Thermodynamics Chemistry Class 11 CBSE Notes Study Material Download pdf-ANAND CLASSES

What is Thermodynamics?

Thermodynamics (thermo means heat and dynamics means motion) is the branch of science which deals with the study of different forms of energy and the quantitative relationships between them.

Thermodynamics which deals with the movement of energy from one form to the other and the relation between heat and temperature with energy and work done.

There are 4 laws of thermodynamics:

1. The zeroth law of thermodynamics.
2. The first law of thermodynamics.
3. The second law of thermodynamics.
4. The third law of thermodynamics.

Important Topics and Subtopics of NCERT  for Class 11 Chemistry Chapter 6 –Thermodynamics

Important Topics of chapter Thermodynamics is as follows :

• Concepts of System and types of systems, surroundings, work, heat, energy
• Extensive and intensive properties
• State functions
• First law of thermodynamics
• Internal energy
• Enthalpy
• Heat capacity
• Specific heat
• Measurement of ∆U and ∆H
• Hess’s law of constant heat summation
• Enthalpy of bond dissociation
• Combustion, formation, atomization
• Sublimation, phase transition, ionization
• Solution and dilution
• Second law of Thermodynamics (brief introduction)
• Introduction of entropy
• State function
• Gibb’s energy change for spontaneous and non- spontaneous processes
• Criteria for equilibrium
• Third law of thermodynamics (brief introduction).

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• Complete Detailed Theory + Critical Points
• Exhaustive Theory with Sub-chapter wise Division
• MCQs with solution
• NTA & NCERT Based Syllabus & Guidelines

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Define Chemical Thermodynamics

Chemical thermodynamics is the study of relation between work, heat and chemical reactions or with the physical changes of the state which are confined to the laws of thermodynamics.

Internal Energy

It referred to the energy content within the system. The energy represents the overall energy of the system and may include many forms of energy such as potential energy, kinetic energy, nuclear energy, bond energy, electronic enenrgy etc.

Factors Affecting the Internal Energy

The internal energy of a system may change when:

• Heat passes into or out of the system,
• Work is done on or by the system or matter enters or leaves the system.

Work

Work done by a system is defined as the quantity of energy exchanged between a system and its surroundings. Work is completely governed by external factors such as an external force, pressure or volume or change in temperature etc.

Heat

Heat in thermodynamics is defined as the kinetic energy of the molecules of the substance. Heat and the thermodynamics together form the basics which helped process designers and engineers to optimize their processes and harness the energy associated with chemical reactions economically. Heat energy flows from higher temperature to lower temperature.

Thermodynamic System and the Surrounding

Thermodynamics defines a system as the part of the universe under study that is the part where observations are made. The surrounding and universe interact with each other and depending on the type of the system, exchange of matter and energies occur. The system’s classification entirely depends on the movement of energy and matter in or out of the system. There are two types of system:

1. Open System

Those systems where the exchange of energy, as well as matter takes place, are treated as an open system. For example, water is boiled on a stove without it being covered, the container behaves as an open system because it receives heat energy from an external source and the matter being released are water vapours.

2. Closed System

A closed system is referred to the system where only energy can be exchanged with the surrounding and not the matter. It can also be said that a closed system has a constant amount of matter and only the energy of the system can be changed. For instance, when we keep a shield bottle of water in the fridge, the loss of energy to the surrounding will result in the temperature of the water inside it going down even though the quantity of water in the bottle remains the same.

3. Isolated System

In case of isolated systems, neither energy nor matter can be exchanged between the system and the surrounding. An example of an isolated system is a Thermos flask.

Laws of Thermodynamics

• First law of thermodynamics: When energy moves into or out of a system, the system’s internal energy changes in accordance with the law of conservation of mass.
• Second law of thermodynamics: The state of the entropy of the entire universe, as an isolated system, will always increase over time.
• Third law of thermodynamics: Entropy of a perfect crystal at absolute zero is zero.

System and Surrounding

(i) System : A specific portion of universe under study which is seperated from rest of the universe with a boundary is called system.

(ii) Surroundings : Rest of the universe which might be in a position to exchange energy and matter with the system is known as surrounding.

Types of System

(i) Open system : System can be open if it can exchange both energy and matter with surroundings.

(ii) Closed system : System can be closed if it can exchange energy but not matter with surroundings.

(iii) Isolated system : System can be isolated if it can neither exchange energy nor matter with surroundings.

Extensive Properties

The properties which depend upon mass of the substance is known as extensive properties i.e., mass, volume, internal energy, enthalpy etc.

Intensive Properties

The properties which are independent of mass of the substance is known as intensive properties i.e., temperature, pressure, density, refractive index.

Thermodynamic State of a System

A state is the condition of a system as specified by its physical properties. We can describe the state of a gas by quoting its pressure (p), volume (V), temperature (T), amount (n) etc. 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.

State Functions

The thermodynamic parameters which depends only on initial and final states of system is known as state function. i.e., internal energy(E), Enthalpy (H), entropy (S), Gibb’s free energy (G).

Path Functions

The thermodynamic parameters where value does not depend merely on initial and final state but depends upon the path followed is known as path funcition. i.e., heat (q), work done (W).

Thermodynamic Process

The sequence followed to change one thermodynamic state of a system into another is called thermodynamic process. The types of thermodynamic processes are:

(a) Isothermal process: It is the process in which temperature is kept constant means temperature of initial and final state of system along with entire path of process is same.

(b) Isobaric process: It is the process in which pressure is kept constant for entire process.

(c) Isochoric process: It is the process in which volume is kept constant.

(d) Adiabatic process: The process in which heat transaction across boundary is not allowed.

