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Chemistry: Chemical Thermodynamics and Energetic

Topics are arranged as per SYLLABI published by Maharashtra State Board of Secondary and Higher Secondary Education.

According to thermodynamic convention, a substance is in its standard state, when it is at a pressure of 1 atm and temperature of 298 K.
1 calorie = 4.3 Joule ; 1 electron volt = Chemical Thermodynamics and Energetics
1 erg = 10-7 Joule

Thermodynamic Equilibrium involves
Chemical Equilibrium Mechanical Equilibrium Thermal Equilibrium
Chemical composition of the system (reactants and products) does not change with time. There is no movement of matter in any part of the system with respect to its surroundings. There is no exchange of heat between system and surrounding.
Number of Moles remains constant. Pressure and Volume Remains Constant. Temperature remains constant.
Thermal Capacity is the heat required to raise the temperature of a body by 1 K.
Concept of System : The term 'Thermo' means heat and 'dynamics' means flow or movement. So thermodynamics is concerned with the flow of heat. The different forms of the energy are inter convertible from one form to another. For example, chemical energy can be converted in to electric energy and electrical energy can be converted into mechanical energy and so on. During the process of transformation, the changes are accompanied by different physical and chemical processes.
The study of flow of heat or any other form of energy accompanying the physical or chemical transformations is called thermodynamics. A system is that part of universe which is under thermodynamic study and the rest of universe is surroundings. The real or imaginary surface separating the system form surrounding is called boundary.

System is the portion of the universe under thermodynamic consideration to study thermodynamic properties. Following are the types of system:

  • Open System
  • Closed System
  • Isolated System
  • Homogeneous System
  • Heterogeneous System
Open Closed Isolated
System that exchanges both matter and energy with the surrounding. System that exchanges only energy but not matter with the surrounding. System that exchanges neither matter nor energy with the surrounding.
Total amount of matter does not remain constant. Total amount of matter remains constant. Total amount of matter remains constant.
Total amount of energy is not constant. Total amount of energy is not constant. Total amount of energy is constant.
Ex: Hot water kept in open beaker. Ex: Hot water kept in a closed glass flask. Ex: Hot water kept in thermos flask.
  • Homogeneous System: A system consisting of only one uniform phase. The properties of Homogeneous System are uniform throughout the phase or system. Ex: Solution of NaCl in Water.
  • Heterogeneous System: A system consisting of two or more phases separated by interfacial boundaries. Ex: Mixture of two or more immiscible liquids (Water and Benzene).
  • Surrounding is the portion of the universe other than under thermodynamics study.
  • Boundary is the wall or interface separating the system from its surrounding. It may be either real or imaginary.
In the homologous series of organic compounds, enthalpy of combustion increases with increase in molecular mass but calorific value decreases.

  • Work is one of ways by which a system can exchange energy with its surrounding by changing the state of the system.
  • Work is not the property of a system. It does not depend upon the states of the system. Hence work is not a state function.
  • Mechanical Work Done due to expansion or contraction against an external opposing pressure is represented as : Chemical Thermodynamics and Energetics; Where P = External Pressure, V2 = Final Volume of the system, V1 = Initial Volume of the system.
  • Maximum Work Done is obtained during isothermal reversible expansion / compression. Chemical Thermodynamics and Energetics or Chemical Thermodynamics and Energetics
    Where n , P, V, T represents moles, pressure, volume and temperature respectively.
  • For the same increase in volume for an ideal gas, work done during adiabatic expansion is less than work done during isothermal reversible expansion.

Key important points about Maximum Work Done:
  • The process is carried out at a constant temperature.
  • During the complete process, driving force is infinitesimally greater than opposing force.
  • Throughout the process, the system exists in equilibrium with the surroundings. Heat absorbed in reversible manner is completely converted into work.
  • Gases with higher molecular weight will give higher value of maximum work.
  • Change in the state of the system takes place in infinite number of small steps.
  • System exists in mechanical equilibrium with its surroundings throughout the process.

