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Thermodynamics is the study of heat, work, and temperature in relation to energy, entropy, and the physical properties of matter and radiation.
- It comes from a Greek word meaning “Movement of heat”.
- It deals with the flow of energy from one place to another and from one form to another.
- The fundamental concept is that heat is a type of energy that corresponds to a certain amount of mechanical work.
- In the 19th century, Count Rumford and James Prescott Joule studied the relationship between work and heat.
- Joule found that the same amount of work done always produces the same amount of heat.
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Key Terms: Thermodynamics, Entropy, Entropy in thermodynamics, Enthalpy, Heat Engine, Carnot Cycle, Internal energy, Thermodynamic processess, Heat engine, Heat capacity, Equation of state
What is Thermodynamics?
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| Thermodynamics deals with heat, work, and temperature, and their relation to energy, radiation, and physical properties of matter. |
Thermodynamics explains how thermal energy gets converted to or from other forms of energy and how matter is affected by this process.
- Thermal energy comes from heat generated by the movement of tiny particles within an object.
- The faster these particles move, the more heat is generated.
- Thermodynamics is not dependent on how and at what rate these energy transformations are conducted.
- It is based on the initial and final states that undergo a change.
- Thermodynamics is a macroscopic science that deals with the bulk system.
Difference Between Mechanics and Thermodynamics
The main differences between mechanics and thermodynamics are listed below:
| Mechanics | Thermodynamics |
|---|---|
| In mechanics, the motion of objects and particles is considered. | In thermodynamics, the internal state of a system and its interaction with its surroundings is considered. |
| Position, velocity, acceleration, forces, torques, etc. are the variables used in mechanics. | Temperature, pressure, volume, entropy, internal energy, etc. are the state variables used in thermodynamics. |
| In mechanics, energy is considered in the form of kinetic, potential, and mechanical. | In thermodynamics, energy is considered in the form of internal, thermal, chemical, and radiant. |
| Newton's laws of motion are the fundamental laws used in mechanics. | Laws of thermodynamics (conservation of energy, entropy, etc.) are used in thermodynamics. |
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Branches of Thermodynamics
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There are four different branches of thermodynamics
- Classical Thermodynamics
- Statistical Thermodynamics
- Chemical Thermodynamics
- Equilibrium Thermodynamics
Classical Thermodynamics
The behavior of matter is studied using a macroscopic approach in classical thermodynamics. Temperature and pressure are taken into account, allowing individuals to determine other characteristics and predict the properties of the substance conducting the process.
Statistical Thermodynamics
Every molecule is examined in statistical thermodynamics, which means that the properties of each molecule and how they interact are used to characterize the behavior of a group of molecules.
Chemical Thermodynamics
Chemical thermodynamics is the study of how work and heat interact in chemical reactions and changes in state.
Equilibrium Thermodynamics
Equilibrium thermodynamics is the study of energy and matter changes as they approach equilibrium.
Basic Concepts of Thermodynamics
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Basic concepts of Thermodynamics have been elaborated below:
Thermodynamics System
A system is a collection of mass and energy, with a well-defined boundary that is to be studied. Based on the movement of mass and energy across the boundary, there are three types of systems:
- Open System: An open system allows unrestricted movement of mass and energy across its boundary.
- Closed System: In a closed system, the exchange of energy is allowed but the exchange of mass across a system boundary is restricted.
- Isolated System: As the name suggests, an isolated system does not allow the exchange of mass or energy across its boundary.
| Examples of Thermodynamics Systems
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Thermodynamic Surroundings
The mass and energy outside of the boundary of a system are known as the surroundings of a system. In other words, the surroundings are what a system interacts with during a thermodynamic process.
Thermodynamic Process
A thermodynamic process is a process, in which a system interacts with its surroundings which leads to a change in either temperature/pressure/volume of the system. There are 4 types of thermodynamic processes:
- Adiabatic: In an adiabatic process, there is no change in the heat of the system.
- Isothermal: In an isothermal process, the temperature of the system remains constant.
- Isobaric: In an isobaric process, the pressure of the system remains constant.
- Isochoric: In an isochoric process, the volume of the system remains constant.
