Thermodynamics Class 11 Chemistry Notes

Thermodynamics Class 11 Notes

Thermodynamics is the study of relations between work, temperature, heat, energy, radiation and physical properties of matter. Chapter 6 of Chemistry Class 11  contains the systems of transference and transformation of energy from one form to another. 

Though this Chapter is extensive, students must study the subjects in-depth as well as mathematical terms. Extramarks provides a summarised and accessible version of Class 11 Chapter 6 Thermodynamics. The chapter contains well-explained core topics with all the necessary details. Numerous forms of energy are interconnected, and under specific circumstances, they can convert from one form to another. The science of thermodynamics focuses on the investigation of these energy transformations. This chapter elaborates on the same.

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Introduction

Thermodynamics is defined as the study of the flow of mass, heat, temperature and energy. The field of thermodynamics focuses on the investigation of these energy transformations. Rather than microscopic systems with a few molecules, the laws of thermodynamics deal with energy transformations in macroscopic systems with many molecules. Thermodynamics is based on the beginning and ending states of a system experiencing the change instead of how quickly these energy transformations take place. Only when a system is in equilibrium or transitions between equilibrium states do the laws of thermodynamics take effect.

 Terminologies to know:

System

The System is the part of the universe in which observations are made. 

Types of system

• Open System: freely exchanges mass and heat energy with the surrounding. Eg- the human body.
• Closed System: only exchanges heat energy with surrounding but the mass is constant. Eg- pressure cooker.
• Isolated System: no exchange of energy with the surrounding takes place. Eg- thermos flask.

Surrounding

The region/universe outside the system is not under observation. The Surrounding consists of everything excluding the system.

It can be explained as: 

System  + Surrounding = Universe

The changes occurring in the system do not have an impact on the rest of the universe. For all practical purposes, the surroundings are the part of the universe that can interact with the system.  The system’s surroundings can be typically defined as the area of space in its vicinity.

States of system

A thermodynamic system’s state is determined by changes in its state variables, namely P (hydrostatic pressure), V (volume), T (temperature), and n (number) (number of moles). The state of a system will change if even one of them changes. 

Note – Since the values of variables like p, V, and T depend only on the state of the system and not on how it is arrived at, these variables are also known as state variables or state functions.

Properties of system

• Intensive Property: Property whose value is not dependent on the mass but depends on concentration. Eg- refractive index, density, concentration, etc.
• Extensive Property: Property whose value depends on the mass and the total number of particles. Eg- volume, energy, etc.

State and Path Function

• State Function: A set of properties that describe the condition of a thermodynamic system which depends on initial and final states but not on the path followed. Eg- enthalpy, temperature, volume, etc.
• Path Function: A series of states through which a system passes and will depend on the path followed while the system is changing. Eg- work, heat, etc.

Thermodynamic equilibrium

A system which remains simultaneously in mechanical, chemical and thermal equilibrium. Its properties do not change with time. 

• Mechanical Equilibrium: Mechanical equilibrium is created when there is no mechanical motion and there is constant pressure and volume.
• Thermal Equilibrium: Thermal equilibrium is created by the heat and temperature remaining constant over time.
• Chemical Equilibrium: Chemical equilibrium is reached when the forward reaction rate is equal to the backward reaction rate.

Internal energy

The total amount of potential and kinetic energy due to the particles in the system is known as Internal Energy (U). This system acts as an ideal gas system which only depends on kinetic energy. 

Hence, Internal Energy is a state function. 

Modes of energy transport

• Heat(Q): It refers to the energy transferred due to differences in temperature within the system and the surroundings. Heat causes the system’s kinetic energy to rise, which in turn raises internal energy.
• Work(W): It refers to the mechanical energy transferred from the system to its surroundings to overcome the external forces acting on it. When a system contracts and expands, its internal energy increases and decreases respectively.

First Law of thermodynamics

According to the First Law of Thermodynamics, energy can only be changed from one form to another and cannot be created or destroyed.

ΔU=Q+W

-W= work done by the system

+W= work done on the system

-Q= heat flows out of the system

+Q= heat flows into the system

Reversibility

It is the characteristic of a process that can be reversed and the system restored to its initial stage without leaving changes in the system or surroundings. For reversibility to take place, there must be no dissipative forces and the system must be in a Quasi-Static State. 

