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D:\Comp Backup\Important\Softwa Thermodynamics CHAPTER THERMODYNAMICS 6 LEARNING OBJECTIVES (i) Explain the terms : system and surroundings. Discriminate between close, open and isolated systems. (ii) Explain internal energy, work and heat. Heat absorbed or evolved is q = CT. Work can be measured by w = –pexV, in case of expansion of gases. Under reversible process, we can put pex = p for infinitesimal changes in the volume making wrev = – p dV. In this condition, we can use gas equation, pV = nRT. (iii) Differentiate between extensive and intensive properties. (iv) Define spontaneous and nonspontaneous processes. (v) State first law of thermodynamics and express it mathematically. (vi) Calculate energy changes as work and heat contributions in chemical systems. (vii) Explain enthalpy. Calculate enthalpy changes for various types of reactions. Enthalpy change, H = E + ngRT, where E is change in internal energy can be found directly from the heat changes at constant pressure, H = qp. There are varieties of enthalpy changes. Changes of phase such as melting, vaporization and sublimation usually occur at constant temperature and can be characterized by enthalpy changes which are always positive. (viii) State and apply Hess’s law of constant heat summation. Enthalpy of formation, combustion and other enthalpy changes can be calculated using Hess's law. Enthalpy change for chemical reactions can be determined by rH (a i f H products) (b i f H reactants) f i and in gaseous state by rH bond enthalpies of the reactants – bond enthalpies of the products. (ix) Explain entropy as a thermodynamic state function and apply it for spontaneity. Entropy is a measure of disorder or randomness. For a spontaneous change, total entropy change is positive. Entropy changes can be measured by the equation q q S rev for a reversible process. rev is independent of path. T T (x) Explain Gibbs energy change (G); Establish relationship between G and spontaneity, G and equilibrium constant. Gibbs energy, G, which is related to entropy and enthalpy changes of the system by the equation: rG = rH – T rS For a spontaneous change, Gsys < 0 and at equilibrium, Gsys = 0. Standard Gibbs energy change is related to equilibrium constant by G RT ln K . INTRODUCTION Thermodynamics (means literally flow of heat) is a physical science dealing with the quantitative relation between heat and mechanical energy. Thus, in broad sense, it deals with the relationship of heat to all other forms of energy such as electrical energy, mechanical energy, chemical energy, light, kinetic energy, etc. The entire formulation of thermodynamics is based on a few (three) fundamental laws which have been established on the basis of human experience of the experiment behaviour of macroscopic aggregates of matter collected over a long period of time. Thermodynamics helps in – (a) Determining feasibility of a particular process i.e., whether or not a particular process will occur under a given set of conditions. (b) Determining the extent to which a reaction would proceed before attainment of equilibrium. (c) Most important laws of physical chemistry such as Raoults’s law, vant’ Hoff law, distribution law, phase rule, law of equilibrium, laws of thermochemistry and expression for elevation in boiling point and depression in freezing point are in accordance with laws of thermodynamics. Thermochemistry is a branch of thermodynamics which deals with the relationships between chemical reactions and the corresponding energy changes. It is based on first law of thermodynamics. There are two laws of thermochemistry : (i) Lavoisier and Laplace law (ii) Hess’s law Gyaan Sankalp 1 Thermodynamics SOME FUNDAMENTAL DEFINITIONS : 1. System : Thermodynamics system is defined as any portion of matter, under consideration, which is separated from the surroundings by real or imaginary boundaries. Thus, a system might be as simple as a gas contained in a flask or as complicated as a rocket shooting towards the moon. A thermodynamics system may be homogeneous or heterogeneous, but has to be macroscopic. 2. Surrounding : Surrounding is the rest of the universe around the system. A system and its surroundings are always separated by real (fixed or movable) or imaginary boundaries, through which matter and energy may be exchanged between the two. Ordinarily, surroundings means a water or air bath. 3. Boundary : The imaginary line which separates the system from the surrounding is called boundary. 