Thermodynamic Equilibrium
A system is called a thermal equilibrium when there is no temperature difference present. In thermal equilibrium, the temperature remains equal at all points.
For example: A hot cup of tea has a higher temperature as compared to the surrounding so it won’t be in thermal equilibrium.
Thermodynamics Properties
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Thermodynamic properties are the characteristic features of a system, capable of specifying the system’s state. Thermodynamic properties can be extensive or intensive.
- Intensive properties: These properties do not depend on the quantity of matter. Pressure and temperature are intensive properties.
- Extensive properties: Their value depends on the mass of the system. Volume, energy, and enthalpy are extensive properties.
Entropy
Entropy’s value depends on the condition of the system.
- For example, the entropy of a solid, where particle movement is restricted, is less than the entropy of gas, wherein the particles occupy the container.
- The measure of a system’s thermal energy per unit temperature that is unavailable for doing useful work.
- The reason is that the work is obtained from ordered molecular motion.
- The amount of entropy is also a measure of the molecular disorders or randomness of a system.
Entropy
| In simple words, Entropy is a physical quantity that remains constant during reversible adiabatic change. |
Change in entropy is given by
dS = δQ/T
Where
- δQ is the heat supplied to the system
- T is the absolute temperature.
Enthalpy
The measurement of the energy is known as Enthalpy.
Numerically, the enthalpy (H), equals the sum of the internal energy (E), the product of the pressure (P), and the volume of the system(V).
H = E + PV
Thermodynamic Potentials
Thermodynamic potentials are quantitative measurements of stored energy in a system.
- Potentials measure energy changes in a system as they evolve from their initial state to their final state.
- Considering the system constraints like temperature and pressure, different potentials are used.
Different types of thermodynamic potentials along with their formula are tabulated below:
| Types of Energy | Thermodynamics Potential |
|---|---|
| Internal Energy | \(U = \int TdS-PdV+\Sigma _i \mu_i dN_i\) |
| Helmholtz free energy | F = U – TS |
| Enthalpy | H = U + PV |
| Gibbs Free Energy | G = U + PV – TS |
What are the Laws Of Thermodynamics?
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The laws in thermodynamics denote the fundamental physical quantities like energy, temperature, and entropy characterizing the system at thermal equilibrium. However, the laws of thermodynamics are mentioned below:
Zeroth Law of Thermodynamics
The zeroth law in thermodynamics states that when two systems are in thermal equilibrium with the third system, the whole system together is said to be in equilibrium.
According to this law, if two bodies A and B are separately in thermal equilibrium with a body C, that is if
TA = TC and TB = TC
then
TA = TB
which means that systems A and B are also in Equilibrium.
First Law of Thermodynamics
The First law states that when energy conservation is applied to a system in which the energy transfers, it represents the relation between the work done, heat supplied, and change occurring in the system's internal energy.
According to the first law of thermodynamics,
ΔQ = ΔU + ΔW
Where
- ΔQ is the heat given to a thermodynamic system
- ΔW is the Work Done
- ΔU is the Internal Energy
The First law of thermodynamics is similar to that of the principle of conservation of energy.
- The change in internal energy in the isothermal process is zero (ΔU = 0). Therefore ΔW = ΔV.
- In the adiabatic process, no change in heat takes place. Therefore ΔU = -ΔW.
- In the Isochoric process, the work done is zero ΔW = 0, therefore ΔQ = ΔU.
Second Law of Thermodynamics
The second law states that it is not possible to find a system, where the absorption of heat from the reservoir is the complete conversion of heat into work.
- The law states that the efficiency of the heat of an engine can never be 0 or 100%.
- There are three statements in the second law of thermodynamics: Kelvin’s statement, Clausius’s Statement, and Planck’s Statement.
| Example of Second Law of Thermodynamics If a room is not tidied or cleaned, it invariably becomes more messy and disorderly with time. When the room is cleaned, entropy decreases, but the effort to clean it has resulted in increased entropy outside the room exceeding the entropy lost. |
Third Law of Thermodynamics
Third law of thermodynamics states that the entropy of a system approaches a constant value as the temperature approaches absolute zero.