  • The Exchange of Energy between system and surrounding due to temperature difference between them is called Heat (q). It is a random form of energy.
  • Heat is not the property of system, hence it is not a state function.
  • Heat is a path function.
Work Heat
Exchange of energy between system and surrounding due to expansion or compression of gas. Exchange of energy between system and surrounding due to temperature difference between them.
Organized form of energy. Random form of energy.
NOT a state function. NOT a state function.
Path Dependent. Path Dependent.
Units of Work and Heat are same.
Erg < Joule < Calories < lit. atm. [Note: 1 J = ergs ; 1 J = 0.239 Calories]

The mechanical work is a form of energy. Internal Energy: It is the total energy constituting potential and kinetic energy of the molecules present in the system. Internal energy is a state function.

Extensive Intensive
Property depends on the size. Properties is independent of the size of a system.
Ex: Mass. Ex: temperature.
The ratio of an extensive property to the mass or the property per unit mass (or mole) is called specific property. The ratio of an extensive property to the number of moles of the substance in the system, or the property per mole of the substance, is called the molar property.

The properties whose values depend only upon the initial and final states of a system and are used to define a state of a system completely are called state functions.

It is a law of conservation of energy which states that energy can neither be created nor destroyed, although it can be converted from one form to another. Total energy of the universe remains constant. So it’s also called as law of conservation of energy. Change in Internal Energy = Chemical Thermodynamics and Energetics
W = Work done on the system ; Q = Heat transferred
For reversible change, under constant temperature Chemical Thermodynamics and Energetics.
Chemical Thermodynamics and Energetics ; n is moles of an ideal gas at temperature T.
For Isochoric Chemical Thermodynamics and Energetics, Chemical Thermodynamics and Energetics ; Chemical Thermodynamics and Energetics
For Adiabatic change Q = 0.
Chemical Thermodynamics and Energetics
Chemical Thermodynamics and Energetics ; Also Chemical Thermodynamics and Energetics ; Chemical Thermodynamics and Energetics

  • 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.
  • In general, if enthalpy of an overall reaction Chemical Thermodynamics and Energetics along one route is Chemical Thermodynamics and Energetics and Chemical Thermodynamics and Energetics, Chemical Thermodynamics and Energetics, Chemical Thermodynamics and Energetics represents enthalpies of reactions leading to same product, B along another route , then we have Chemical Thermodynamics and Energetics
  • Exothermic compounds are more stable than endothermic compounds.
  • Greater is the amount of heat evolved in the formation of a compound, more will be its stability.
  • The compounds whose enthalpies of formation have positive values are called endothermic compounds and those having a negative value are called exothermic compounds.

Enthalpy of a system is defined as the total energy of a system consisting of internal energy of the system and that arises due to its pressure and volume. It is represented as : H = U + PV ; where U is Internal Energy.

  • It represents total heat content of the system, at constant pressure.
  • It is a thermodynamic state function.
  • It is an extensive property.
  • Absorption of heat by a system increases its enthalpy.
  • It is also called as heat content of the system.
Bond Dissociation Combustion Formation Atomization Sublimation
Chemical Thermodynamics and Energetics Chemical Thermodynamics and Energetics Chemical Thermodynamics and Energetics
Change of heat required to break one mole of particular bond of gaseous molecules forming free gaseous atoms or radicals at constant temp. and pressure. Change accompanying the complete combustion of one mole of a substance in standard state in excess of oxygen or air. Change accompanying formation of 1 mole of a substance in standard state from it’s constituent elements in their standard state. Change accompanying the dissociation of the molecules in one mole of a gaseous substance into free gaseous atoms at constant temperature and pressure. Change of matter from solid state directly into gaseous state.
It is always negative. It can be positive or negative. Heat is absorbed.
Every covalent bond in a polyatomic molecule has its own bond enthalpy. Used to calculate calorific value of fuel or food. More Negative value, more stable is the substance. Heat generated is also called as Heat of Dissociation. Endothermic.

It involves a change in the physical state of a matter. In this chemical property remains an unchanged but physical properties change. The Phase change of a substance always takes place at constant pressure and temperature.
During Phase transition, both the phases exist at equilibrium.

