Basics of Thermodynamics: Easy learning by Dr. Anjana Sen

Transcription

1 Basics of Thermodynamics: Easy learning by Dr. Anjana Sen Part 1: Theory and concept Part 2: Definitions and equations Part 3: Laws of Thermodynamics

2 Part 1: theory and concept Thermodynamics means conversion of thermal energy (heat) to mechanical energy (work). Thermodynamics deals with properties of systems where temperature is an essential coordinate. Attention! What is possibility of reaction? Thermodynamics. What is rate of reaction? Kinetics. Why do we perform evaluation of thermodynamics parameters? in order to elucidate molecular mechanisms of interactions. in order to facilitate rational design of interactions. Attention! 3 important E s in thermodynamics. (1) Energy (2) Equilibrium (3) Entropy Thermodynamic variables (1) Temperature (2) Pressure (3) Volume isolated system Part 2: definitions and equations A system that is not subject to outside influence. State Function A function relating to the state variables that depends only on the current equilibrium state of the system. State Function does not depend on the path taken to arrive at the present state or specific value.

3 Heat The flow of energy which changes the temperature in a system. Extent of temperature change depends on (1) The amount of heat (2) The identity of the object heat of reaction gaining heat (melting) (vaporization) solid state liquid state gaseous state (freezing) (condensation) losing heat

4 Internal Energy is defined as the sum of the kinetic and potential energies of the particles that form the system. Change in Internal Energy of a system is equal to the sum of the heat gained / lost by the system + the work done by / on the system. heat transferred work done Enthalpy is an extensive thermodynamic quantity and a state function. It is defined as the total heat content of the system under certain very specific conditions. Enthalpy is evaluated as the sum of the internal energy + pressure multiplied volume. Enthalpy reflects capacity to do non-mechanical work.

5 Simple description of Enthalpy change in Enthalpy = heat evolved (released/absorbed) at constant pressure. At constant pressure, ΔH = q p At constant volume, ΔE = q v Effect of temperature on Enthalpy Temperature increases molecular interaction increases internal energy rises ΔH increases

6 Endothermic heat absorbed Exothermic heat emitted

7 Please note that absolute value of Enthalpy does not exist total Enthalpy of a system cannot be measured only the difference in Enthalpy ΔH is measured Why do we use the concept of Enthalpy? Because it s the preferred expression of changes in the system energy. Because it simplifies the description of energy transfer. common examples Enthalpy of hydrogenation Enthalpy of neutralization Enthalpy of denaturation Enthalpy of vaporization Enthalpy of sublimation Enthalpy of combustion Enthalpy of atomization Enthalpy of formation Enthalpy of hydration Enthalpy of reaction Enthalpy of solution Enthalpy of fusion Discussing now Entropy definition of Entropy Entropy is a thermodynamic quantity and a state function. Entropy is a measure of: the amount of energy, which is unavailable to do useful work. the multiplicity of a system. how far the equilibrium has progressed.

8 Thermodynamic Definition of Entropy It s the transfer of heat energy at uniform absolute temperature of a closed system in a reversible process. Physical meaning of Entropy Entropy is a measure of spreading and sharing of thermal energy within a system. Entropy applies only to molecular-level systems exchanging thermal energy with the surroundings. What is Isentropic Process? The process where Entropy of the system remains constant, ΔS = 0 What are bulk properties? composition, volume, energy, pressure, temperature,.. There can be trillions of different microscopic states, at constant and defined bulk properties. role of Entropy order & chaos Entropy defines existing number of different microscopic states in a system. Role of Entropy is sorting of energy into different states in a system. Entropy describes the course of a process whether it s a spontaneous process with a probability of occurring in a defined direction.

9 Entropy increases due to external interaction: exchange of heat with surroundings. due to internal changes: disturbance inside the system. Entropy is a measure of microscopic disorder of a system. Entropy is a measure of disorder or randomness (different possible arrangements) of a system. larger Entropy more possible microscopic states more disorder

10 Increasing Entropy provides the basic reason for the events happening in the universe. more microscopic states greater molecular disorder enhanced flexibility greater molecular mobility higher degrees of freedom larger Entropy

11 ΔS = change in entropy can be + or increasing disorder = positive decreasing disorder = negative

12 not a naturally occurring process definition of Gibbs free energy Discussing now Gibbs Free Energy

13 ΔG determines: direction of the chemical reaction extent of the chemical reaction feasibility of the chemical reaction Energy is partitioned into the maximum number of states possible Entropy increases here additional Entropy decreases here Discussing now Heat Capacity

14 definition of Heat Capacity In a defined system, it s the amount of heat needed to raise the temperature by 1

15 Discussing now Equilibrium Equilibrium of a reaction: The reaction has stopped progressing The converted amount of reactants remains constant The amount of leftover reactants remains constant definition of equilibrium constant

16 Potential Energy energy absorbed Activation Energy Potential Energy energy released Activation Energy Discussing now Exothermic / Endothermic Exothermic Reaction reactants products reaction progress Endothermic Reaction products reactants reaction progress

17 exothermic endothermic more energy is released by formation of new bonds less energy is consumed by breaking old bonds weaker bonds are replaced by stronger bonds less energy is released by formation of new bonds more energy is consumed by breaking old bonds stronger bonds are replaced by weaker bonds Discussing now favourable / spontaneous +ΔS favors spontaneous change! favorable occurrence of the chemical process unfavorable conditions ΔH 0 < 0 ΔH 0 > 0 ΔS 0 > 0 ΔS 0 < 0 spontaneous ΔG 0 < 0 K eq > 1 non-spontaneous ΔG 0 > 0 K eq < 1 equilibrium ΔG 0 = 0 K eq = 1

18 is the reaction spontaneous? ΔG = ΔH TΔS if ΔG is negative, the reaction is spontaneous if ΔH is negative (exothermic), the reaction is spontaneous if ΔS is positive, the reaction is spontaneous reactants favorable enthalpy ΔH favorable entropy +ΔS + ΔG = ΔH TΔS products always negative products + ΔG = ΔH TΔS always positive unfavorable enthalpy +ΔH unfavorable entropy ΔS reactants

19 reactants + ΔG = ΔH TΔS reactants + ΔG = ΔH TΔS favorable enthalpy ΔH unfavorable entropy ΔS unfavorable enthalpy +ΔH favorable entropy +ΔS + ΔG = ΔH TΔS products + ΔG = ΔH TΔS products Enthalpy Entropy Free Energy exothermic, ΔH < 0 increased disorder, ΔS > 0 spontaneous, ΔG < 0 endothermic, ΔH > 0 increased order, ΔS < 0 not spontaneous, ΔG > 0

20 Part 3: Laws of Thermodynamics Zeroth law of thermodynamics If 2 systems are in thermal equilibrium with a 3 rd system, they are in thermal equilibrium with each other. This law defines the notion of temperature. 1st law of thermodynamics This is the law of conservation of energy. Energy cannot be created or destroyed in an isolated system. Energy only changes form. net heat net work 2nd law of thermodynamics Heat always flows from hot to cold. Entropy of any isolated system always increases. Entropy approaches a maximum value at equilibrium. 3rd law of thermodynamics Entropy approaches a constant value, when temperature approaches absolute zero. Entropy at absolute zero = logarithm of the product of the quantum ground states. The perfect crystal has only one minimum energy state and absolutely no molecular motion.