- Entropy of a pure crystalline substance at absolute zero temperature is zero.
- This statement holds true if the perfect crystal has only one state with minimum energy.
| Examples of Third Law of Thermodynamics:
|
Applications of Thermodynamics
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The following are the daily life examples of Thermodynamics
- Taking a bath: On taking a bath, the heat released from the water to our body feels warmth.
- Photosynthesis: The sun's solar energy is absorbed by plants before it is converted into chemical energy. This process not only feeds the plants but also creates oxygen, which is necessary for our life.
- Melting of ice cubes: The melting of an ice cube is a common example of the first law of thermodynamics. If you leave an ice cube out in the open, it will melt and turn to water. This occurs because the ice absorbs heat from the surrounding air, cooling it and converting the ice to water.
- Turning on a heater: When you switch on the heater, electrical energy passes through the heating coil and is transformed into heat energy. Cold water flows through a separate dip tube and is heated by the heat energy developed, resulting in warm water for your relaxing bath.
Things to Remember
- Thermodynamics is the study of the conversion of heat energy into other forms of energy
- There are 3 laws of thermodynamics and one zeroth law of thermodynamics
- Entropy is the measure of the degree of randomness of a system.
- Enthalpy is the measure of the energy of a system.
- According to the First law of thermodynamics: ΔQ = ΔU + ΔW
- Internal energy is the sum of the kinetic and potential energies of all the constituent molecules of the system.
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Previous Year Questions
- When 50cm3 of 0.2N H2SO4 is mixed with 50cm3 of 1N KOH, the heat… [KCET 2004]
- 6 moles of an ideal gas expand isothermally and reversibly from a volume of… [VITEEE 2006]
- When ideal gas expands in vacuum, the work done by the gas is equal to… [VITEEE 2006]
- Name the process in which Boyle's law is applicable? [JEE Advanced 2019]
- A thermodynamical system is changed from state… [BITSAT 2007, 2013]
- Which of the accompanying P-V diagrams best represent; an isothermal… [KEAM 2004]
- 800cc volume of a gas having γ=5/3 is suddenly compressed adiabatically to… [KEAM 2004]
- A gas under constant pressure of… [KCET 2016]
- Super cooled water is liquid water that has been cooled below its normal… [MET 2013]
- A cylinder contains hydrogen gas at pressure of 249 kPa and temperature… [NEET 2020]
- Two cylinders A and B of equal capacity are connected to each other via a stop… [NEET 2020]
- Three moles of an ideal gas expanded spontaneously into vaccum. The work… [NEET 2010]
- The following two reactions are known… [NEET 2010]
- A sample of 0.1g of water at 100∘C and normal pressure… [NEET 2018]
- Thermodynamic processes are indicated in the following diagram… [NEET 2017]
- A carnot engine having an efficiency of 1/10 as heat engine, is used as a… [NEET 2017]
- A Carnot engine whose sink is at 300 K has an efficiency of… [NEET 2006]
- The molar specific heat at constant pressure of an ideal gas is… [NEET 2006]
- If enthalpies of formation for… [NEET 1995]
- An ideal gas heat engine operates in a Carnot cycle between… [NEET 2003]
Sample Questions
Ques: Explain Isentropic Condition? (1 Mark)
Ans: It is a thermodynamic state when the entropy for the given output points remains constant. There are two processes involved in isentropic conditions that are reversible and adiabatic.
Ques: How many types of systems are there in Thermodynamics? State some descriptions about all. (3 Marks)
Ans: There are three types of systems – Open system, Closed System, and isolated system.
Open system – The open system can exchange both matters and energy with its surroundings
Closed System – It can exchange energy only with its surroundings.
Isolated system – No exchange takes place in Isolated systems in their surroundings.
