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Summary
Key concepts
- The branch of thermodynamics which deals with the study of process in which chemical energy is involved is called chemical thermodynamics
- Application of thermodynamics :
- We can predict feasibility of the reaction that is if two substances are mixed then the reaction between them will takes place or not.
- If reaction does take place then what are the energy changes involved during the reaction.
- If in a chemical reaction, equilibrium is going to get attained then what will be the equilibrium concentrations of different reactants & products, can be calculated with thermodynamics
- Limitations of Thermodynamics: Laws of thermodynamics are applicable to matter in bulk or on system as a whole, these can not be applied on individual particles(temperature, pressure, enthalpy etc have meanings only for system as a whole).
- System: It is any part of universe that is under thermodynamic study at that instant
- Surroundings :The remaining part of the universe, other than system is called surroundings.
Universe = system + surroundings - Boundary: Any real (or) imaginary, rigid (or) non – rigid surface that separates system and surroundings.
- Types of system: Systems are classified on the basis of their interaction with the surroundings as follows:
- 1. Open System : The system where matter and energy are exchanged with surroundings. Boundary is not sealed and not insulated Ex: All living beings, Reactants in open vessel. Δ Q ≠ 0 ; Δ m ≠ 0
- 2. Closed system: The system where only the energy but not the matter is exchanged with the surroundings. Boundary is sealed but not insulated. Ex:A closed steel container having hot water. Δ Q ≠ 0 ;Δ m = 0
- 3. Isolated system: The system which does not exchange either the matter or energy with the surroundings. Boundary is sealed and insulated. Ex: A perfectly insulated, closed flask containing water. Δ Q = 0 ; Δ m = 0
- State functions or state variables : The Thermodynamic properties whose values depend only upon the initial and final states of the system and are independent of the path are called state functions. eg: Internal energy (E), Enthalpy (H), Entropy (S), Gibb's energy (G), Pressure (P), Temperature (T), Volume (V), Number of moles etc.
- Path Function: The property of a system that depends on the path of the process. eg: work, heat.
- Thermodynamic Process: The operation which brings about the changes in the state of the system is termed as thermodynamic process.
- Isothermal Process : A process in which temperature of system does not change throughout the studies. For an isothermal process dT = 0 and dU = 0. An isothermal process is achieved by using thermostatic control.
- Adiabatic Process: A process in which exchange of heat between system and surroundings does not take place. For an adiabatic process q = 0. It can be achieved by insulating the boundaries of system.
- Isobaric Process: A process in which pressure of the system remains constant throughout the studies. For an isobaric process ΔP = 0 .
- Isochoric Process: A process in which volume of the system remains constant throughout the studies. For an isochoric process ΔV = 0
- Cyclic Process: A process in which initial state of system is regained after a series of operations. For a cyclic process ΔU = 0 and ΔH = 0
- Reversible Process: A reversible process (or quasistatic process) is one in which all changes occurring at any part of the process are exactly reversed when change is carried out in opposite direction. It gives rise to maximum work
- Irreversible Process: An irreversible process is one in which direction of the change cannot be reversed by small change in variables. An irreversible process is a real one and all process which naturally occur are irreversible. It involves a) A fast change during investigation, b) Driving force is much higher than opposing force
- c) Wrev > Wirr (Expansion)
- Extensive Property : It is the property of a substance that depends on the quantity or size of matter present in the system. Ex:, Mass, volume of a gas, Internal energy, Enthalpy, entropy, heat capacity, Gibbs energy, heat content, no of moles etc
- Intensive Property : It is the property of a substance that does not depend on the quantity or size of matter present in the system. Ex: Density, molar properties (such as molar volume, molar entropy, molar heat capacity) surface tension, viscosity, specific heat, refractive index, pressure, temperature, boiling point, freezing point, vapor pressure etc.,
- The product, ratio and sum of two extensive properties is intensive. Ex: Mass and volume are extensive but density = (M/V) is an intensive property
- Internal Energy (E or U) : It is the sum of all types of potential and kinetic energies of constituent particles of a given substance at given temperature.
It may be chemical, electrical, mechanical or any other type of energy. Denoted by U of the system, which may change, when a) Heat passes into (or) out of the system b) Work is done on (or) by the system c) Matter enters (or) leaves the system. It is an extensive property and a state function. It is impossible to determine the absolute value of 'U' of a substance. But the change of Internal energy of a system ( Δ U) can be determined.
