AP Chemistry B Exam Prep Session Thermodynamics

Three Laws of Thermodynamics: ΔE = q + w q describes the heat absorbed (+, endothermic) or released (-, exothermic) by system; also known as enthalpy w describes the work done on system (+); or done by the system (-) The energy of the universe is constant. heat lost = heat gained law of conservation of energy

Three Laws of Thermodynamics: 2. In any spontaneous process there is always an increase in the entropy of the universe. Spontaneous processes occur without outside intervention & can be of variable speed. Entropy, S = measure of disorder or randomness. Exothermic reactions tend to be spontaneous. Increases in entropy tend to be spontaneous. S solid < S liquid <<< S gas

ΔS = Sproducts – Sreactants S (+) entropy increases S (-) entropy decreases Predict ΔS sign: H2O(s)  H2O(l) CO2(g) CO2(s) NaCl(s)  NaCl(aq) 2 NaHCO3(s)  NaCO3(s) + CO2(g) + H2O(g) O2(g) + 2 SO2(g)  2 SO3(g) Entropy

Three Laws of Thermodynamics: The entropy of a perfect crystal at zero Kelvin is zero.

ΔG = ΔH - T ΔS Gibbs Free Energy ΔG (-) spontaneous change ΔG (0) equilibrium, no driving force Values can be calculated by using accepted values ΔG = Ʃ products – Ʃreactants Hess’s Law and equations.

Spontaneity: enthalpy vs entropy ΔG = ΔH - T ΔS

Sample multiple choice ?s Some questions in this section start with a list of choices followed by numbered statements (a) ΔH >0, ΔS > 0 1. Must be true for a reaction (b) ΔH > 0, ΔS < 0 that is spontaneous at all (c) ΔH < 0, ΔS > 0 temperatures (d) ΔH < 0, ΔS < 0 2. True for the evaporation (e) ΔH = 0, ΔS < 0 of water at 25˚C and 1 atm 3. True for the combustion of liquid pentane, C5H12(l) to form H2O(g) and CO2(g) at 1 atm.

4. If ΔG for a certain reaction has a negative value at 298 K, which of the following must be true? I.The reaction is exothermic. II.The reaction occurs spontaneously at 298 K. III.The rate of the reaction is fast at 298 K. (a) I only (b) II only (c) I and II only (d) II and III only (e) I, II, and III

A Sample AP Free Response Question Topic: Thermodynamics http://apcentral.collegeboard.com/apc/public/repository/ap- chemistry-course-description.pdf

Wrapping up: Your Best Resource AP Central http://apcentral.collegeboard.com/apc/Controller.jpf Use the site to access tutorials, information, practice problems, the Equations Sheets and Physical Constants Sheet