Ques: Is it possible to maintain a constant temperature by continuously heating the system? (2 Marks)
Ans: Yes, it can possible if the work done by the system is against the surroundings should compensate the heat-
ΔT = 0
ΔQ = ΔU + ΔW
Therefore, ΔQ = ΔW
Ques: The enthalpy of combustion of methane, graphite, and dihydrogen at 298k are -890.3 KJ mol -1, -395.5 KJ mol -1, and -285.8 KJ mol -1 respectively. Enthalpy of formation of CHJg will be? (4 Marks)
- CH 4 + 2O 2 (g) →CO 2(g)+ 2H 2O(l); ΔCH- = -890.3 KJ mol -1
- C(s) + O 2 → CO 2(g); ΔCH- = -393.5KJ mol -1
- H 2(g) + 1/2O 2(g) → H 2O(l); ΔCH- = -285.8KJ mol -1
The equation we aim at:
C(s) + 2H 2O(g) → CH 4(g); ΔfH- = ?
Equation (1) +2 x Equation (3) – Equation (1) and the correct value of ΔH f- is:
= (-393.5) + 2 x (-285.8) – (-890.3)
= -74.8 KJ mol -1
Ques: Enthalpy of combustion of carbon to carbon dioxide is -393.5 KJ mol -1. Calculate the heat released upon formation of 35.5g of CO 2 from carbon and oxygen gas. (3 Marks)
Heat released in the formation of 44g of CO 2 = -393.5 KJ
Heat released in the formation of 33.5g of CO 2 = (393.5 KJ) x (35.2g)/(44g) = 314.8KJ
Ques: Give an example of an Isolated System. (1 Mark)
Ans: Coffee held in a thermos flask is an isolated system because it can neither exchange energy nor matter with the surroundings.
Ques: What is the enthalpy change(Δ H)- for positive and negative: (2 Marks)
Ans:
- ΔH is positive for an endothermic reaction that absorbs heat from the surroundings.
- ΔH is negative for exothermic reactions which evolve heat to the surroundings.
Ques: Which of the following statements is correct? (2 Marks)
i. The presence of reacting species in a covered beaker is an example of an open system.
ii. There is an exchange of energy as well as a matter between the system and the surroundings in a closed system.
iii. The presence of reactants in a closed vessel made up of copper is an example of a closed system.
iv. The presence of reactants in a thermos flask or any other closed insulated vessel is an example of a closed system.
Ans. Correct Answer is (iii) The presence of reactants in a closed vessel made up of copper is an example of a closed system.
Explanation: There is no exchange of matter in a closed system (for example, the presence of reactants in a closed vessel made of conducting material, such as copper), but there is an exchange of energy between the system and its surroundings.
Ques: What are heat capacities at constant volume and constant pressure? What is the relationship between them? (3 Marks)
Ans. Heat capacity at constant volume (Cv): The amount of heat supplied to a system to raise its temperature by one degree Celsius while keeping the volume of the system constant is referred to as its heat capacity at constant volume (Cv).
Heat capacity at constant pressure (Cp): The amount of heat supplied to a system to raise its temperature by one degree Celsius while keeping the external pressure constant is referred to as its heat capacity at constant pressure (Cp).
Cp – Cv = R is the relationship between Cp and Cv.
Ques: Calculate the maximum work obtained when 0.75 mol of an ideal gas expands isothermally and reversible at 27°C from a volume of 15 L to 25 L. (3 Marks)
Ans. For an isothermal reversible expansion of an ideal gas
w = – nRT log V2/V1 = – 2.303 nRT log V2/V1
Putting n = 0.75 mol; V1 = 15 L; V2 = 25 L, T = 27 + 273 = 300 K R = 8.314 JK-1 mol-1.
w = – 2.303 × 0.75 × 8.314 × 300 log 25/15
w = -955.5J.
Ques: Define the following terms: (5 Marks)
(i) Standard enthalpy of formation
(ii) Standard enthalpy of combustion
(iii) Enthalpy of atomization
(iv) Enthalpy of solution
(v) Lattice enthalpy
Ans. (i) Standard enthalpy of formation.
The change in enthalpy when one mole of a compound is formed from its elements in their standard states under standard conditions, i.e. at 298K and 101.3kPa pressure, is referred to as the standard enthalpy of formation.
(ii) Standard enthalpy of combustion
The enthalpy change when one mole of a compound is completely burned in oxygen with all reactants and products in their standard state under standard conditions is defined as standard enthalpy of combustion (298K and 1 bar pressure).
