- A spontaneous process is a physical or chemical change that occurs with
no intervention.
- For many spontaneous processes, some energy must be supplied to get
the process started.
- Entropy and changes in heat play roles in determining whether a
chemical process occurs spontaneously under a given set of conditions.
What is entropy?
- Molecules are more likely to exist in a high state of disorder (mixed)
than in a low state of disorder (unmixed).
- The term for disorder is entropy.
- Entropy (S) is a measure of the disorder or randomness of the particles
that make up a system.
- The tendency toward disorder or randomness is summarized in the law
of disorder.
- The law of disorder states that spontaneous processes always proceed
in such a way that the entropy of the universe increases.
- This law is also called the second law of thermodynamics.
Predicting changes in entropy:
- The change in enthalpy for a reaction is equal to the enthalpy of
the products minus the enthalpy of the reactants.
- The change in entropy during a reaction is similar.
- The change in entropy equals the entropy of the products minus
the entropy of the reactants.
Change in S (system) = S (products) - S (reactants)
- Therefore, if the entropy of a system increases during a reaction,
S (products) > S (reactants) and Change in S (system) is positive.
- If the entropy of a system decreases during a reaction,
S (products) < S (reactants) and Change in S (system) is negative.
- You can predict if Change in S (system) is positive or negative by examining
the equation for a reaction or process.
1) Entropy changes associated with changes in state can be predicted.
- In solids, molecules are tightly packed and have limited movement, but they
have some freedom to move in liquids.
- In gases, molecules are unrestricted in their movements.
- Therefore, entropy increases as a substance changes from a solid to a liquid
and from a liquid to a gas.
- For example, the entropy of the system increases (Change in S system is
positive) as water vaporizes.
H2O (l) ----> H2O (g) Change in S (system) > 0
2) The dissolving of a gas in a solvent always results in a decrease in
entropy.
- Gas particles have more entropy when they can move freely in the gaseous
state than when they are dissolved in a liquid or solid that limits their
movements and randomness.
- For example, Change in S (system) is negative for the dissolving of carbon
dioxide in water.
CO2 (g) ----> CO2 (aq) Change in S (system) < 0