Sem2Midterm-3.1 Laws of Thermodynamics PDF

THERMODYNAMICS 8 December 2024  Terminologies: SYSTEM - object or collection of objects under study SURROUNDINGS - everything around the system A system can be: OPEN - exchanges matter and energy CLOSED - exchanges energy only ISOLATED - seals matter and heat from exchange with another...

THERMODYNAMICS 8 December 2024  Terminologies: SYSTEM - object or collection of objects under study SURROUNDINGS - everything around the system A system can be: OPEN - exchanges matter and energy CLOSED - exchanges energy only ISOLATED - seals matter and heat from exchange with another system  OPEN System: exchanges matter and energy Real systems ○ human body ○ automobile engine ○ beaker  CLOSED System: exchanges energy only not real; imaginary ○ The earth ○ Covered beaker ○ Hot compress  ISOLATED System: neither energy nor matter can be exchange may be a portion of larger systems ○ The physical universe ○ Combustion of glucose in a bomb calorimeter ○ A closed thermos bottle or a sealed vacuum flask  First Law of Thermodynamics ➔ Conservation of Energy ➔ Energy is neither created nor destroyed; it can only be redistributed or changed from one form to another  First Law of Thermodynamics ➔ The change in the internal energy of a system is equal to the amount of ○ ΔU = change in internal heat supplied to the energy (Joules) ○ Q = heat added to the system minus the amount system (Joules) of work done on its ○ W = work done by the surroundings system (Joules) Flow of Heat  Sign Conventions PHYSICS (ΔU = Q - W) CHEMISTRY (ΔU = Q + W) ❖ +Q when heat is lost from the system ❖ -Q when heat is lost from the system ❖ -Q when heat is added to the system ❖ +Q when heat is added to the system ❖ -W when work is done on the system ❖ +W when work is done on the system ❖ +W when work is done by the system ❖ -W when work is done by the system +Q added to -Q lost from -Q added to the the system the system +Q lost from the system system SURROUNDIN SYSTEM SURROUNDIN -W done onGthe system +W done on -W done by G +W done by the system the system the system  Sample Problem: (a) 100 J of work is done on a system. Internal energy increases by 74 J. How much energy is transferred as heat? ➔ 26 J is absorbed heat by the surroundings or lost heat by the system.  Sample Problem: (b) A sample of gas does 150 J of work against its surroundings and loses 90 J of internal energy in the process. Does the gas gain or lose heat and how much? ➔ 60 J is gained heat by the gas  Thermodynamic Processes change in the temperature, pressure, volume or other properties of a system related to heat and energy transfer Thermodynamics Processes & PV Diagrams ISOCHORIC PROCESS constant volume closed system PV=nRT; P ∝ T W = 0; ΔU = Q energy change is due to heat transfer Example: heating a sealed gas container Thermodynamics Processes & PV Diagrams ISOBARIC PROCESS constant pressure PV=nRT; V ∝ T ΔU = Q + W; e.g.: heating a pot of water on a +W = system expands (+ΔV) stove -W= system contracts (-ΔV) Thermodynamics Processes & PV Diagrams ISOTHERMAL PROCESS constant temperature PV=nRT; P ∝ 1/V ΔU = 0; Q = W Q transferred is used to do W not increasing U e.g.: gas expanding slowly in a cylinder with a piston at a constant temperature Thermodynamics Processes & PV Diagrams ADIABATIC PROCESS no heat exchanged; insulated system PV=nRT; only nR is constant no external pressure W = 0 ; ΔU = -W e.g.: rapid expansion of a gas from a pressurized tank REVERSIBLE PROCESS - the system and its surroundings are returned to their original state at the end of the process ➔ Leaves no trace on either system or surroundings ➔ Net of heat and net work exchange between system and surrounding is zero IRREVERSIBLE PROCESS - process that leaves traces on either system or