Chem 108 1st Edition Lecture 26Current LectureChapter 18: ThermodynamicsReview of Thermochemistry: - Energy is exchanged between system and surroundings - Chemical reactions are systems - Signs of energy changes are system centric - Reactions can be exothermic (q<0) or endothermic (q>0)- At constant pressure, ΔH = qP - Calorimeters measure q- Energy is a state function. State function: function only of initial and final state of system, not on path from initial to final state - First Law of Thermodynamics: Energy of the Universe is Constant Δsystem + Δsurroundings= 0 o Δsystem = q + wo Energy cannot be created or destroyed- Calculation of q: calorimetry: q = mCsΔT - Calculation of w: PV work: w = PΔV (constant P)Second Law of Thermodynamics:- The second law of thermodynamics states that the total entropy of a system and its surroundings always increases for a spontaneous process.- The net change in entropy of the system, ΔS, equals the sum of the entropy created (or destroyed) during the spontaneous process and the change in entropy associated with the heat flow.- ΔSuniverse=ΔSsystem+ΔSsurroundings ≥0- ΔSsys = (q/T) q is measured in joulesEntropy and Molecular Disorder:- Entropy (S): A measure of the distribution of energy in a system at a specific temperature. Energy distribution affected by molecular motion, volume- Entropy is essentially related to energy dispersal. The entropy of a molecular system may be concentrated in a few energy states and later dispersed among many more energy states. The entropy of such a system increases. - Ex: In the case of the cup of hot coffee, as heat moves from the hot coffee, molecular motionbecomes more disordered. In becoming more disordered, the energy is more dispersed.These 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.- This is also why to recharge a battery with 100 kJ of useful energy will require more than 100kJ, because as the second law of thermodynamics states the entropy always increases and energy is always dispersed, so there is energy lost to heat.Spontaneous Processes:- Spontaneous process: One that proceeds in a given direction without outside intervention. o ΔSuniv > 0 process is spontaneous- Nonspontaneous process: Occurs only as long as energy is added to the system.o ΔSuniv < 0 process is nonspontaneous- Spontaneity depends on dispersion of energy that occurs during a process.- | ΔSsurr| > | ΔSsys|- Although many spontaneous processes are exothermic (e.g., combustion), not true for all spontaneous reactions- Non-spontaneous processes do not violate the first law of thermodynamics - The Second Law of Thermodynamics predicts spontaneityEntropy and Microstates: - Microstate: A unique distribution of particles among energy levels.- The motion of molecules is quantized: Different molecular states related to molecular motion are separated by specific energies.- Three types of motion: o Translational—movement through space. o Rotational—spinning motion around axis to bond. o Vibrational—movement of atoms toward/away from each other. - As temperature increases, the amount of motion increases.- The number of times a molecule occupies an accessible energy level follows a Boltzmann distribution: Number of energy states increases as volume increases.- Accessible energy levels move closer together if you increase volumeo Number of accessible energy states
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