Unit 3: Stability and Change

Chemists explain the observed instability or stability of a physical system (i.e., its tendency to change or not) under specific conditions using energy concepts. Chemists define changes in these systems according to their starting and ending compositions. All systems tend toward equilibrium, a state where all change appears (visually) to have stopped but is still occurring without any net changes in composition. Chemists use energy concepts to describe two aspects of change toward equilibrium—the extent of the change and the rate of that change. This module introduces students to the energy concepts needed to describe how the composition of a system changes on the way to equilibrium (i.e., enthalpy, entropy, and the Gibbs free energy) and the energy concepts needed to describe a system’s relative rate of change (i.e., activation energy). These ideas are used to explain and predict physical changes (e.g., physical state changes, dissolutions, protein folding, etc.). Chemists recognize that matter can not only change phase (liquid, solid, gas), but also transform in more profound ways—into different substances. These chemical changes are also both knowable and predictable. Chemists explain chemical changes mostly as rearrangements of atoms and redistributions of electron density, based on interactions between positive and negative charge centers. After this module, students should be able to: Use models of structure and energy to describe how systems undergo chemical and/or physical changes toward achieving equilibrium.