# Chapter 19

Review sections 5.2 – 5.7 in the textbook, especially topics including: The 1st Law of Thermodynamics, Enthalpy, and how q and ΔH are related.

In Chapter 5, the term enthalpy was introduced and in this chapter we will use enthalpy and introduce a new concept, entropy, to tell a more complete story of Thermodynamics.

First, we will look into combustion reactions (which are exothermic) and see that as we progress from reactants to products there is a lowering in the potential energy stored in the chemical bonds, and the potential energy is converted to thermal energy (release of heat).

19.1 Thermodynamics

The first law of Thermodynamics provides the means for accounting for energy, but it gives no hint as to why a particular process occurs in a given direction. A process is considered to be spontaneous if it occurs without outside intervention, and the driving force for a spontaneous process is an increase in entropy. We can define entropy as the measure of randomness or disorder and entropy can be expressed mathematically using macro/microstates and we can also compare the entropy of various systems.

19.1 Spontaneous Process and Entropy

19.2 Mathematical Definition of Entropy

Show PHET tutorial of an ideal gas.

19.3 Macro and Micro States

19.3 Comparing Entropy of Various Systems

The second law of Thermodynamics states that in any spontaneous process there is always an increase in entropy of the universe.

19.3 2nd Law of Thermodynamics

The third law of Thermodynamics states that the entropy of a pure crystal at zero degress kelvin is zero. The change in entropy of a reaction can be calculated from the standard entropy of each substance.

19.3 3rd Law of Thermodynamics

Gibbs Free Energy is a state function combining enthalpy and entropy in the form of G = H – TS. J. Willard Gibbs developed this equation, which provides a convenient way to use the change in enthalpy and change in entropy to predict whether a given reaction occurring at constant pressure and constant temperature will be spontaneous. The derivation of this equation can be seen below.

19.5 Free Energy (G)

We can also consider the various situations for the relative signs of ΔH and ΔS to predict how ΔG will change with temperature. We can use the chart generated in the video below to investigate the effect of temperature on the spontaneity of a chemical reaction.

19.5 Predicting the Sign of Delta G

19.6 Free Energy Under Non-standard Conditions

19.6 Thermochemistry of the Haber Process