4. Types of system : (i) Open System : It is the system which is capable of exchanging both matter and energy (as heat) with the surroundings. For example, plants and living beings are all open systems, since they are capable of exchanging both matter and energy with their surroundings. Similarly, an open reaction vessel (like flask, beaker, test-tube) is also an example of open system. (ii)Closed System : It is the system which is capable of exchanging only energy (as heat or work) with the surrounding, but exchange of matter is not possible. For example, a sealed flask containing a gas (or matter) is a closed system. (iii) Isolated System : It is the system which can exchange neither matter nor energy with the surroundings. For such a system, the matter and energy remain constant. We can say that in an isolated system, all matter is "trapped", i.e., no mass can escape, and no energy can leak in or out. 5. State of a System and state variable : (i) The existence of a system under a given set of conditions is called a state of systems. (ii)The properties which change with change in the state of system are called as state variables e.g., pressure, volume and temperature etc. The first and last state of a system are called initial state and final state respectively. 6. State function and Path Function : A physical quantity is said to be state function if its value depends only upon the state of the system and does not depend upon the path by which this state has been attained. For example, a person standing on the roof of a five storeyed building has a fixed potential energy, irrespective of the fact whether he reached there by stairs or by lift. Thus the potential energy of the person is a state function. On the other hand, the work done by the legs of the person to reach the same height, is not same in the two cases i.e., whether he went by lift or by stairs. Hence work is a ‘path function’. 7. Extensity and Intensive properties : An extensive property of a system is that which depends upon the amount of the substance or substances present in the system. e.g., mass, volume, energy etc. An intensive property of a system is that which is independent of the amount of the substance present in the system e.g., temperature, pressure, density, velocity etc. 8. Thermodynamic process : (a) Isothermal System : Isothermal system is a one in which reaction is carried out at constant temperature, i.e., T = 0. When a system undergoes an isothermal process, the system is usually kept in contact with a constant temperature bath (called thermo- stat) and the system maintains its temperature constant by exchange of heat with the thermostat. (b) Adiabatic System : Adiabatic system is a one in which no heat can leave or enter the system (i.e., thermally insulated). Thus, for carrying out adiabatic process, the system is carefully insulated from the surroundings. It may be pointed out that in an adiabatic process, temperature of the system may increase or decrease. (c) Isobaric Process : Isobaric Process is one in which reaction is carried out at constant pressure, i.e., P = 0. For example, a reaction taking place in an open vessel is always at atmospheric pressure and hence, such a reaction is isobaric. (d) Isochoric Process : Isochoric process is one in which the volume of the system is kept constant, i.e.,V = 0. (e) Cyclic Process : Cyclic Process is the overall process when the system in a given state goes through a number of different processes and finally returns to its initial state. (f) Reversible Process : A process which can be performed in the reverse direction, the whole series of changes constituting the process being exactly reversed, i.e., the direction of a reversible process can be reversed by an infinitesimal change in the state of the system. (g) Irreversible Process : Irreversible process is one which goes from the initial to final state in a single step and cannot be carried in reverse order. 9. Internal energy (E or U) : The total energy stored in a substance by virtue of tis chemical nature and state is called its internal energy, i.e., it is the sum of its translation, vibrational, rotational, chemical bond energy, electronic energy, nuclear energy of constituent atoms and potential energy due to interaction with neighbouring molecules. It is also called intrinsic energy. E = Et + Er + Ev + Ee + En + EPE Internal energy is a state property and its absolute value can’t be determined. However, change in internal energy (difference between the internal energies of the products and that of reactants) can be determined experimentally using a bomb calorimeter. Internal energy of a system depends upon : (a) the quantity of substance (b) its chemical nature and (c) temperature, pressure and volume. 2 Gyaan Sankalp Thermodynamics (i) For a given system, E is directly proportional to its absolute temperature. (E T) (ii) At constant volume, the quantity of heat supplied to a system (isochoric process) is equal to the increase in its internal energy, i.e., QV = E (iii) In the adiabatic expansion of a gas, it gets cooled because of decrease in internal energy. (iv) In cyclic process the change in internal energy is zero (E = 0) since E is a state function.
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