Class Notes (807,339)
Chemistry (406)
CHEM 1050 (158)
Lori Jones (63)
Lecture 18

# CHEM 1050 Lecture 18: Copy of Chapter 18.

3 Pages
99 Views

School
University of Guelph
Department
Chemistry
Course
CHEM 1050
Professor
Lori Jones
Semester
Winter

Description
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
More Less

Related notes for CHEM 1050

OR

Don't have an account?

Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Join to view

OR

By registering, I agree to the Terms and Privacy Policies
Just a few more details

So we can recommend you notes for your school.