Δ U = heat absorbed (or) released in a process at constant volume and temperature Δ U = Qv (Δ U = Ufinal – Uinitial) ; Δ U of a chemical reaction is determined in a Bomb calorimeter. For any chemical reaction Δ U = UP – UR; UP = Total internal energy of the products, UR = Total internal energy of the reactants a) For exothermic reaction, ΔU is negative (UP < UR;) b) For endothermic reaction, ΔU is positive (UP > UR) - Heat : Heat is a form of energy. It flows from one system to another because of difference in temperature. Heat absorbed by the system is positive, i.e., q > 0. Heat given out by the system is negative i.e., q < 0
- Work Done in Isothermal Reversible Expansion of Ideal Gas : Total work done on the gas in finite steps is equal to sum of all the infitismal workdone in each step – Σ Δp V With change in infitismal pressure on the system the volume decreases by infitismal amount dV, then total workdone in finite steps from V1 to V2 is given by
Wrev= –2.303nRT logV2/V1 = –2.303nRT logP1/P2 - Free Expansion Expansion of a gas in vacuum: Pext = 0 is called free expansion. No work is done during free expansion of an ideal gas whether the process is reversible (or) irreversible. For isothermal (T = constant) expansion of an ideal gas into vacuum W = 0 since Pext = 0
Work Done in Isothermal Irreversible Expansion: W = – PExt ΔV - Work Done in Adiabatic Process: qp = 0; W = ΔU
In P – V diagram (called indicator diagram, the area under P – V curves represents work done. If expansion W = –ve and if compression W = +ve - Enthalpy (H) : The total heat content of a system at constant pressure and temperature is called enthalpy. It is a state function and an extensive property. It is calculated as the sum of internal energy and the product of pressure and volume.
H = U + PV ,It is impossible to determine the absolute value of enthalpy. Δ H of a process can be calculated as Δ H = Δ U + W, For finite changes at constant pressure, we can write equation ΔH = ΔU + P ΔV - Heat Capacity and Specific Heat: Heat capacity (C) of a substance is the amount of heat required to raise its temperature through one degree
C = q/dT; q = heat absorbed by the system, dT = rise in temperature - Heat capacity at constant volume (Cv) gives the measure of the change of internal energy (U) of a system with temperature. ΔU = n Cv dT
- Heat capacity at constant pressure (CP) gives the measure of the change of enthalpy (H) of a system with temperature. ΔH = n CpdT
- Specific Heat Capacity (c): The quantity of heat required to raise the temperature of 1 gram of substance through 1K (or 1°C), specific heat capacity(C) = (q/m); q = cm Δ T
- Relation Between Cp and Cv for an Ideal Gas: Cp – Cv = R
- Calorimeter : For this water at higher temperature (t2°C) of known mass (m2) and water at lower temperature (t1° C) and mass (m1) are used. These two are mixed in the calorimeter and the resultant temperature (t3°C) is noted.
W = [m2 (t2 – t3)/(t3 – t1) – m1]
Heat liberated = (W + volume of reaction mixture) × rise in temperature - Exothermic Reaction: A chemical reaction, which occurs with the evolution of heat
- Endothermic Reactions : A chemical reaction, which occurs with the absorption of heat from the surroundings
- Heat of Reaction : The quantity of heat liberated or absorbed at constant temperature when the reactants undergo a complete transformation into the products as per the stoichiometric equation
- Molar enthalpy of Fusion(ΔH0fuss): The enthalpy change that accompanies melting of one mole of a solid substance in standard state is called standard enthalpy of fusion or molar enthalpy of fusion,
- Molar Enthalpy of Vapourization (ΔH0vap): Amount of heat required to vapourize one mole of a liquid at constant temperature and under standard pressure (1bar) is called its standard enthalpy of vapourization.
- Molar enthalpy of Sublimation (ΔHsub ): The change in enthalpy when one mole of a solid substance sublimes at a constant temperature and under standard pressure Ex: 1) solid CO2 or "dry ice" sublimes at 195K with 0 1 H K
- Enthalpy of Formation : The amount of heat energy released or absorbed, when one mole of a compound is formed in its standard physical state by the combination of elements taken in their standard physical states, is known as the standard heat of formation of the compound. The following general equation can be used for the enthalpy change calculation.
- Enthalpy of Combustion : The quantity of heat evolved when one mole of a substance burns completely in excess of oxygen at a given temperature and constant volume is called the heat of combustion of the substance. The heat of combustion is always negative,
- Enthalpy of Atomization(ΔaHO): The heat required to dissociate one mole of a simple molecule in the gaseous state into its constituent atoms is called enthalpy of atomization.