(e) Reversible process and Irreversible process: In thermodynamics, a process is said to be reversible when energy change in each step of the process can be reversed by changing the variables such as pressure, volume or temperature acting on them. In such a process, the driving and opposing forces differ infinitesimally and the process can be reversed completely by increasing the opposing force by an infinitesimally small amount.

Any process which does not take place in the above mentioned manner is said to be an irreversible process. In an irreversible process the driving and opposing force differ by a large amount.

(f) Cyclic process: It is the process which run in close loop means process in which initial and final states are identical.

Internal Energy

Every substance is associated with definite amount of energy that is called internal energy. It is an extensive property and a state function. Internal energy of ideal gases is a function of temperature only.

Pressure-Volume Work

It is the work done when the gas expands or contracts against the external pressure. Consider a cylinder containing one mole of an ideal gas fitted with a frictionless and weightless piston having an area of cross-section A. The total volume of the gas is V_i and the initial pressure of the gas inside P.

Let the external pressure acting on the piston is pex. If the external pressure P_ex is slightly greater than P piston moves downward till the pressure inside the cylinder becomes equal to P_ex. Let this change be achieved in a single step and the final volume be V_f. During this compression, suppose the piston moves a very small distance Δl. Thus, the work done on the gas is given by,

W = – P_ex  ΔV

Heat

The change in internal energy of a system can be brought about by the transfer of heat from the surroundings to the system or vice-versa. This exchange of energy between the system and surroundings is possible as a result of the temperature difference between them. This energy called heat is represented by Q.

First Law of Thermodynamics

First law of thermodynamics states the law of conservation of energy in a different manner. According to this law, whenever a quantity of one kind of energy disappears an equivalent amount of energy appears in some other form.

According to first law of thermodynamics,

ΔQ = ∆U + ∆W

Where, Q = Heat change
W = Work done
ΔU = Change in internal energy

Enthalpy (H)

The total heat content of a system at constant pressure is known as its enthalpy. Mathematically it is the sum of internal energy and pressure-volume energy

ΔH = ΔU + PΔV

It is an extensive property and a state function. Increase in enthalpy H is equal to heat absorbed at constant pressure.

Heat Capacity

Heat capacity is amount of heat require to raise the temperature of a system by unity. It is represented as “C”. It is an extensive property and temperature dependent.

Types of heat capacity

(i) Specific heat: For 1 gram system the heat loose or gained by system to bring one unit change in temperature is called specific heat denoted by ‘s’. specific heat is the intensive property.

(ii) Molar heat capacity: It is heat capacity for a system having 1 mole of material. It is represented as C_m. It is an intensive property.

(iii) Heat capacity at constant volume: Heat capacity of a system in isochoric condition is called heat capacity at constant volume, it is represented as C_v means molar heat capacity at constant volume.

(iv) Heat capacity at constant pressure: Heat capacity of a system in isobaric condition. It is represented by C_p means molar heat capacity at constant pressure.

Relation between C_p and C_v

$C_p-C_v=R$

Hess’s Law

According to Hess’s law, 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.

I^st method

C(s) + O₂(g) ⟶ CO₂(g) = ΔH

II^nd method

C(s) + 1/2O₂(g) ⟶ CO(g) = ΔH₁

CO(s) + 1/2O₂(g) ⟶ CO₂(g) = ΔH₂

According to Hess’s law,

ΔH = ΔH₁ + ΔH₂

Application of Hess’s Law

1. Calculation of enthalpy of formation.
2. Determination of standard enthalpies of reactions.

Bond Dissociation Energy

The energy required to break one mole bond of a particular type in gaseous molecule is known as bond dissociation energy. For example, we consider the dissociation of water,

H – OH(g) ⟶ H(g) + OH(g) = ΔH = 498 kJ/mol

Entropy

Entropy is a measure of degree of randomness or disorder in a system. Entropy is an extensive property and a state function.. Its value depends upon the amount of substance present in the system.

Second Law of Thermodynamics

This states that the entropy of the universe always increases in every spontaneous (natural) change.

Free Energy (G)

Gibb’s free energy is defined as,

ΔG = ΔH – TΔS

H is enthalpy, S is entropy and T is the temperature on Kelvin scale.

Summary

1. System : A part of universe which is under investigation.
2. Surroundings : The rest of the universe which is not a part of the system.
3. State of the system : The conditions of existence of a system when its macroscopic properties have definite values.
4. State functions : The thermodynamic quantities which depend only on the initial and final state of the system.
5. Energy is exchanged between the system and the surroundings as heat if they are at different temperatures.
6. The properties of the system whose value is independent of the amount of substance are called intensive properties. e.g., temperature, pressure, viscosity, surface tension, dielectric constant, specific heat capacity.
7. The properties of the system whose value depends upon the amount of substance present in the system are called extensive properties. e.g., mass, volume, surface area, energy, enthalpy, entropy, free energy, heat capacity.
8. Work is also a mode of transference of energy between system and the surroundings. Work done by the system on the surroundings is given by pΔV.
9. Internal energy (U) : The energy associated with the system at a particular conditions of temperature and pressure.
10. Enthalpy (H) : It is sum of internal energy and pressure-volume energy of the system at a particular temperature and pressure. It is also called heat content (H = E + pV).
11. Hess’s law : The enthalpy change in a particular reaction is the same whether the reaction takes place in one step or in a number of steps.
12. Bond enthalpy : The average amount of energy required to break one mole of the bonds of a particular type in gaseous molecules.
13. Entropy (S) : It is a measure of randomness or disorder of the system. Thus, the order is Gas > Liquid > Solid.

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