• Quasi-Static State: A system in which motion happens at an extremely slow rate but appears to be static at all times and seems to be in equilibrium with its surroundings.

Expansion work

It is the work that is completed as a result of changes in a system’s volume. Regardless of expansion or compression, we consider external pressure to be the driving force.

W=−∫PexdV

For reversible processes- external pressure=pressure of the gas,

W=−∫PgasdV

Sign conventions

• W- When the system’s volume is decreasing, the value is positive; when it is rising, it is negative.

• ΔU- When a system’s temperature, product pressure, or volume is decreasing, it is negative; otherwise, it is positive.
• Q- is determined by the first law of thermodynamics.

Cyclic process

A process in which the initial and final states are the same, i.e., it comes back to its original/initial state, is called a Cyclic Process. 

Here,

ΔU=0    &     Qnet=−Wnet

Enthalpy

A thermodynamic system’s enthalpy, which is a property, is determined by the total amount of energy used and stored within the system. 

ΔH=U+PV

At constant P, ΔH=Qp

At constant V, ΔU=Qv

Molar heat capacity

• At constant pressure(CP)- The amount of heat required to raise the temperature of one mole of a substance by 1°C or 1K at constant pressure. 

CP=QP /nΔT

• At constant volume(CV)- The amount of heat required to raise the temperature of one mole of a substance by 1°C or 1K at constant volume. 

Cv=Qv /nΔT

Hence ΔH=nCPΔT and ΔU=nCVΔT.

Types of thermodynamic processes

• Isothermal process- A process that takes place at a constant temperature.

Here, ΔU=0 and ΔH=0.

W=−2.303nRTlogV2/V1=−2.303nRTlogP1/P2

  Q=2.303nRTlogV2/V1=2.303nRTlogP1/P2

• Adiabatic process- Heat exchanged with the surrounding is zero. Here,

TVγ−1=C,  TγP1−γ=C,  PVγ=C

C- constant

Q=0⇒W=ΔU

Now, ΔU=nCVΔT=(P2V2−P1V1)/(γ−1)=(nRΔT)/(γ−1)

   and

ΔH=nCPΔT

• Isochoric process- A process that takes place at constant volume.

Here, W = 0, 

ΔH=nCPΔT and ΔU=nCVΔT=QV.

• Isobaric process- A process that takes place at constant pressure.

Here, W=−PΔV=−nRΔT  , ΔH=nCPΔT=QP and ΔU=nCVΔT.

• Irreversible process- The system and surroundings do not return to their original position when this process is initiated. Work done is given as 

W=−∫PexdV

• Free expansion- The expansion of a gas in an empty space without heat transfer or work. The external pressure of the gas is zero. It is an isothermal and adiabatic process. 
• Polytropic Process- A thermodynamic process that is represented as PVn= constant.

For the isothermal process, n=1

For adiabatic process, n=γ

Thermochemical equation

A thermochemical equation is a balanced equation that gives you information about the reactants, products, and energy changes. 

Types of reaction

• Endothermic reactions- These are the reactions that absorb energy. 

ΔH=+ve 

• Exothermic reactions- These are the reactions that emit energy.

ΔH=-ve

For any chemical reaction, 

Δ HReaction= Δ HProducts– Δ HReactants

Hess Law of Constant Heat Summations

The Hess law of constant heat summation governs the overall change in enthalpy for the solution, which is the sum of all changes regardless of how the reaction is carried out in steps.

Enthalpy of reactions

• Enthalpy of bond dissociation: the energy required to break one mole of a substance’s chemical bond
• Enthalpy of combustion: the change in heat when one mole of substance undergoes combustion in presence of oxygen.
• Enthalpy of formation: the change in heat when one mole of a substance is formed from its elements in their standard states.
• Enthalpy of atomization: the amount of energy necessary to break down one mole of a substance into gaseous atoms.
• Enthalpy of sublimation: the amount of heat needed at STP to convert one mole of a substance from a solid to a gaseous state.
• Enthalpy of phase transition: the release or absorption of a particular standard enthalpy when a phase transitions from one phase to another. 
• Enthalpy of ionisation: the amount of energy needed for a gaseous atom to lose an electron in its ground state.
• Enthalpy of the solution: the change in heat when one mole of a compound is dissolved in a solvent
• Enthalpy of dilution: the change in enthalpy during a dilution process of an element in solution under constant pressure.

