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1 THERMODYNAMICS Thermodynamics: is the branch of science which deals with deals with the study of different forms of energy and the quantitative relationship between them. Significance of Thermodynamics: to answer questions - 1. How do we determine the energy changes involved in a chemical reaction/process? 2. Will it occur or not? 3. What drives a chemical reaction/process? 4. To what extent do the chemical reactions proceed? TERMS: System- the part of universe in which observations are made and remaining part of universe is the Surroundings. Universe= System + Surrounding Open System: can exchange both matter & Energy with the surrounding Closed System: can exchange only energy with the surrounding but not matter, Isolated System: can neither exchange matter nor energy with the surrounding. State variables or state functions :Variables like p, V, T are called state variables or state functions because their values depend only on the state of the system and not on how it is reached. Isothermal Process: temperature remains constant, i.e. T=0 Adiabatic Process: no heat exchange between the system and the surrounding, i.e. Q=0 Isochoric process: volume of the system remains constant, i.e., V=0 Isobaric Process: pressure of the system remains constant, i.e., P=0 Cyclic Process: energy of the system remains the same, i.e., U=0 Internal Energy (U) U = Ee + En + Ec+ Ep + Ek Internal energy is a state function. First Law of thermodynamics: Energy can neither be created nor destroyed although it may be converted from one form to another form, Mathematically: U = q + w HEAT VS WORK Energy transfer as a result of temperature difference Endothermic = (+ q) Exothermic = (+ q) Energy expended to move an object against a force Work done on a system = (+ w) Work done by the system = (-w) Work (w): when the gas expands or contracts against external pressure (usually the atmospheric pressure). Sign: w it taken as +ve if work is done on the system. I.e. for compression and it is taken as -ve if work is done by the system ie. For expansion. W= - P ext x (V 2 V 1 ) = - P ext x V

2 1.For isothermal expansion of an ideal gas into vacuum; w = 0 since pext = 0. So: U = q - pext V =q - 0=q If process is carried out at constant volume, V=0, then U = q - pext V =q - 0=qv(heat exchange at constant volume) 2.For isothermal irreversible change; q = - W= Pext x (Vf Vi) = - Pext x V 3. For isothermal reversible change- 3. For adiabatic change, q = 0; therefore, U = wad 4. U = q - Pext x V; If a process is carried out at constant volume; V = 0, then ; U = qv A process or change is said to be reversible, if a change is brought out in such a way that the process could, at any moment, be reversed by an infinitesimal change. A reversible process proceeds infinitely slowly by a series of equilibrium states such that system and the surroundings are always in near equilibrium with each other. Processes other than reversible processes are known as irreversible processes. APPLICATION OF FIRST LAW Enthalpy (H): is the heat content of the system; H=U + PV Enthalpy Change ( H): is the sum of the increase in internal energy of the system and the pressure volume work done; H= U + P V H is -ve for exothermic H is +ve for endothermic reactions Heat of Reaction at Constant Volume (qv): q = U-w, w= -P V; therefore; q = U + P V at constant volume V=0, Therefore, qv= U Heat of Reaction at Constant pressure: qp = U + P V at constant pressure = (Uf Ui) + P (Vf Vi) = (Uf + PVf) (Ui+ PVi) = Hf Hi = H qp= H Since, pδv = ΔngRT, Therefore ΔH = ΔU + ΔngRT Where, Δng= np (gaseous)-nr(gaseous) Extensive property - Property whose value depends on the quantity of matter present in the system. Eg; mass, V, U, H, C, etc. Intensive properties: properties which do not depend on the quantity of matter present in the system, eg; temperature, density, pressure etc. Heat Capacity (C): of a system is the amount of heat required to raise the temperature of the system through 10C.

3 = C is also not a state function. Molar Heat Capacity (C m ): the amount of heat required to raise the temperature of 1 mole of the system through 1 0 C. Specific Heat capacity or Specific Heat (c): is the amount of heat required to raise the temperature of 1 gram of the system through 1 0 C. q = c m T = C T Relationship between Cp and C V for an ideal gas: Cp -C V = R MEASUREMENT OF U & H: Bomb calorimeter-used for measurement of U Calorimetry: is the measurement of the energy changes associated with chemical or physical processes. The vessel in which calorimetry is carried out is called calorimeter. Enthalpy ( r H): The enthalpy change accompanying a reaction rh = (sum of enthalpies of products) (sum of enthalpies of reactants) = aih products- bih reactants ai & bi are the stoichiometric coefficients of the products and reactants in the balanced chemical equation Standard enthalpy of reaction is the enthalpy change for a reaction when all the participating substances are in their standard states. The standard state of a substance at a specified temperature is its pure form at 1 bar. Enthalpy Change during Phase transformation: The enthalpy change during melting of one mole of a solid substance in standard state is called standard enthalpy or molar enthalpy of fusion, Δ fus Hº. Amount of heat required to vaporize 1 mole of a liquid at constant temperature and under standard pressure (1bar) is called its standard or molar enthalpy of vaporization, vap Hº Standard enthalpy of sublimation, subhº is the change in enthalpy when one mole of a solid substance sublimes at a constant temperature and under standard pressure (1bar). Standard enthalpy of formation ( f Hº): The standard enthalpy change for the formation of one mole of a compound from its elements in their most stable states of (also known as reference states) is called Standard Molar Enthalpy of Formation. r H 0 =[Sum of the standard enthalpies of formation of products]-[ Sum of the standard enthalpies of formation of reactants] r H 0 = a i f H 0 (Products)- b i f H 0 (Reactants) Thermochemical equations

A balanced chemical equation together with the value of its ΔrH and specification of the physical state (along with allotropic state) of the substance in an equation. 1.

