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Lecture 18

CHEM 1050 Lecture 18: Copy of Chapter 18.

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University of Guelph
CHEM 1050
Lori Jones

Chapter 18: Thermodynamics and Equilibrium Entropy  Entropy (S) - state function - is a measure of randomness or disorder of a system.  It measures how dispersed the energy of a system is among the different possible ways that system can contain the same amount of energy. SI unit: joules per Kelvin (J/K).  It is an extensive quantity - dependent on the size of the system.  There are two types of disorders: 1. Positional  Positional disorder involves the distribution of species in space.  Ex. mixing of two gases or the expansion of a gas into a vacuum. 2. Thermal  Thermal disorder involves the distribution of energy among species, or distribution of species among energy levels.  Ex. Heat flow from hot to cold objects. Spontaneous Processes and Entropy  Spontaneous Process - physical/chemical change that occurs by itself until equilibrium is attained.  Non–Spontaneous Processes require external work to make the process occur.  Change in Entropy (S) can be calculated by subtracting initial entropy from final entropy.  Comparison of entropy in different states: S = S final  S initial  Solids < Liquids < Aqueous Solutions < Gases  The reaction side with the more moles of gas, has a greater entropy.  The more the room there is for energy to disperse (create disorder), the higher the entropy. Second Law of Thermodynamics  Second Law of Thermodynamics - total entropy of universe MUST  for a spontaneous process.  Heat flow (q) is also a flow of entropy, because it is a dispersal of energy.  A chemical reaction will favor the side with minimum enthalpy and maximum entropy (disorder).  When enthalpy and entropy OPPOSE each other (one favors reactants and one favors products), the reaction will reach a state of equilibrium.  For spontaneous processes, S univ 0 S universe= S system + S surroundings  For non-spontaneous processes, S univ 0  For processes at equilibrium, univ= 0  Entropy Change for a phase transition at an absolute temperature:  Third Law of Thermodynamics  Third law of thermodynamics - a substance that is perfectly crystalline at 0 K has an entropy of zero.  Absolute Entropy(S) - standard entropy - is the entropy value for the standard state of the species.  S values for stable forms of pure elements at 298K and 1 atm are not 0 and are ALWAYS positive.  Entropy increases in the following situations:  Entropy decreases when:  Molecule is broken into two or more smaller molecules.  Gas is dissolved in a solvent.  Solid or liquid dissolved into water.  Reaction in which there is an increase in moles of gas.  When pressure is increased.  Decrease in the moles of gas.  Expansion of gas or when solution is diluted.  Solid changes to liquid or gas or a liquid changes to a gas.  Increase in mass or molecular flexibility. ΔS = (Σn S products  (Σn S reactants Entropy, Pressure and Temperature  Entropies are strongly dependent on pressure (for gases) and concentration (for solutes).  When pressure , volume  causing disorder to , therefore entropy .  When pressure , volume  causing disorder to , therefore entropy .  When temperature , motion , causing disorder to , therefore entropy .  When temperature , motion , causing disorder to , therefore entropy . Free Energy and Spontaneity  Free Energy (G) is the thermodynamic energy that can do useful work. It is also called Gibbs free energy.  Change in Gibbs free energy (G) - state function - is a measure of the change in disorder of the universe.  For a chemical reaction:  If H Rxnis negative and S Rxnis positive, then the reaction will be spontaneous ,as both enthalpy and entropy favour the reaction.  If H Rxnis positive and SRxn is negative, then the reaction will be non-spontaneous, as both enthalpy and entropy disfavour the reaction (the reverse reaction will be spontaneous and occur instead).  If H Rxnis positive (endothermic) & S Rxnis positive ( disorder), then reaction is ENTROPY driven.  If H Rxnis negative(exothermic) & S Rxnis negative ( disorder), then reaction is ENTHALPY driven.  The total change in free energy can be defined by the eq
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