The second law of thermodynamics

According to the second law, if a physical process is irreversible, the combined entropy of the system and its surroundings must always rise over time. This law was necessary to explain the feasibility of the first law of thermodynamics.

Types of processes

• Spontaneous Process: it is the process which occurs naturally and does not require external work and energy input to carry it out. 
• Non-Spontaneous Process: it does not occur naturally and requires an input of external work and energy.

Entropy

Entropy, which measures molecular disorder or randomness, is the system’s thermal energy per unit temperature. It is a state function represented by S. 

This is the process in which the total randomness of the universe tends to increase. Thus,

ΔS=Qrev/T

 For spontaneous change,

ΔSTotal=ΔSSystem+ΔSSurrounding>0 

For reversible processes where the entropy of the universe remains constant, ΔSTotal=0.

Entropy changes in the thermodynamic processes

The entropy changes in any environment can be mathematically demonstrated as,

ΔS=nCVlnT2/T1+nRlnV2/V1

• Isothermal process –

ΔS=nRlnV2/V1

• Isochoric process –

ΔS=nCVlnT2/T1

• Isobaric process –

ΔS=nCPlnT2/T1

• Adiabatic process –

ΔS=0

Gibbs free energy

It gives us a parameter from the perspective of the system to judge the spontaneity of the process. 

It can be represented at a constant temperature as,

ΔGsys=ΔH−TΔSsys 

At constant temperature and pressure, 

ΔG=−TΔSTotal  

For the process to be spontaneous, ΔG<0.

Third law of thermodynamics

The third law of thermodynamics states that the entropy of a system approaches a constant value as the temperature approaches absolute zero.

Class 11 Thermodynamics Notes Chemistry Chapter 6

Extramarks believes in incorporating joyful learning experiences through its own repository of resources. The following topics are included in Extramarks Revision Notes for Class 11 Chemistry Chapter 6 Thermodynamics in a concise format. It is available in easily understandable language that helps the student to clear their concepts and prepare well for the exams. 

• Types of Thermodynamic Equilibriums

The three major types of equilibrium, namely- thermal, mechanical and chemical.

• Energy Transfer Modes

There are two modes of energy transfer, namely- heat and work. Heat is the energy transfer due to temperature differences. On the other hand, work is the energy transfer due to differences in external forces between two systems.

• First Law of Thermodynamics

“Energy can neither be created nor destroyed. It can only be changed from one form to another.” This is what the first law states. 

Extramarks Revision Notes contain the basic laws along with their equations in a clear and organised format. This will help them understand the important points easily and make them revise quickly.

• Concept of Enthalpy

The amount of heat energy emitted by a system under constant pressure is known as enthalpy. This change in enthalpy is relative to the heat absorbed or released in a specific reaction in a system.

• Thermodynamic Processes

There are several types of Thermodynamic processes described in Extramarks Revision Notes which are-

• Isothermal process
• Adiabatic process
• Isochoric process
• Isobaric process
• Concept of Entropy

It is the measurement of thermal energy in a system relative to per unit temperature that does not produce quantifiable work.

• Hess Law of Constant Heat Summation

The Hess Law of Constant Heat Summation, which states that regardless of how many steps or stages are involved in a reaction, the total change in enthalpy for that reaction equals the sum of all changes, is found in the thermodynamics Revision Notes by Extramarks. Such an aspect leads to a state function.

• Born Haber Cycle

Ionic compounds are created from elemental atoms in their natural state through the Born Haber Cycle. It demonstrates the conditions necessary for the ionic bond to form.

For students who must study a broad syllabus for their exams, thermodynamics can be challenging. The revision notes by Extramarks provide a clear understanding of important concepts with their simple definitions and structured outline. These Notes bring clarity to concepts which in turn becomes useful when it comes to answering the most difficult questions that students may think it’s out of the syllabus. Chemistry faculty experts ensure that these Notes are complete in every way for students to learn and grasp with better understanding and score well. They need not look for additional resources to supplement their studies. To maximise their potential, students may register themselves on Extramarks’ website and make the most of it.