Chapter 18. Entropy, Free Energy and Equilibrium

CHAPTER 18. ENTROPY, FREE ENERGY AND EQUILIBRIUM Thermodynamics: Study of energy changes accompanying chemical or physical processes. ⇒ Thermodynamics tells us the direction and extent of the reaction, but it does not tell us how fast the reaction occurs. THE FIRST LAW OF THERMODYNAMICS Law of Conservation of Energy: The total amount of energy in the universe is constant. Energy can be converted from one form to another, but energy cannot be created or destroyed in ordinary chemical and physical processes. 1st Law of Thermodynamics: ∆E = q + w E = internal energy; sum of the kinetic and potential energies of the particles in a system. q = heat absorbed by the system (+) or heat released by the system to the surroundings (-). w = work done on the system (+) or by the system (-); work is the energy used when a force moves an object through a distance d. q at constant pressure: qp = ∆H ∆H = Enthalpy: Heat gained or lost by a system at constant pressure. State Function: Property of a system that depends only on its present state, which is determined by variables such as temperature and pressure. ∆E and ∆H are state functions. ∆H °f = Standard Molar Enthalpy of Formation: The heat change for the formation of 1 mole of a compound from its elements in their most stable forms under standard conditions. ⇒ ∆H °f = 0 for an element in its most stable form at 25 °C (298 K) and 1 atm pressure. ∆H °rxn = Σn∆H °f (products) - Σm∆H °f (reactants) 18.2 SPONTANEOUS PROCESSES Types of processes: 1) Spontaneous– naturally occurs under a given set of conditions. 2) Nonspontaneous - does not occur under a given set of conditions. ⇒ If a process is spontaneous in the forward direction, then the reverse process will be nonspontaneous. Examples of spontaneous processes: • • • Iron rusts when exposed to air and water. A waterfall runs downhill, not up. Ice melts at T > 0 °C. What makes a process spontaneous? • Tendency for a system to go to a lower state of energy. ⇒ Many spontaneous processes are exothermic (e.g. combustion reactions have ∆H < 0), but some spontaneous processes are endothermic (e.g. ice melting has ∆H > 0). • Tendency for a system to become more disordered or random. ⇒ Entropy of a system typically increases, ∆S > 0 (+∆S: increase in disorder) Thus, spontaneity is a balance between enthalpy and entropy. 1 18.3 ENTROPY Entropy, S: a measure of the disorder or randomness of a system. Entropy increases as the number of possible microstates increases. Microstate: microscopic arrangement of atoms or particles. What is the order of entropy values for solid, liquid and gaseous states? Solid state is highly ordered ⇒ smallest number of microstates ⇒ lowest entropy. Gas molecules move randomly in a disorderly manner ⇒ largest number of microstates ⇒ highest entropy. Standard Entropy, S°, the absolute entropy of a substance at 1 atm and 25 °C. J Units: . K mol ⇒ Entropy of an element in its most stable form is NOT zero. Trends for Standard Entropy values • • entropy of solid < liquid < gas more complex molecules have higher entropies (↑vibrational energy) E.g. Which do you predict to have the highest standard entropy, S°? A. H2O(s) B. Na(s) C. Br2(l) D. H20(g) E. C3H8(g) 18.4 THE SECOND LAW OF THERMODYNAMICS Second Law of Thermodynamics - The entropy of the universe always increases for a spontaneous process. ⇒ For a spontaneous process: ∆Suniv = ∆Ssys + ∆Ssurr > 0. ⇒ For an equilibrium process: ∆Suniv = ∆Ssys + ∆Ssurr = 0. Entropy Changes in the System Standard Entropy of Reaction: ∆S !rxn = ΣnS°(products) - ΣmS°(reactants) ∆S is usually + (∆S > 0) when 1. A solid changes to a liquid or gas, or a liquid changes to a gas. 2. The number of gas molecules increases (# product gas molecules than reactant gas molecules). 