CHE 141 1st Edition Lecture 27Outline- Predicting Entropy and Entropy Changes- The Entropy Change for a Chemical Reaction delta S reaction- The 3rd Law of Thermodynamics- Standard Molar Entropy S- Heat Transfer and Entropy Changes of the Surroundings- Temperature Dependence of delta SsurrPredicting Entropy and Entropy Changes- Increasing entropy means increasing randomness - Entropy increases when a solid (vibrational energy only) becomes a liquid (rotational and transitional energy)- Entropy increases when a liquid becomes a gas (expands to fill available volume)- Entropy increases when a solute dissolves (more disorder)- Entropy decreases when a gas dissolves in a solvent- Entropy increases with increasing temperature- At higher temperatures:o Higher KEThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.o More vibration, rotational and translational motiono More microstateso Higher entropy- Delta S >0=entropy increases- Delta S<0=entropy decreases- We can predict the entropy change of a chemical reaction according to:o Rule 1: entropy increases when a substance changes to a more disordered state of mattero Rule 2: entropy increases when mixtures are formed from pure substanceso Rule 3: entropy increases when the moles of gases increasesThe Entropy Change for a Chemical Reaction delta S reaction- The standard entropy change for a reaction is the change in entropy for a process in which all reactants and products are in their standard states- Since entropy is a state function: delta S=Sproducts-SreactantsThe 3rd Law of Thermodynamics- We can define an absolute zero for entropy- Consider a perfect crystal at 0K- Only one distribution of particles is possible therefore there is only one microstate- Substitute W=1 into S=kBlnWS=kB(ln 1)- Ln(1)=0 so S=0- The 3rd law of thermodynamics states the entropy of a perfect crystal is zero at absolute zero, S=0 at T=0KStandard Molar Entropy S- We can measure all standard entropy values against the absolute zero defined by the 3rd law- Standard molar entropy is the value of S for one mole of a pure substance in its standard state- The SI units for standard molar entropy are joules per mole per kelvin- The values are listed per mole as entropy is an extensive property it depends on the amount of substance- Molar mass: entropy increases with increasing molar mass- Ssolid<Sliquid<Sgas- Allotypes: More freedom of movement=more entropy- Dissolution: Dissolution of a crystalline solid into solution corresponds to an increase in entropy- Molar complexity: Standard molar entropy increases with molecular size- More bonds=more internal motion=higher entropy- It is often the case that molar mass and molecular complexity both increaseHeat Transfer and Entropy Changes of the Surroundings- The entropy change for the universe is the sum of the entropy changes for the system and surroundings: delta Suniv=delta Ssys + delta Ssurr- Consider a process with delta Ssys<0 such as water freezing- For delta Suniv>0, delta Ssurr must be >0 and greater in magnitude than delta S sys- The entropy of the surroundings increase because the process is exothermic- The release of heat energy into the surroundings, increasing the entropy of the surroundings- An exothermic process increases the entropy of the surroundings- An endothermic process decreases the entropy of the surroundings- Exothermic process delta Hsys<0- Heat is transferred to the surroundings- Motion of molecules in the surroundings increases- Entropy of the surrounding increases- Endothermic process delta Hsys>0- Heat is absorbed from the surrounding- Motion of molecules in the surroundings become less energetic - Entropy of the surroundings decreasesTemperature Dependence of delta Ssurr- Water freezing is spontaneous and exothermic at 0 degrees Celsius - This is not true at all temperatures: water is nonspontaneous above 0 degrees Celsius - Delta Ssurr is not constant- Units of entropy are JK-1- The magnitude of the increases in entropy of the surroundings is temperature dependent- The greater the temperature, the smaller the increase in entropy for a given amount of energy- At low temperature the negative delta Ssys is overcome by the large delta Ssurr resulting in a positive delta Suniv and a spontaneous process- At high temperature, the negative delta Ssys is not overcome by the small delta Ssurr resulting in a negative delta Suniv and a spontaneous process- Delta suniv=delta Ssys+delta
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