Overview
This lecture explains how the tendency for systems to become more disordered (entropy) makes certain outcomes more probable, using gas molecules and paper as examples, before introducing the link between entropy, enthalpy, and spontaneity of reactions.
Probability and Gas Molecule Distribution
- Four gas molecules in one compartment and none in the other is the initial state.
- Opening the valve allows the gas molecules to move randomly to either side.
- The most probable final state is the gas evenly spread out (option C), not all on one side.
- All molecules on one side (A or B) can occur but are much less likely.
- The spread-out state (C) is more disordered and has more possible arrangements.
Counting Microstates and Likelihood
- Each gas molecule arrangement has the same probability, but there are more arrangements for gases spread out.
- With 4 molecules: 1 way for all left, 1 way for all right, 6 ways to split (2 and 2 across sides).
- The chance of getting a spread-out state is higher because there are more combinations (microstates).
Entropy and Disorder Analogy
- Arranging two pieces of paper in order is limited, but disordering them has many possibilities.
- Disordered states are favored because there are more ways to achieve them.
- In random processes, disordered outcomes are more likely due to higher numbers of microstates.
Spontaneity, Enthalpy, and Entropy
- Nature favors processes that decrease enthalpy (energy) and increase entropy (disorder).
- Next, these principles will be combined to predict if a reaction is spontaneous (happens by itself).
Key Terms & Definitions
- Entropy — a measure of the disorder or randomness in a system.
- Microstate — a specific detailed arrangement of the components in a system.
- Spontaneous process — a process that occurs naturally without outside intervention.
- Enthalpy — the total heat content of a system.
Action Items / Next Steps
- Prepare to combine concepts of enthalpy and entropy to predict reaction spontaneity in future lessons.