3. A molecule is broken into 2 or more smaller molecules. E.g. Predict whether ∆S is + or – for the following reactions A. 2SO2(g) + O2(g) 2SO3(g) ∆S is ____ B. CaCO3(s) CaO(s) + CO2(g) ∆S is ____ C. AgBr(s) - Ag+(aq) + Br (aq) ∆S is ____ Entropy Changes in the Surroundings Entropy of the surroundings is related to the heat gained or lost by the system: ∆Ssurr = - ∆H sys T Third Law of Thermodynamics - The entropy of a perfect crystal is zero at 0 K. 2 • • • At 0 K there are no translational, vibrational or rotational motions. As a substance is heated, entropy increases due to increased molecular motion and increased kinetic energy. Entropy of a substance is determined by measuring the specific heat of a substance at different temperatures. 18.5 GIBBS FREE ENERGY For spontaneous processes: ∆Suniv = ∆Ssys - ∆H sys T >0 Multiplying both sides by –T gives us: -T∆Suniv = ∆Hsys - T∆Ssys < 0 ∆G = Gibbs free energy change ∆G = ∆H - T∆S ∆G is the maximum amount of energy available to do work on the surroundings. Relationship between ∆G and spontaneity 1. ∆G < 0, spontaneous process in the forward direction; the process releases energy. 2. ∆G > 0, nonspontaneous process in the forward direction. 3. ∆G = 0, system is at equilibrium; no net change occurs. Standard Free Energy Changes ∆G °f = standard free energy of formation: the free energy change that occurs when 1 mole of a compound is formed from its elements in their standard states. ⇒ ∆G °f = 0 for an element in its most stable form at 1 atm pressure and 25 °C. ∆G !rxn = standard free energy of reaction: the free energy change that results when reactants in their standard states are converted to products in their standard states. ∆G !rxn = Σn∆G °f (products)- Σm∆G °f (reactants) The Gibbs equation provides another way to obtain the standard free energy for a reaction: ∆G° = ∆H° - T∆S° Affect of Temperature on Spontaneity ∆H ∆S + ∆G Always negative − Reaction is spontaneous at all temperatures + − Always positive Reaction is nonspontaneous at all temperatures. − − - at low T Reaction is spontaneous at low T, but becomes nonspontaneous at high T. + at high T + + + at low T - at high T Reaction Outcome Reaction is nonspontaneous at low T, but becomes spontaneous at high T. 3 How do we determine the temperature at which a process goes from nonspontaneous to spontaneous (or vice versa)? Set ∆G° = 0: 0 = ∆H° - T∆S° T = ∆H°/∆S° ⇒ Note that this temperature is an approximation because we are assuming that ∆H° and ∆S° don’t change with temperature. Phase Transitions Between Physical States At the temperature at which the phase transition occurs, the system is at equilibrium: ∆H - T∆S = 0 ⇒ ∆S = ∆H T 18.6 FREE ENERGY AND CHEMICAL EQUILIBRIUM ∆G = actual free energy change; free energy change that results when reactants and products are not in their standard states. Free Energy, ∆G, Changes During the Course of a Reaction: 1. ∆G° is a large negative number (∆G° < 0). Spontaneous process; reaction mix is mostly products. G°(reactants) -∆G° G°(products Reaction Progress → 2. ∆G° is a large positive number (∆G° > 0).Nonspontaneous process; reaction mix is mostly reactants. G°(products +∆G° G°(reactants) Reaction Progress → ⇒ G = 0 at the minimum point; this is the equilibrium mixture of reactants and products. ⇒ ∆G (actual amount of free energy) changes as the reaction progresses; but ∆G° (standard free energy) has a fixed value. Relationship between ∆G and ∆G°: ∆G = ∆G° + RT ln Q At equilibrium, ∆G = 0 and Q = K: ∆G° = -RT ln K or rearranging K = e-∆G°/RT Relationship between ∆G° and K 1. ∆G° is negative: K > 1 (product favored) 2. ∆G° is zero: K = 1 3. ∆G° is positive: K < 1 (reactant favored) 4