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45607328-Thermodynamics - Thermodynamics NARAYAN TUTORIALS...

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Unformatted text preview: Thermodynamics NARAYAN TUTORIALS IITian Sushil Kumar The branch of science which deals with study of different forms of energy and their interconversion is called thermodynamics. A system in thermodynamics refers to that part of universe in which observations are made. The remaining portion of universe which is not part of system constitutes the surroundings. The surroundings include everything other than the system. The wall (real or imaginary) that separates the system from the surroundings is called boundary. Types of system Exchange of energy Exchange of matter Open Yes Yes Closed Yes No Isolated No No The state of a thermodynamic system is described by its measurable or macroscopic (bulk) properties. The state of the surroundings can never be completely specified. State variables are the measurable properties of system required to describe the state of the system. Examples are temperature, pressure, volume etc. Various types of processes:Type of process Definition Isothermal ( T = 0) Process in which temperature of system remains constant Adiabatic ( Q= 0) Process in which there is no transfer of heat between the system and surroundings Isobaric ( P= 0) Process in which pressure of system remains constant Isochoric( V= 0) Process in which volume of system remains constant Sign Convention: Work done by the system W = positive Work done on the system W = negative Heat transferred From surroundings to the system Q = positive Heat transferred From system to the surroundings Q = negative First law of Thermodynamics:First law of thermodynamics is also called as law of conservation of energy i.e. energy can neither be created nor destroyed. It also states that the energy of an isolated system is constant. Q = U+ W Expression of Work: W=0 Isochoric W= P(V2±V1) = nR(T2±T1) Isobaric W = nRT ln(V2/V1) = 2.303nRT log(V2/V1) Isothermal W = (P1V1-P2V2)/( ± 1) = (nRT1-nRT2)/( ± 1) Adiabatic Reversible and Irreversible process:- 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. Free Expansion:- Expansion of a gas in vacuum (Pex= 0) is called free expansion. No work is done during free expansion of an ideal gas whether the process is reversible or irreversible. Enthalpy:- The enthalpy of a system may be defined as the sum of the internal energy and the product of its pressure and volume. It is denoted by the symbol H and is given by H = U + PV. H= U+P V NARAYAN TUTORIALS Page 1 Thermodynamics NARAYAN TUTORIALS IITian Sushil Kumar Types of reaction:Type of reaction Definition H Exothermic reactions Reactions in which heat is evolved during the reaction Negative Endothermic reactions Reactions in which heat is absorbed during the reaction positive Extensive and Intensive property: An extensive property is a property whose value depends on the quantity or size of matter present in the system. Examples are mass, volume, internal energy, enthalpy, heat capacity, etc. An intensive property is a property whose value does not depend on the quantity or size of matter present in the system. Examples are temperature, density, pressure etc. Specific heat capacity: Specific heat, also called specific heat capacity is the quantity of heat required to raise the temperature of one unit mass of a substance by one degree Celsius (or one Kelvin). Q = m x S x T; At constant pressure, QP = nCP T = H; At constant volume QV = nCV T = U For ideal gas: CP ± CV = R Calorimetry: -Calorimetry is an experimental technique that helps determining energy changes associated with chemical or physical processes. Reaction enthalpy: - The enthalpy change accompanying a reaction is called the reaction enthalpy ( rH). rH = (sum of enthalpies of products) - (sum of enthalpies of reactants) The 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 pressure of 1 bar. It is denoted by ( rH ). Thermochemical equation: - A balanced chemical equation together with the value of its rH is called a thermochemical equation. 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 A rH B = rH1 + rH2 + rH3 rH1 rH3 C rH2 D Different types of Enthalpy: Type of enthalpy Standard Enthalpy of fusion or Molar enthalpy of fusion Standard Enthalpy of vaporization or Molar enthalpy of vaporization Standard Enthalpy of sublimation Enthalpy of formation or Standard NARAYAN TUTORIALS Definition The enthalpy change that accompanies melting of one mole of a solid substance in standard state The enthalpy change that accompanies vaporizing of one mole of a liquid at constant temperature and under standard pressure (1bar) The enthalpy change when one mole of a solid substance sublimes at a constant temperature and under standard pressure (1bar). The enthalpy change for the formation of one mole of a Symbol fusH VapH subH fH Page 2 Thermodynamics NARAYAN TUTORIALS molar enthalpy of formation compound from its elements in their most stable states of aggregation. Enthalpy change when 1 mole of substance is completely burnt in excess of oxygen or air. Enthalpy change in breaking one mole of bonds of a substance completely into atoms in gaseous state. Amount of energy required to break one mole of bond of a particular type between atoms in gaseous state Enthalpy of solution of a substance is the enthalpy change when one mole of substance dissolves in a specified amount of solvent. solH = latticeH + hydH Enthalpy change which occurs when one mole of an ionic compound dissociates into its ions in gaseous state. Enthalpy change when one gram equivalent of an acid is completely neutralized by one gram equivalent of a base. H+(aq)+OH-(aq) H2O(l) ; H=-13.7kcal Enthalpy of combustion Enthalpy of atomisation Bond dissociation enthalpy Enthalpy of solution Lattice enthalpy Enthalpy of Neutralisation IITian Sushil Kumar cH aH bondH solH latticeH neuH Born-Haber Cycle is used to determine lattice enthalpy of ionic compounds since they cannot be determined by experiment directly. Spontaneous and non-spontaneous process: A process that has natural tendency of occurrence in a particular direction and is reversible only by application of some external agency is known as spontaneous process. The processes which are forbidden and are made to take place only by supplying energy continuously from outside the system are called non-spontaneous process. Entropy: - Entropy (S) is the measure of randomness of a system. It is a state function. Entropy increases from solid to gas. Entropy is maximum for gases as they have maximum disorder. According to Second law of thermodynamics, entropy of the universe always increases during a spontaneous change. For reversible process entropy change is given by: S = ( Qrev)/ T ; Qrev = is heat absorbed or released during the reaction and T is the temperature of the reaction At constant pressure Qrev = H; S = ( H)/T For spontaneous process Stotal = S system + Ssurr > 0 Gibbs energy: - Gibbs function or Gibbs energy is denoted by G. G=H-TS ; G = H - T S The criteria for spontaneous reaction in relation to G at constant pressure and constant temperature is If G < 0, process is spontaneous If G = 0, process is in equilibrium If G > 0, process is non-spontaneous. Relation between Gibbs energy change G0and equilibrium constant (K):G0 = -2.303RT logK NARAYAN TUTORIALS Page 3 Thermodynamics NARAYAN TUTORIALS Effect of temperature on Spontaneity of Reactions: H0 S0 G0 + - (at low T) + (at high T) + + + (at low T) + + - (at high T) + + (at all T) NARAYAN TUTORIALS IITian Sushil Kumar Description Reaction spontaneous at all temperature Reaction spontaneous at low temperature Reaction nonspontaneous at high temperature Reaction nonspontaneous at low temperature Reaction spontaneous at high temperature Reaction nonspontaneous at all temperatures Page 4 ...
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