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(i) The enthalpy change that accompanies melting of one mole of a solid substance at its melting point is called molar enthalpy of fusion. 

(ii) Amount of heat required to vaporize one mole of a liquid at constant temperature and understand pressure (1 bar) is called molar enthalpy of vapourisaton.

Enthalpy of atomization: It is defined as the enthalpy change accompyning the breaking of one mole of a substance completely into its atoms in the gas phase. 

H₂(g) → 2H(g); ΔaH° = 435.0kJ mol^-1 

Lattice enthalpy: Lattice of an comp; the enthalpy change which occure when one mole of an ionic compound dissociates into its gaseous ionic state. 

NaCl(s) → Na⁺ (g) + Cl^– (g); ΔH = +788 ks / mol

C₂H₆(g) will have highest heat of combustion because it has highest molecular weight and second highest calorific value. 

ΔH_c = calorific value of kJ/g × mol. wt. = 52 × 30 = 1560 kJ/mol.

A cyclic process consists of a series of changes which return the system back to its initial state. 

In non-cyclic process the series of changes involved do not return the system back to its initial state. 

(1) In cyclic process change in internal energy is zero and temperature of system remains constant. 

(2) Heat supplied is equal to the work done by the system. 

(3) For cyclic process P–V graph is a closed curve and area enclosed by the closed path represents the work done. 

If the cycle is clockwise work done is positive and if the cycle is anticlockwise work done is negative. 

Irreversible process : Any process which is not reversible exactly is an irreversible process. 

Examples of irreversible processes are : 

(i) Sudden expansion or contraction 

(ii) Heat transfer between bodies

(i) Source and sink have infinite heat capacities. 

(ii) Each process of the engine’s cycle is fully reversible.

Properties of thermal radiation :

(1) Thermal radiations are also called infra-red radiations. 

(2) Medium is not required for the propagation of radiations. 

(3) Every body whose temperature is above zero Kelvin emits thermal radiation. 

(4) Their speed is equal to that of light. 

(5) They follow laws of reflection refraction, interference diffraction and polarisation.

The specific heat of a gas in an isothermal process is Infinite. 

Work is converted into heat.

The working of an air conditioner is exactly similar to that of a refrigerator, but the volume of the chamber/room cooled by an air conditioner is far greater than that in a refrigerator. The evaporator coils of an air conditioner are inside the room, and the condenser outside the room. A fan inside the air conditioner circulates cool air in the room.

The coefficient of performance, K, of an air conditioner is defined as K = \(|\frac{Q_C}W|\), where Q_C is the heat absorbed and W is the work done. The time rate of heat removed is the heat current, H = \(\frac{|Q_C|}t\), where t is the time in which heat |Q_C|, is removed.

\(\therefore\) K = \(|\frac{Q_C}W|\) = \(\frac{|Q_C|/t}{|W|/t}\) = \(\frac HP\).

where H = |Q_C|/t is the heat current and P( = |W|/t) is the time rate of doing work, i.e., power.

In a refrigerator, Q_C is the heat absorbed by the working substance (refrigerant) at a lower temperature T_C , W is the work done on the working substance, and Q_H is the heat rejected at a higher temperature T_H . The absorption of heat is from the contents of the refrigerator and rejection of heat is to the atmosphere. Here, Q_C is positive and W and Q_H are negative. In one cycle, the total change in the internal energy of the working substance is zero.

∴ Q_H + Q_C = W ∴ Q_H = W – Q_C 

∴ -Q_H = Q_C – W 

Now, Q_H < 0, W < 0 and Q_C > 0 

∴ |Q_H| = |Q_C| + |W| 

The coefficient of performance (CoP), K, or quality factor, or Q value of a refrigerator is defined as

K = \(\frac{|Q_C|}{|W|}\) = \(\frac{|Q_C|}{|Q_C|-|Q_H|}\)

[Note: K does not have unit and dimensions or its dimensions are [M°L°T°.]

(b) expansion work

1. Mechanical work is defined as force multiplied by the displacement through which the force acts.

2. Whenever a force (F) acts on an object and the object undergoes a displacement (x) in the direction of the force, then the mechanical work is said to be done.

3. Mathematically w = F . x 

w = F. x = N.m 

w = Joule

• When a charged body moves from one potential region to another electrical work is done.
• If the electrical work done is QV. Where V is the potential difference and Q is the quantity of electricity.

w = QV 

w = Coulomb . Volts 

w = Joule

• The symbol of heat is q. 
• if the heat flows into the system from the surroundings, the energy of a system increases. Hence it is taken to be positive (+ q). 
• if heat flows out of the system into the surroundings. energy of the system decreases. Hence it is taken to be negative (- q).

1. When an object is raised to a certain height against the gravitational field, gravitational work is done on the object.

2. For example, if an object of mass ‘m’ is raised through a height ‘h’ against acceleration due to gravity ‘g’ , then the gravitational work carried out is ‘mgh’.

w = m.g.h 

w = Kg .ms^-2.m 

w = Kg m² s^-2 

w = Joule

• A system which can exchange both matter and energy with its surroundings is called an open system.
• Hot water contained in an open beaker is an example for open system.
• In this system, both water vapour and heat is transferred to the surroundings through the imaginary boundary.
• All living things are open systems because they continuously exchange matter and energy with the surroundings.

• Work is defined as the force (F) multiplied by the displacement (x).
• – w = F. x
• Minus (-) sign indicates the work done by the system
• Unit of work: The SI unit of work is Joule (J).

• A system which can exchange only energy but not matter with its surroundings is called a closed system.
• Here the boundary is sealed but not insulated. Hot water contained in a closed beaker is an example for a closed system.
• In this system heat is transferred to the surroundings but no water vapour can escape from this system.
• A gas contained in a cylinder fitted with a piston constitutes a closed system.

• A system which can exchange neither matter nor energy with its surroundings is called an isolated system.
• Here boundary is scaled and insulated.
• Hot water contained in a thermos flask, is an example for an isolated system

1. System: A system is defined as any portion of matter under thermodynamic consideration. which is separated from the rest of the universe by real or imaginary boundaries. 

For example, water taken in a beaker, balloon filled with air, seed, plant, flower and bird.

2. Surroundings: Everything in the universe that is not the part of system and can interact with system is called as surroundings.

(a) -30.39 J

∆V = expansion in volume = 100 to 250 

∆V = V₂ – V₁ = 250 – 100 

∆V = 150 ml = 0.15 litre 

Work done = ? 

Pressure = 2 atm 

w = -P∆V 

= -2 x 0.15 litre x 101.3 JL^-1 atm^-1 

= -30.39 J.

Answer: (a) -900 J

• Thermodynamics suggests feasibility of reaction but fails to suggest rate of reaction. It is concerned only with the initial and the final states of the system. It is not concerned with the path by which the change occurs.
• It does not reveal the mechanism of a process.

The scope of thermodynamics:

• To derive feasibility of a given process. 
• It also helps in predicting how far a physical (or) chemical change can proceed. until the equilibrium conditions are established.

Answer: (b) q – w

(c) temperature should be low