Fusion Vaporization or Evaporation Sublimation
Change of a matter from solid state to liquid state. Change of matter from liquid state to gaseous state. Change of matter from solid directly into gaseous state.
Heat is absorbed. Heat is absorbed. Heat is absorbed.
Endothermic Endothermic Endothermic

It is the change or amount of heat absorbed accompanying the removal of one electron from each atom or ion in one mole of gaseous atoms or ions is called enthalpy of ionization.
The enthalpy change accompanying the removal of one electron from each gaseous atom in one mole is called first ionization enthalpy.
The enthalpy change accompanying the removal of one electron from each gaseous univalent positive ion in one mole is called second ionization enthalpy.
Normally second ionization enthalpy is larger than first ionization enthalpy.

Solution is a homogenous system consisting of more than one component. The component in the large amount is called a solvent and the component in the small amount is called solute. The properties of solution depend upon the concentration of a solution which is the proportion of solute in the solution.
Enthalpy of a solution is defined as the enthalpy change when one mole of a substance is dissolved in a large excess of a solvent, so that further dilution will not change the enthalpy at constant temperature and pressure.
Ex: Chemical Thermodynamics and Energetics; Chemical Thermodynamics and Energetics = -75.14 kJ/mol
Enthalpy of solution of specified concentration is the change when mole of a substance is dissolved in a specified quantity of a solvent to give a solution of specified concentration at constant temperature and pressure. Ex: 1 mol of HCl (g) dissolved in 50 moles of water. Chemical Thermodynamics and Energetics; Chemical Thermodynamics and Energetics = -73.26 kJ/mol
Enthalpy of Dilution is defined as the enthalpy change that occurs when a solution of one concentration is diluted to form the solution of another concentration at given temperature and pressure.

A State Function is a thermodynamic quantity whose value depends only on the state at the moment, i. e., the temperature, pressure, volume, etc… The value of a state function is independent of the history of the system. The fact that internal energy is a state function is extremely useful because it we can measure the energy change in the system by knowing the initial energy and the final energy. Entropy is a measure of randomness of a system.

A spontaneous process is an irreversible process and may only be reversed by some external agency. Temperature dependence of free energy can be summarized in table. Chemical Thermodynamics and Energetics

Chemical Thermodynamics and Energetics Chemical Thermodynamics and Energetics Chemical Thermodynamics and Energetics Spontaneity of Reaction Reaction
+ - - Spontaneous
+ - + Non-Spontaneous at all temp. Endothermic
- - - or + spontaneous below specific temp
+ + - or + Spontaneous above specific temp.
+ - + Never spontaneous
- - - Spontaneous at low temperature Exothermic
+ + - Spontaneous only at high temperature Endothermic
- + - Spontaneous at all temperature Exothermic
When Chemical Thermodynamics and Energetics is zero; there is no change in the system; the reaction does not proceed to the right; nor does the reaction proceed to the left. In this state, a balance exists among all the chemical components and we say that the system is in equilibrium. The value of the reaction quotient is the equilibrium constant, K
Chemical Thermodynamics and Energetics

Second law of thermodynamics - Spontaneity of processes;

  • All spontaneous processes are thermodynamically irreversible.
  • It is impossible to convert heat completely into equivalent amount of work without leaving some effect elsewhere.
  • Without the help of an external agency, heat can not by itself flow from a cold body to hot body.
  • The entropy of the universe is continuously increasing.
So, according to this law, the entropy of the universe always increases in the course of every spontaneous change. Chemical Thermodynamics and Energetics at equilibrium.
A spontaneous process is an irreversible process and may only be reversed by some external agency.
Spontaneous Non Spontaneous
Process which takes place by itself or after proper initiation under a given set of conditions. Process which can neither take place by itself nor by initiation.
Natural Process
Cannot be reversed without help of external agency.
Flow of heat from hot reservoir to cold reservoir. Flow of water down the hill. Flow of heat from cold reservoir to hot reservoir. Flow of water up the hill.