4 A balanced chemical equation together with the value of its ΔrH and specification of the physical state (along with allotropic state) of the substance in an equation. 1. The coefficients in a balanced thermochemical equation refer to the number of moles of reactants & products involved in the reaction. 2. Δ r Hº will have units as kj mol When a chemical equation is reversed, the value of ΔrHº is reversed in sign. Hess s Law of Constant Heat Summation: If a reaction takes place in several steps then its standard reaction enthalpy is the sum of the standard enthalpies of the intermediate reactions into which the overall reaction may be divided at the same temperature. rh 0 =rh1+rh2 + rh3 ENTHALPIES FOR DIFFERENT TYPES OF REACTIONS: Standard enthalpy of combustion is the enthalpy change per mole of a substance, when it undergoes combustion and all the reactants and products being in their standard states at the specified temperature. Standard Enthalpy of atomization (Δ a H 0 ): is the enthalpy change on breaking one mole of bonds completely to obtain atoms in the gas phase. In case of diatomic molecules, enthalpy of atomization is also the bond dissociation enthalpy. In some cases, the enthalpy of atomization is same as the enthalpy of sublimation. Bond Enthalpy ( bond H 0 ): is the amount of energy released when one mole of bonds is formed from the isolated atoms in the gaseous state. Or The amount of energy required to dissociate one mole of bonds present between the atoms in the gaseous molecules. Relationship between rhº and bond enthalpies of the reactants and products in gas phase reactions: Δ r H= bondenthapies reactants - bondenthapies products Lattice Enthalpy: The lattice enthalpy of an ionic compound is the enthalpy change which occurs when one mole of an ionic compound dissociates into its ions in gaseous state. Na + Cl - (s) Na + (g) + Cl - (g) ; latticeh= +788kJmol -1 Born-Haber Cycle is the diagram which is helps determine lattice enthalpies we construct an enthalpy diagram called a

5 SPONTANEITY OF REACTION Spontaneity means having the potential to proceed without the assistance of external agency. Spontaneous processes cannot reverse their direction on their own. Summary: A spontaneous process is an irreversible process and may only be reversed by some external agency. Is decrease in enthalpy a criterion for spontaneity? CRITERIA FOR SPONTANEITY: common examples of spontaneous processes Water freezes spontaneously below 0º C, and ice melts spontaneously above 0º C at 1 atm pressure Flow of heat from hotter to colder object, but no other way around. A water fall runs downhill, but never uphill Sugar dissolves in a cup of coffee but it does not reappear in its original form Rusting of iron Conclusion: spontaneous processes occur in one direction but not the other direction. Are the exothermic reactions only spontaneous? No, there are some endothermic reactions that are spontaneous too. E.g.; like melting of ice, evaporation of water. Conclusion: Exothermicity favors spontaneity but does not guarantee the spontaneity. Energy is not the only factor for predicting the spontaneity. There is one more factor called Entropy Entropy(S): is a measure of randomness or disorder of the system. Ssolid < Sliquid < Sgas is a state function. increases with increase in temperature Entropy change ( S): is the amount of heat (q) exchange isothermally and reversibly divided by the absolute temperature (T) at which the heat is exchanged. S = S VAP q T T VAP b H f u s i o n S fusion = Tm Second Law of Thermodynamics: The entropy of universe is continuously increasing due to spontaneous process taking place in it. S T = S univ = S system + S surrounding If S total > 0, irreversible spontaneous process, If S total < 0 ; non spontaneous process If, S= 0, process is at equilibrium Gibbs free energy (G): is the maximum amount of energy available to a system during the process that can be converted into useful work. It is a measure of capacity to do useful work.

6 G = H - TS Change in free energy G: G = H - T S (i) If G is -ve, the process is spontaneous. (ii) If G is +ve, the process is non spontaneous. (iii) If, G= 0, process is at equilibrium Effect of Temperature on Spontaneity of Reactions: r H Θ r S Θ r G Θ Description* +ve +ve -ve spontaneous at all temperature -ve -ve at low T spontaneous at low temperature -ve -ve + at high T no spontaneous at high temperature +ve +ve + at low T no spontaneous at low temperature +ve +ve at high T spontaneous at high temperature +ve -ve + at all T no spontaneous at all temperatures Standard Free energy of formation ( fgº) It is free energy change when 1 mole of compound is formed from its constituting elements in their standard state. r G p f G ( pr oduc t s ) G ( r e ac t a n ts) Gibbs Energy Change and Electrical Work = -nfe cell, where E cell = standard cell potential G r GIBBS ENERGY CHANGE & EQUILIBRIUM: G 2.303RT log K r R f

6.Thermodynamics. Some Important Points and Terms of the Chapter

6.Thermodynamics. Some Important Points and Terms of the Chapter 6.Thermodynamics Some Important Points and Terms of the Chapter 1. System and the Surroundings: A system in thermodynamics refers to that part of universe in which observations are made and remaining universe