Lecture Slides Chapter 12

Thermodynamics: Why Chemical Reactions Occur

Chemical Thermodynamics

Chemical thermodynamics is the study of the energetics of chemical reactions. It focuses on how energy is transferred and transformed during chemical processes, and helps predict whether reactions will occur spontaneously under given conditions.

• Thermodynamics deals with energy changes, especially heat and work, in chemical systems.

• Chemical thermodynamics specifically examines the energy changes associated with chemical reactions.

Spontaneous and Nonspontaneous Processes

Understanding whether a process occurs naturally or requires external energy is fundamental in thermodynamics.

• Spontaneous process: A process that occurs without continuous energy input from outside the system.

• Nonspontaneous process: A process that only occurs as long as energy is continuously added to the system.

• The spontaneity of a process depends on the dispersion of energy that occurs during the process.

Predicting Spontaneity: Chemical Potential Energy and Free Energy

Thermodynamics predicts whether a process will occur under the given conditions by comparing the chemical potential energy before and after the reaction.

• Processes that occur are called spontaneous.

• Nonspontaneous processes require energy input to proceed.

• Spontaneity is determined by comparing the chemical potential energy of the system before the reaction with the free energy of the system after the reaction.

• If the system after the reaction has less potential energy than before, the reaction is thermodynamically favorable.

• Spontaneity ≠ fast or slow: A reaction can be spontaneous but occur very slowly (e.g., diamond turning into graphite).

Directionality of Process

The direction in which a process proceeds is determined by its spontaneity and reversibility.

• A spontaneous process is irreversible because there is a net release of energy and it proceeds in one direction.

• A reversible process will proceed back and forth between two states; at equilibrium, there is no net change in free energy.

• If a process is spontaneous in one direction, it must be nonspontaneous in the reverse direction.

Exothermic and Endothermic Processes

Most spontaneous processes occur because they release energy (exothermic), but some spontaneous processes can be endothermic.

• Exothermic: Spontaneous processes usually proceed from a system of higher potential energy to a system at lower potential energy, releasing energy.

• Endothermic: Some spontaneous processes absorb potential energy yet have a net increase in entropy, making them spontaneous.

Example: The melting of ice at temperatures above 0°C is spontaneous even though it absorbs heat (endothermic), due to the increase in entropy.

Key Terms and Concepts

• Chemical potential energy: The energy stored within the chemical bonds of a substance.

• Free energy (G): The energy in a system that is available to do work at constant temperature and pressure.

• Enthalpy (H): The heat content of a system at constant pressure.

• Entropy (S): A measure of disorder or randomness in a system.

Important Equations

• Change in free energy: $\Delta G = \Delta H - T\Delta S$

• Spontaneity criterion: If $\Delta G < 0$, the process is spontaneous.

Summary Table: Spontaneity and Energy Changes

Process Type	Energy Change	Spontaneity
Exothermic	Energy released ($\Delta H < 0$)	Often spontaneous
Endothermic	Energy absorbed ($\Delta H > 0$)	Can be spontaneous if $\Delta S$ is large and positive

Additional info: The notes above are based on textbook slides and introductory content for Chapter 12 of a General Chemistry course, focusing on thermodynamics and the energetics of chemical reactions.