Glossary
Endergonic (ΔG° > 0)
A reaction that absorbs free energy and is nonspontaneous under the given conditions, requiring energy input to proceed.
Example:
Photosynthesis, where plants convert light energy into chemical energy, is an endergonic process that requires continuous light input.
Endothermic (ΔH° > 0)
A process that absorbs heat from its surroundings, resulting in a positive enthalpy change.
Example:
An instant cold pack feels chilly because the chemical reaction inside is endothermic, drawing heat from your skin.
Enthalpy Change (ΔH°)
The heat absorbed or released by a system at constant pressure during a chemical reaction or physical process.
Example:
When calculating the total energy released by burning propane, you are determining the enthalpy change for the combustion reaction.
Enthalpy-Driven
Describes a reaction whose spontaneity is primarily due to a significant release of heat (negative ΔH°), even if it results in a decrease in entropy.
Example:
Combustion reactions are typically enthalpy-driven because the large amount of heat released makes them highly spontaneous.
Entropy Change (ΔS°)
The change in the disorder or randomness of a system during a chemical reaction or physical process.
Example:
When ice melts into liquid water, the molecules become more disordered, leading to a positive entropy change.
Entropy-Driven
Describes a reaction whose spontaneity is primarily due to a significant increase in the system's disorder (positive ΔS°), even if it is endothermic.
Example:
The dissolving of salt in water is often entropy-driven; even though it might absorb a little heat, the increased disorder of ions in solution makes it spontaneous.
Equilibrium Constant
A value that expresses the ratio of product concentrations to reactant concentrations at equilibrium, indicating the extent to which a reaction proceeds.
Example:
A large equilibrium constant (K >> 1) for a reaction indicates that products are heavily favored at equilibrium, suggesting a spontaneous reaction.
Exergonic (ΔG° < 0)
A reaction that releases free energy and is spontaneous under the given conditions.
Example:
The cellular respiration process, which breaks down glucose to produce ATP, is an exergonic reaction that powers biological functions.
Exothermic (ΔH° < 0)
A process that releases heat to its surroundings, resulting in a negative enthalpy change.
Example:
The burning of a candle is an exothermic reaction, as it releases heat and light into the room.
Gibbs Free Energy (ΔG°)
A thermodynamic potential that measures the maximum reversible work that a system can perform at constant temperature and pressure; it is the primary indicator of a reaction's spontaneity.
Example:
Calculating the Gibbs Free Energy for the synthesis of ammonia helps determine if the reaction is spontaneous under specific conditions.
Negative ΔS°
Indicates a decrease in the disorder or randomness of a system.
Example:
The formation of a highly ordered crystal from a solution results in a negative ΔS° as the system becomes more organized.
Nonspontaneous
Describes a process or reaction that requires continuous external energy input to occur.
Example:
Pushing a ball uphill is a nonspontaneous process because it requires constant effort to overcome gravity.
Positive ΔS°
Indicates an increase in the disorder or randomness of a system.
Example:
The sublimation of dry ice (solid CO₂ turning directly into gas) results in a positive ΔS° because the gas phase is much more disordered than the solid.
Spontaneous
Describes a process or reaction that occurs naturally without continuous external energy input once initiated, if at all.
Example:
A ball rolling downhill is a spontaneous process, as it moves from a higher potential energy state to a lower one without needing a constant push.
Standard Free Energies of Formation (ΔG°f)
The change in Gibbs free energy that accompanies the formation of one mole of a substance from its constituent elements in their standard states.
Example:
Using standard free energies of formation for reactants and products allows chemists to calculate the overall ΔG° for a complex reaction without direct measurement.
Thermodynamic Favorability
A concept that predicts whether a chemical reaction or physical process will occur spontaneously under a given set of conditions without external intervention.
Example:
The rusting of iron is a thermodynamically favorable process, meaning it will happen naturally over time in the presence of oxygen and water.