1. Prairie High School Chemistry
Entropy and Enthalpy
Prairie High School
Chemistry

2. First Law of Thermodynamics
The total amount of energy (and mass) in the universe is constant.
In any process energy can be changed from one form to another; but it can never be created nor destroyed.
“You can’t get something for nothing”

3. Enthalpy (Heats) of Reaction
REVIEW: The amount of heat released or absorbed by a chemical reaction at constant pressure (as one would do in a laboratory) is called the enthalpy or heat or reaction.
We use the symbol H to indicate enthalpy. +H indicates that heat is being absorbed in the reaction (it gets cold) endothermic
Positive H is endothermic and negative H is exothermic

4. Enthalpy of Formation Standard Enthalpy of Formation H f 
The amount of heat absorbed (endothermic) or released (exothermic) in a reaction in which one mole of a substance is formed from its elements in their standard states.
Also called heat of formation.
Going from a solid to a liquid

5. Hess’s Law: Adding Reactions
The overall heat of reaction (Hrxn) is equal to the sum of the Hf (products) minus the sum of the Hf (reactants):
H = rxn °  H (products) f° -  H (reactants)
Therefore, by knowing Hf of the reactants and products, we can determine the Hrxn for any reaction that involves these reactants and products
See problem…

6. Entropy
The final state of a system is more energetically favorable if:
1. Energy can be dispersed over a greater number and variety of molecules.
2. The particles of the system can be more dispersed (more disordered).

7. Chaos= Entropy
The dispersal of energy and matter is described by the thermodynamic state function entropy, S.
The greater the disorder (dispersal of energy and matter, both in space and in variety) the higher the entropy. Adding heat to a material increases the disorder.
Example: ice is well structured, so it has a low entropy. Gas is very disordered, having a large entropy.

8. Entropy
+S means that entropy is increasing, meaning its getting more disordered.
Mother nature likes this…
- S means that the system is getting less disordered. Mother nature doesn’t like, but it can happen.

9. Rules of Entropy
1) Entropy increases as one goes from a solid to a liquid, or more dramatically, a liquid to a gas.
2) Entropy increases if a solid or liquid is dissolved in a solvent.
3) Entropy increases as the number of particles (molecules) in a system increases:
N2O4(g)  2NO2(g)
S° = 304 J/K (1 mole)
S° = 480 J/K (2 moles)

10. Spontaneous Reaction?
Neither entropy (S) or enthalpy (H) alone can tell us whether a chemical reaction will be spontaneous or not.
An obvious (?) conclusion is that one needs to use some combination of the two.

11. Gibbs Free Energy
The combination of entropy, temperature and enthalpy explains whether a reaction is going to be spontaneous or not.
The symbol G is used to define the Free Energy of a system.
"Free" energy refers to the amount of energy available to do work once you have paid your price to entropy.
Note that this is not given simply by H, the heat energy released in a reaction.

12. Calculations Gº = Hº  T Sº
When G is negative, it indicates that a reaction or process is spontaneous.
A positive G indicates a non-spontaneous reaction.