Click, click, click - you picture has been taken! Our episode starts with describing entropy with respect to the dispersal of matter and compares it to the snapshots or microstates.
Click, click, click - you picture has been taken! Our episode starts with describing entropy with respect to the dispersal of matter and compares it to the snapshots or microstates (0:30). The phase changes are examples of increases of dispersal of matter (1:06). Entropy can also be defined as an increase of the dispersal of energy and therefore we tie it to KMT (3:50). The change in entropy can be calculated by subtracting the sum of the entropy of the reactants from the sum of the entropy of products (4:53).
Question: What is the sign of ΔSo for the formation of NaCl from its elements?
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Hi and welcome to the APsolute Recap: Chemistry Edition. Today’s episode will recap entropy.
Lets Zoom Out:
Unit 9 - Applications of Thermodynamics
Topic 9.1 & 9.2 - Entropy and Entropy Change
Big idea - Energy
Click, click, click - you’ve got your picture taken! And not just one!
What does that have to do with Entropy? In statistical thermodynamics, we can compare entropy to how many microstates - the arrangements of position and kinetic energy - are possible. And that’s like taking a snapshot of the molecular view of a system - the more different the arrangement of particles and snapshots are, the higher the entropy!
Let’s zoom in:
Generally speaking, entropy increases with an increase in dispersal of matter or energy. With regards to matter, you often see it compared to a messy room - indicating that the matter is more dispersed and therefore the entropy is increased. Defining entropy as the “disorder” of a system can be misleading. In AP Chemistry, we prefer the concepts of microstates - or snapshots as mentioned in the introduction. Imagine you have two connected round flasks and two particles. You can have four arrangements - microstates: Both particles on the left, both particles on the right, one on the left, one on the right, and vice versa. Now, four possible arrangements is rather low, but we also do not usually only have 2 particles. Now imagine having 1 mole - 6.022 x 1023 particles! There are A LOT of possible microstates - which means we have a greater dispersal of matter - more options - and therefore a greater entropy.
Now we can easily tie this to phase changes: from solid to liquid as well as from liquid to gas, the particles move more freely They occupy a greater volume and therefore you have more microstates - a greater dispersal of matter and an increase in entropy. You also have an increase in entropy when you increase the volume available to a gas - because there are more possible positions. And, since we are already talking about gases: You also increase entropy in a chemical reaction that has more moles of gas on the product side than the reactant side.
Let’s stick with gases and take a look at the energy perspective. As mentioned before, entropy also increases with a greater dispersal of energy. This ties ALL THE WAY back to the Kinetic Molecular Theory - listen to episode 18 if you need a refresher! Kinetic Molecular Theory and the Maxwell-Boltzmann distribution tell us that when we increase the temperature of a system, the curve broadens and shows that the distribution of the kinetic energy increases. This means we have a greater dispersal of energy and therefore we increase the entropy when we increase the temperature.
Entropy itself is denoted as a capital S and most of the time measured in joules. If the entropy increases, for example when we melt ice or when a solid decomposes to form two gases, we have a positive value for S. If the entropy decreases, for example when we cool a gaseous system or when we decrease the volume, we have a negative value for S.
Similarly to enthalpy, we can calculate the CHANGE in entropy for the reaction by taking into account the number of moles from the balanced chemical equation and subtracting the sum of the entropy of the reactants from the sum of the entropy of products. Products minus reactants - AGAIN! Same, same, but different!
One difference: every substance has a value for entropy, there is no zero value for elements in their standard form like there is for enthalpy.
To recap:
Entropy, S, is a measurement of the dispersal of matter or energy. Entropy increases with an increase in the dispersal of matter, like with phase changes from solid to liquid to gas. For gases, entropy also increases with an increase in volume, because there are more possible microstates. Taking into account the Kinetic Molecular Theory, entropy also increases with an increase in temperature. An increase in entropy is denoted as positive, a decrease in entropy with a negative sign. The change in entropy can be calculated using sum of entropy of products minus sum of entropy of reactants.
Coming up next on the APsolute RecAP Chemistry Edition: Gibb’s Free Energy.
Today’s Question of the day is about the change in entropy.
Question: What is the sign of ΔSo for the formation of NaCl from its elements?