surroundings ➔ do not return to the initial state by themselves ➔ Factors: friction, unrestrained expansion, mixing of two gases, heat transfer through finite temperature difference  2nd Law of Thermodynamics & Heat Transfer ➔ Clausius Statement: Heat flows naturally from hot to cold objects ◆ Hot to cold - natural spontaneous process ◆ Cold to hot - non-spontaneous; needs an energy to do work 2nd Law of Thermodynamics & Entropy ➔ Entropy a measure of disorder or “unavailable energy” within a system ◆ Low entropy - system with organized and concentrated energy, ready for work (e.g., ice cube, diamond, a new car) ◆ High entropy - system with dispersed and unused energy, unavailable for work (e.g., steam, scrambled eggs, a cluttered room)  2nd Law of Thermodynamics & Entropy ➔ 2nd Law of Thermodynamics states that in any isolated system, entropy will always increase or constant, but never decrease ➔Q = ΔS x T where: ◆ ΔS of universe = 0 Q - heat added or released (J) for reversible process ◆ ΔS of universe > 0 ΔS - change in entropy (J/K) for irreversible process T - temperature (K) Phase Change Latent heat of vaporization (L v) Latent heat of fusion Note: (L f) Lv > L f HEAT ENERGY ADDED  2nd Law of Thermodynamics & Entropy Q = ΔS x T Q = mL Q = mcΔT 1. Calculate the entropy change of melting 15 g of ice at 0oC. The latent heat of fusion for ice is 334 J/g  2nd Law of Thermodynamics & Entropy Q = ΔS x T Q = mL Q = mcΔT 2. Calculate the entropy change in heating 2 kg of water from 0oC to 100oC. The specific heat capacity of water is 4184 J/kgCo.  Q = ΔS x T Q = mL Q = mcΔT 2nd Law of Thermodynamics & Entropy 3. 3kg of water at 70oC is mixed with 3kg of water at 30oC. The specific heat capacity of water is 4184 J/kgCo. a) What is the final temperature of the mixture? b) Calculate the entropy change that occurs when mixing the two samples of water.  Q = ΔS x T Q = mL Q = mcΔT 2nd Law of Thermodynamics & 3. 3kgEntropy o o o of water at 70 C is mixed with 3kg of water at 30 C. The specific heat capacity of water is 4184 J/kgC. a) What is the final temperature of the mixture? b) Calculate the entropy change that occurs when mixing the two samples of water. 2nd Law of Thermodynamics & Entropy 2nd Law of Thermodynamics & Heat Engines ➔ Heat Engines are devices that convert thermal energy into mechanical work; ◆ Heat (QH)- supplied to the engine by an external source; hot reservoir ◆ Some heat - used to do work on an object ◆ Heat (QC)- released at lower temp. than the input temp. to an external place - cold reservoir 2nd Law of Thermodynamics & Heat Engines ➔ Heat Engines are devices that convert thermal energy into mechanical work; ◆ 2nd Law- not all the heat in the system can be converted into work ◆ Engine Efficiency - less than 100% conversion of heat to work  2nd Law of Thermodynamics & Heat Engines ◆ Carnot Engine- a theoretical model; most efficient heat engine within specific operating temperatures ◆ Entropy Increase and Work - less than 100% conversion of heat to work  2nd Law of Thermodynamics & Heat Engines 1. Calculate the maximum efficiency of a heat engine with operating temperatures of 300oC and 500oC.  2nd Law of Thermodynamics & Heat 2.Engines A carnot engine is operating at temperatures of 400K and 700K a) If 14 000 J of heat energy is absorbed by the engine, how much heat energy is discarded into the cold reservoir? b) How much mechanical work is performed by the engine? c) Calculate the efficiency of this carnot engine.  Topic for Revision - Quiz 3.1 Thermodynamics Heat and Temperature ◆ 1st Law ◆ Zeroth Law ΔU, W and Q ◆ Thermal Expansion Endothermic vs Exothermic Thermodynamic processes Quiz 3.1 - Heat, Temperature, and PV diagrams and Laws of ◆ 2nd Law Thermodynamics heat transfer Jan 15, 2025 (Wed) entropy heat engine