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Chemistry 100 Chapter 19 . Spontaneity of Chemical and Physical Processes: Thermodynamics. What Is Thermodynamics?. Study of the energy changes that accompany chemical and physical processes. Based on a set of laws.
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Chemistry 100 Chapter 19 Spontaneity of Chemical and Physical Processes: Thermodynamics
What Is Thermodynamics? Study of the energy changes that accompany chemical and physical processes. Based on a set of laws. In chemistry, a primary application of thermodynamics is as a tool to predict the spontaneous directions of a chemical reaction.
What Is Spontaneity? Spontaneity refers to the ability of a process to occur on its own! Can the Niagara Falls suddenly reverse? “Ice will melt, water will boil,” Neil Finn, Tim Finn of Crowded House/Plant ‘It’s Only Natural’. Water spontaneously freezes on a cold winter day!
The First Law of Thermodynamics The First Law deals with the conservation of energy changes. E = q + w The First Law tells us nothing about the spontaneous direction of a process.
Entropy and Spontaneity • Need to examine • the entropy change of the process as well as its enthalpy change (heat flow). • Entropy – the degree of randomness of a system. • Solids – highly ordered low entropy. • Gases – very disordered high entropy. • Liquids – entropy is variable between that of a solid and a gas.
Entropy Is a State Variable Changes in entropy are state functions S = Sf – Si Sf = the entropy of the final state Si = the entropy of the initial state
Entropy Changes for Different Processes S > 0 entropy increases (melting ice or making steam) S < 0 entropy decreases (examples freezing water or condensing steam)
The Solution Process Highly ordered – low entropy Disordered or random state – high entropy The formation of a solution is always accompanied by an increase in the entropy of the system! For the dissolution of NaCl (s) in water NaCl (s) Na+(aq) + Cl-(aq)
The Entropy Change in a Chemical Reaction • Burning ethane! C2H6 (g) + 7/2O2 (g) 2CO2 (g) + 3H2O (l) • The entropy change rS np S (products) - nr S (reactants) • np and nr represent the number of moles of products and reactants, respectively.
Finding S Values Appendix C in your textbook has entropy values for a wide variety of species. Units for entropy values J / (K mole) Temperature and pressure for the tabulated values are 298.2 K and 1.00 atm.
Finding S Values Note – entropy values are absolute! Note – the elements have NON-ZERO entropy values! e.g., for H2 (g) fH = 0 kJ/mole (by def’n) S = 130.58 J/(K mole)
Some Generalizations For any gaseous reaction (or a reaction involving gases). ng > 0, rS > 0 J/(K mole). ng < 0, rS < 0 J/(K mole). ng = 0, rS 0 J/(K mole). For reactions involving only solids and liquids – depends on the entropy values of the substances.
The Second Law of Thermodynamics • The entropy of the universe (univS) increases in a spontaneous process. • univS unchanged in an equilibrium process
What is univS? univS = sysS + surrS sysS = the entropy change of the system. surrS = the entropy change of the surroundings.
How Do We Obtain univS? We need to obtain estimates for both the sysS and the surrS. Look at the following chemical reaction. C(s) + 2H2 (g) CH4(g) The entropy change for the systems is the reaction entropy change, rS. How do we calculate surrS?
Calculating surrS Note that for an exothermic process, an amount of thermal energy is released to the surroundings!
Calculating surrS Note that for an endothermic process, thermal energy is absorbed from the surroundings!
Connecting surrS to sysH For a constant pressure process qp = H surrS surrH = -sysH surrS = -sysH / T For a chemical reaction sysH = rH surrS = -rH/ T
The Use of univS to Determine Spontaneity • Calculation of TunivS two system parameters • rS • rH • Define a systemparameter that determines if a given process will be spontaneous?
The Definition of the Gibbs Energy The Gibbs energy of the system G = H – TS For a spontaneous process sysG = Gf – G i Gf = the Gibbs energy of the final state Gi = the Gibbs energy of the initial state
Gibbs Energy and Spontaneity Note that these are the Gibbs energies of the system under non-standard conditions sysG < 0 - spontaneous process sysG > 0 - non-spontaneous process (note that this process would be spontaneous in the reverse direction) sysG = 0 - system is in equilibrium
Standard Gibbs Energy Changes • The Gibbs energy change for a chemical reaction? • Combustion of methane. CH4 (g) + 2 O2 (g) CO2 (g) + 2 H2O (l) • Define • rG = np fG (products) - nr fG (reactants) • fG = the formation Gibbs energy of the substance
Gibbs Energy Changes fG (elements) = 0 kJ / mole. Use tabulated values of the Gibbs formation energies to calculate the Gibbs energy changes for chemical reactions.
The Third Law of Thermodynamics Entropy is related to the degree of randomness of a substance. Entropy is directly proportional to the absolute temperature. Cooling the system decreases the disorder.
The Third Law of Thermodynamics The Third Law - the entropy of any perfect crystal is 0 J /(K mole) at 0 K (absolute 0!) Due to the Third Law, we are able to calculate absolute entropy values.
At a very low temperature, the disorder decreases to 0 (i.e., 0 J/(K mole) value for S). The most ordered arrangement of any substance is a perfect crystal!
Applications of the Gibbs Energy • The Gibbs energy is used to determine the spontaneous direction of a process. • Two contributions to the Gibbs energy change (G) • Entropy (S) • Enthalpy (H) G = H - TS
Gibbs Energies and Equilibrium Constants rG < 0 - spontaneous under standard conditions rG > 0 - non-spontaneous under standard conditions
The Reaction Quotient Relationship between QJ and Keq Q < Keq - reaction moves in the forward direction Q > Keq - reaction moves in the reverse direction Q = Keq - reaction is at equilibrium
rG° refers to standard conditions only! For non-standard conditions - rG rG < 0 - reaction moves in the forward direction rG > 0 - reaction moves in the reverse direction rG = 0 - reaction is at equilibrium
Relating Keq to rG rG = rG +RT ln Q rG = 0 system is at equilibrium rG = -RT ln Qeq rG = -RT ln Keq
Phase Equilibria At the transition (phase-change) temperature only - trG = 0 kJ tr = transition type (melting, vapourization, etc.) trS = trH / Ttr