It states that the entropy of all crystalline solids may be taken as zero at the absolute zero of temperature. It is used in the calculation of absolute entropy of any substance in any state at any temperature.
As a limitation of third law; some solids do not have zero entropy even at absolute zero ex: glassy solids, crystals of CO, NO, etc; solids containing mixture of isotopes.

  • Kelvin Scale of temperature is also known as Thermo-dynamic scale of temperature. In this scale, the reference points are absolute zero of temperature and triple point of water (Chemical Thermodynamics and Energetics).
  • Standard Enthalpy Ho of any element at 1 atm and 25oC is zero; but standard entropy (So) is not zero.
  • Standard enthalpy of formation and standard free energy of formation of an elementary substance is taken as zero but standard entropy is not zero.
  • Calorific Value is the enthalpy change or amount of heat liberated when one gram of a substance undergoes combustion.
  • Exothermic process : Evolution of heat.
  • Endothermic process : Absorption of heat.
  • The state of a system is defined by specifying the values of state variables like temperature, pressure, volume etc. By changing these variables, the properties of a system change, hence, the state of system changes.
  • Statistical Thermodynamics deals with study of properties of microscopic system like small number of atoms or molecules.
  • Joule-Thomson Effect: When a real gas is allowed to expand adiabatically from a region of high pressure to a region of low pressure through a fine hole or porous plug, it is accompanied by cooling except H2 and He which get warmed up.
  • The temperature at which a real gas shows no cooling or heating on adiabatic expansion, is called inversion temperature. Below this temperature it shows cooling effect, while above this temperature it shows heating effect.
  • Lavoisier and Laplace Law: The enthalpy of decomposition of a compound into its elements is equal to its enthalpy of formation (with opposite sign).
  • Enthalpy of combustion is measured by Bomb calorimeter.
  • Absolute value of internal energy, enthalpy and free energy cannot be determined while absolute value of entropy of any substance in any state at any temperature can be determined.
  • Bond energy is defined as the average amount of energy produced when one mole of bonds are formed from the isolated gaseous atoms or the amount of energy required when one mole of bonds are broken so as to get the separated gaseous atoms. Average value is taken, because energy required to break a particular kind of bond is not same in polyatomic molecules.
  • Bond dissociation energies of four C-H bonds in CH4 are not equal. Hence an average value is taken.
  • 1 cal > 1 Joule > 1 erg > 1 eV
  • When rubber band is stretched , entropy decreases because the macromolecules get uncoiled and hence arranged in a more ordered manner i.e. randomness decreases.
  • When an egg is boiled, the entropy increases because denaturation occurs, resulting into a change of proteins from helical form into random coiled form.
  • Enthalpy change when one mole of the solute is dissolved in a definite number of moles of the solvent is called integral heat of solution.
  • Heat of fusion is the heat required to convert one mole of solid substance into liquid at the boiling point.
  • Heat of vaporisation is the heat required to convert one mole of the liquid into vapour at the boiling point.
  • Heat of sublimation is the heat required to convert one mole of solid directly into vapour below the boiling point.
  • If a system returns to its original stage after undergoing a number of successive changes, it is said to be cyclic process. The most common example of such process is that of Carnot cycle, which consists of the following four stages:
    • Isothermal Expansion
    • Adiabatic Expansion
    • Isothermal Compression
    • Adiabatic Compression
  • Resonance Energy is the difference between the calculated value and the experimentally observed value of the enthalpy of formation or the bond dissociation energies of a compound.
  • Work done on the system is positive; Work done by the system is negative.
  • Heat absorbed by the system is positive; Heat given out by the system is negative.
  • Energy absorbed by the system (internal energy increases) is positive.
  • Energy released by the system (internal energy decreases) is negative.
  • For spontaneous process Chemical Thermodynamics and Energetics is negative. For non-spontaneous process Chemical Thermodynamics and Energetics is positive. For the process in equilibrium Chemical Thermodynamics and Energetics is zero.
  • When an element exhibits allotropy, the standard enthalpy of formation is taken as zero for the allotropic form which is most stable under standard conditions (298K and 1 atmosphere pressure.) Ex: In case of Carbon Chemical Thermodynamics and Energetics =0 for graphite.