Enthalpy and Entropy

1. Enthalpy and Entropy

2. Enthalpy (H) Spontaneous change • change that will occur because of the nature of the system once it is initiated • occurs primarily in one direction, but can occur in both • does not necessarily occur quickly

3. Enthalpy (H) • total energy content of a system at constant pressure • (-) exothermic; reactants contain more energy than the products • (+) endothermic; products contain more energy than the reactants • changes that involve a decrease in energy are favored (exo) • ΔH = enthalpy change • ΔH = Hproducts– Hreactants • more energy released  the greater the decrease in enthalpy  more likely the reaction will occur

4. Enthalpy A potential energy graph ofan endothermic reaction: A potential energy graph ofan exothermic reaction:

5. Activation Energy (Ea) • minimum amount of energy that must be put into a system to start a chemical change Endothermic reactions • not as favored as exothermic reactions, but they do occur • occur if sufficient source of energy is available

6. Kinetic Energy • some particles have more kinetic energy than others • some reactions occur w/o added energy due to some of the particles having a higher kinetic energy than the necessary activation energy for the reaction

7. Enthalpy (H) Hess’s Law • overall enthalpy change in a reaction is equal to the sum of the enthalpy changes for the individual steps in the process • ΔHrxn= ΔHof products - ΔHof reactants • ΔHrxn • heat of reaction • heat absorbed or released during a reaction

8. Entropy (S) • measure of the degree of disorder • tendency in nature is more disorder • change in temperature affects entropy • chemical changes that produce gases are favored when looking only at entropy

9. Entropy (S) • absolute zero is O entropy • J/K or J/K•mol (Hess’s Law changes) • ΔS = Sproducts - Sreactants • positive value means that entropy increases • negative value means that entropy decreases

10. Enthalpy (H) vs Entropy (S) • decrease in entropy can be overcome with a decrease in enthalpy (exothermic reactions) • reactions resulting in lower enthalpy occur because the heat energy released by the reaction causes a larger increase in the entropy of the surroundings • ex. Refrigerators, air conditioners • Thermodynamically favored or spontaneous • opposite change will not occur unless reaction conditions change

11. Entropy (S) • Effects of entropy and enthalpy • dominant one determines the outcome example • ice melts if the temperature is above 0oC • change in entropy encourages melting, but the endothermic change in enthalpy discourages melting • entropy change is dominant over the enthalpy change at higher temperatures • Condensing of steam below 100oC • change in entropy discourages condensation, but the exothermic change in enthalpy encourages condensation • enthalpy change is dominant over entropy change at lower temperatures

12. Enthalpy vs. Entropy Summarization • ΔH ΔS spontaneity • exo (-) disorder (+) always (-) • exo (-) order (-) lower temp • endo (+) disorder (+) higher temp • endo (+) order (-) never (+)

13. Enthalpy vs. Entropy Free energy (G) • relates entropy and enthalpy • energy available to cause a change • ΔG is negative change is spontaneous • ΔG is positive change is not spontaneous • ΔG = 0 forward nor reverse reaction is favored(dynamic equilibrium; p + r exist) • ΔG = ΔH - TΔS • temperature must be in Kelvin

14. Hess’ Law Rules • If you multiply the chemical reaction by a number, then multiply ΔH by the same number. • If you flip the chemical reaction, then change the sign of ΔH.