- Enthalpy of Bond Dissociation: The amount of energy required to break 1 mole of a particular bond in a given compound and to separate the resulting gaseous atoms or ions or radicals is bond dissociation energy.
- Enthalpy of Solution (ΔH0sol): The Enthalpy change when one mole of substance is dissolved in a specified amount of solvent is called enthalpy of solution
Δ Hsol = Δ H0lattice + Δ H0hyd - 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
- Enthalpy of Ionization in Aqueous Solutions: The enthalpy change in the formation of an ion at unit activity (or concentration) from its elements in aqueous solution is enthalpy of ionization.
- Enthalpy of Dilution : The change of enthalpy when a solution containing one mole of a solute is diluted from one concentration to another is called enthalpy of dilution
- Enthalpy of Hydration (ΔHhyd): The enthalpy change accompanying the hydration of one mole of an anhydrous salt by combining with specific number of moles of water.
- Hess's Law of Constant Heat Summation: The heat energy released or absorbed in a process is same whether the process occurs in one step or in several steps
- Spontaneous Process : A process is said to be spontaneous if it occurs on its own without the intervention of any external agency of any kind. Spontaneous (or) natural processes are thermodynamically irreversible.
- Entropy(S) : "Entropy is a measure of randomness (or) disorderness of the particles of a system"
Entropy is a state function and an extensive property Δ S = Δ Sfinal – Δ Sinitial
Δ Sproducts – Δ S reactants : and Δ S = qrev/T - Integrated expression for entropy change in reversible process:
- ΔS= 2.303 n Cp logT2/T1 + 2.303 n R logP1/P2 = 2.303 n Cv logT2/T1 + 2.303 nRlogV2/V1
- For isothermal process ΔS = 2.303nRlogP1/P2 = 2.303 n R logV2/V1
- For isobaric process ΔS = 2.303 n Cp logT2/T1
- For isochoric process ΔS = 2.303 n Cv logT2/T1
- Entropy of Fusion : It is the change in entropy when one mole of a solid changes to a liquid at its melting point. rev fusion ΔSfus = Qrev/T = ΔHfus / melting point
- Entropy of Vapourisation: It is the change in entropy when one mole of a liquid changes to vapour at its boiling point. ΔSvap = Qrev/T = ΔHvap/bolling point
- Entropy of Sublimation: It is the change of entropy when one mole of solid changes into vapour at a particular temperature. ΔSsub = Svapour – Ssolid = ΔH/Tsub
- Second law of Thermodynamics: It is stated in various forms. Heat cannot flow from a colder body to a hotter body on its own. Heat cannot be converted into work completely without causing some permanent changes in the system or in the surroundings. All spontaneous processes are thermodynamically irreversible and entropy of the system increases.
It is impossible to construct a machine working in cycles and transfers heat from a lower temperature region to a higher temperature region without inter vension of an external agency (such an imaginary machine is called perpetual motion machine of second kind). - Gibb's Energy (or) Gibb's Function (G):
- ΔH = –ve may be a condition but not a necessary and sufficient condition for the spontaneous nature of a reaction.
- ΔS = + ve is a condition but is not necessary and sufficient condition for the spontaneous nature of the reaction.
- Gibbs introduced another thermodynamic function which involved both enthalpy (H) and entropy (s) function. This is known as free energy function G is referred as Gibbs energy (or) Gibbs function. G= H – TS .Gibbs function is an extensive property and a state function
- ΔG gives a criteria for spontaneity at constant pressure and temperature.
- If ΔG is negative (< 0), the process is spontaneous.
- If ΔG is positive (> 0), the process is non spontaneous.
- If ΔG = 0 the process is at equilibrium.
- ΔG0 of a reaction can be calculated from the following equation. ΔG = Σ ΔfG – Σ ΔfG
- Δ G =[sum of standard energies of formation of products] – [sum of standard energies of formation of reactants
- Gibb's Energy Change and Equilibrium ΔrG is related to the equilibrium constant of the reaction as follows: ΔrG = – RT ln Q = 2.303 log Keq
- Change and Electrical Work Done in a Cell: ΔG = – nFEcell F = Faraday = 96,500 coulomb E = E.M.F of the cell; n = number of electrons involved in balanced electrochemical reaction.
- Third law of Thermodynamics: "The entropy of a pure and perfectly crystalline substance is zero at the absolute zero temperature.(– 273°C)