(iii) Enthalpy of atomization
This is the enthalpy change that occurs when one mole of a substance is completely broken down into its atoms in the gas phase.
(iv) Enthalpy of solution
The heat change that occurs when one mole of a substance dissolves in a specified amount of a solvent is defined as the enthalpy of solution. The enthalpy of solution at infinite dilution is the enthalpy change observed when dissolving two moles of a substance in an infinite amount of solvent.
(v) Lattice enthalpy
The enthalpy change that occurs when one mole of an ionic compound dissociates into its ions in a gaseous state is referred to as the lattice enthalpy of an ionic compound.
Ques: A refrigerator transfers heat from a cold body to a hot body. Does this not violate the second law of thermodynamics? (1 Mark)
Ans: No. This is because external work is being performed.
Ques: Can the temperature of an isolated system change? (1 Mark)
Ans: In an adiabatic process the temperature of an isolated system changes. It increases when the gas is compressed adiabatically.
Ques: What is the nature of the P – V diagram for isobaric and isochoric processes? (2 Marks)
Ans: The P – V diagram for an isobaric process is a straight line parallel to the volume axis while for an isochoric process, it is a straight line parallel to the pressure axis.
Ques: What are the main features of the second law of thermodynamics? (2 Marks)
Ans: Second Law of Thermodynamics tells that entire heat energy can’t be converted into mechanical energy or heat can’t itself flow from lower temperature to higher temperature body.
Ques: Kelvin and Clausius’s statements of the Second law of thermodynamics are equivalent. Explain? (3 Marks)
Ans: Suppose we have an engine that gives a continuous supply of work when it is cooled below the temperature of its surroundings.
This is a violation of Kelvin’s statement. Now if the work done by the engine is used to drive a dynamo that produces current and this current produces heat in a coil immersed in hot water. We would then have produced a machine that causes the flow of heat from a cold body to a hot body without the help of an external agent. This is a violation of Clausius’s statement. Hence both statements are equivalent.
Ques: No real engine can have an efficiency greater than that of a Carnot engine working between the same two temperatures. Explain. (3 Marks)
Ans: A Carnot engine is an ideal engine from the following points of view:
- There is no friction between the walls of the cylinder and the piston.
- The working substance is an ideal gas i.e. the gas molecules do not have molecular attraction and they are points in size.
However these conditions cannot be fulfilled in a real engine and hence no heat engine working between the same two temperatures can have an efficiency greater than that of a Carnot, engine.
Ques: Explain the first law of thermodynamics. What are the sign conventions? (3 Marks)
Ans: It states that if an amount of heat dQ is added to a system then a part of it may increase its internal energy by an amount dU and the remaining part may be used up as the external work dW done by the system i.e. mathematically,
dQ = dU + dW = dU + PdV
Sign conventions:
- Work done by a system is taken as positive while the work done on the system is taken as -ve.
- The increase in the internal energy of the system is taken as positive while the decrease in the internal energy is taken as negative.
- Heat added (gained) by a system is taken as positive and the heat lost by the system is taken as negative.
Ques: State the principle of a refrigerator. (3 Marks)
Ans: A refrigerator may be regarded as Carnot’s ideal heat engine working in the reverse direction. Thus when a Carnot engine works in opposite direction as a refrigerator, it will absorb an amount of heat Q2 from the sink (contents of the refrigerator) at a lower temperature T2.
As heat is to be removed from the sink at a lower temperature, an amount of work equal to Q1 – Q2 is performed by the compressor of the refrigerator to remove heat from the sink and then to reject the total heat Q1 = (Q2 + Q1 – Q2) to the source (atmosphere) through the radiator fixed at its back. It is also called a heat pump.
Ques: An ideal gas in a cylinder is compressed adiabatically to one-third of its original volume. During the process. 45 J of work is done on the gas by the compressing agent. By how much did the internal energy of the gas change in the process? How much heat flowed into the gas? (2 Marks)
Ans: Here, ΔQ = 0, ΔW = – 45 J, ΔU = ?
According to first law of thermodynamics,
ΔQ = ΔU + ΔW, we get
ΔU = 0 – ΔW
= – (- 45) = 45 J
As the process is adiabatic, so the heat flow is zero.
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