders builds a human pyramid.
Spontaneous? Y / N ______
b. Table salt dissolves in water.
Spontaneous? Y / N ______
c. A cup of cold coffee in a room becomes steaming hot.
Spontaneous? Y / N ______
d. A person peels an orange, and you smell it from across the room.
Spontaneous? Y / N ______
4. When ice melts, its volume decreases. Despite this fact, the entropy of the system increases. Explain a. why the entropy increases and b. why under most circumstances, a decrease in volume results in an entropy decrease.
5. For each process, tell whether the entropy change is positive or negative.
a. A glassblower heats glass (the system) to its softening temperature.
b. A teaspoon of sugar dissolves in a cup of coffee (the system consists of both sugar and coffee)
c. Calcium carbonate precipitates out of water in a cave to form stalactites and stalagmites (consider only calcium carbonate to be the system.)