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Chapter Notes
Subject: Chemistry
Class: XI
Chapter: Thermodynamics
Top concepts
1. The branch of science which deals with study of different forms of
energy and their interconversion is called thermodynamics.
2. A system in thermodynamics refers to that part of universe in which
observations are made.
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3. The remaining portion of universe which is not part of system
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constitutes the surroundings. The surroundings include everything
other than the system.
4. The wall (real or imaginary) that separates the system from the
surroundings is called boundary.
5. Types of the System
ing
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Types of the System Exchange of Exchange of matter
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energy
Open Yes Yes
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Closed Yes No
Isolated No No
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6. The state of a thermodynamic system is described by its measurable
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or macroscopic (bulk) properties. The state of the surroundings can
never be completely specified.
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7. State variables are the measurable properties of system required to
describe the state of the system. Examples are temperature, pressure,
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volume etc.
8. Various types of processes
Type of process Definition
Isothermal Process in which temperature of system
remains constant
Adiabatic Process in which there is no transfer of heat
between the system and surroundings
Isobaric Process in which pressure of system
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remains constant
Isochoric Process in which volume of system remains
constant
9. Every substance possesses definite amount of energy which depends
on factors such as chemical nature, temperature and pressure etc.
Internal energy, U of the system represents the total energy of the
system
10. Work done in a adiabatic process and sign convention
Work done (wad) Sign of w
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By the system -
On the system +
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11.Heat changes and sign convention
Heat(q)transferred
From surroundings to
Sign of q
+ ing
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the system
From system to the -
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surroundings
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12. First Law of Thermodynamics: U = q + w
13. First law of thermodynamics is also called as law of conservation of
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energy i.e. energy can neither be created nor destroyed. It also states
that the energy of an isolated system is constant.
14.Absolute value of the internal energy can never be determined only
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change in internal energy can be determined.
15.A process or change is said to be reversible, if a change is brought out
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in such a way that the process could, at any moment, be reversed by
an infinitesimal change.
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16.For an isothermal reversible process
Vf
q=- w rev 2.3.30nRTlog
Vi
17.For isothermal irreversible change q = -w = pex Vf -Vi
18.Expansion of a gas in vacuum (pex = 0) is called free expansion. No
work is done during free expansion of an ideal gas whether the
process is reversible or irreversible
19.For adiabatic change, q = 0 and U = w ad
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20.The enthalpy of a system may be defined as the sum of the internal
energy and the product of its pressure and volume. It is denoted by
the symbol H and is given by H = U + PV
21.Change in enthalpy H = U + pV
22.Enthalpy change and sign convention
Type of reaction Definition H
Exothermic reactions Reactions in -
which heat is
evolved during
the reaction
Endothermic reactions Reactions in +
which heat is
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absorbed during
the reaction
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23.An extensive property is a property whose value depends on the
quantity or size of matter present in the system. Examples are mass,
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volume, internal energy, enthalpy, heat capacity, etc
24.An intensive property is a property whose value does not depend on
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the quantity or size of matter present in the system. Examples are
temperature, density, pressure etc.
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25.Specific heat, also called specific heat capacity is the quantity of heat
required to raise the temperature of one unit mass of a substance by
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one degree Celsius (or one Kelvin).
26.Relation between amount of heat, q, required to raise the
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temperatures of a sample, specific heat of the substance, c and
change in temperatures is given as q = mc T = CT
27.At constant pressure as q p = C p T = H
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28. At constant volume as q V = CV T = U
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29. For a mole of an ideal gas, C p - C V = R
30. Calorimetry is an experimental technique that helps determining
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energy changes associated with chemical or physical processes
31.The enthalpy change accompanying a reaction is called the reaction
enthalpy ( r H )
r H= (sum of enthalpies of products)- (sum of enthalpies of reactants)
32.The standard enthalpy of reaction is the enthalpy change for a reaction
when all the participating substances are in their standard states. The
standard state of a substance at a specified temperature is its pure
form at pressure of 1 bar.
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33.Different types of enthalpy
No. Type of Definition Symbol
enthalpy
1 Standard The enthalpy change that accompanies fus H
Enthalpy of melting of one mole of a solid
fusion substance in standard state
or
Molar
enthalpy
of fusion
2 Standard The enthalpy change that accompanies vap H
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Enthalpy of vaporizing of one mole of a liquid at
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vaporization constant temperature and under
or standard pressure (1bar)
Molar
enthalpy
of ing
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vaporization
3 Standard The enthalpy change when one mole of sub H
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Enthalpy of a solid substance sublimes at a
sublimation constant temperature and under
standard pressure (1bar)
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4 Enthalpy of The enthalpy change for the formation f H
formation of one mole of a compound from its
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or elements in their most stable states of
Standard aggregation
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molar
enthalpy of
formation
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5 Enthalpy of Enthalpy change when 1 mole of comb H
combustion substance is completely burnt in excess
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or
of oxygen or air
cH
7 Enthalpy of Enthalpy change in breaking one mole a H
atomisation of bonds of a substance completely into
atoms in gaseous state
8 Bond Amount of energy required to break bond H
dissociation one mole of bond of a particular type
enthalpy between atoms in gaseous state
9 Enthalpy of Enthalpy of solution of a substance is sol H
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solution the enthalpy change when one mole of
substance dissolves in a specified
amount of solvent
34.A balanced chemical equation together with the value of its r H is
called a thermochemical equation
35.Hesss Law of Constant Heat Summation: 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
36. Born-Haber Cycle is used to determine lattice enthalpy of ionic
compounds since they cannot be determined by experiment directly.
37.The lattice enthalpy of an ionic compound is the enthalpy change
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which occurs when one mole of an ionic compound dissociates into its
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ions in gaseous state
38. sol H lattice H hyd H
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39.A process that has natural tendency of occurrence in a particular
direction and is reversible only by application of some external agency
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is known as spontaneous process.
40.The processes which are forbidden and are made to take place only by
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supplying energy continuously from outside the system are called non-
spontaneous process.
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41.Entropy (S) is the measure of randomness of a system. It is a state
function.
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42.Entropy increases from solid to gas. Entropy is maximum for gases as
they have maximum disorder.
43.According to Second law of thermodynamics entropy of the universe
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always increases during a spontaneous change
44.For reversible process entropy change is given by
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q
S = rev where
T
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q rev is heat absorbed or released during the reaction
T is the temperature of the reaction
45. At constant pressure qrev= H
H
S=
T
46. For spontaneous process S total =S sys S surr 0
47.Decrease in enthalpy and increase in entropy of system independently
cannot serve the criteria for spontaneity
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48.Gibbs function or Gibbs energy is denoted by G.
G=H-TS
49. The criteria for spontaneous reaction in relation to G at constant
pressure and constant temperature.
If G < 0, process is spontaneous
If G = 0, process is in equilibrium
If G > 0, process is non-spontaneous
50.Relation between Gibbs energy change ( r G0 ) and equilibrium
constant (K) r G0 =-RTlnK
51. In case of endothermic reactions enthalpy of reaction ( r H ) is large
and positive. In such a case, value of equilibrium constant(K) will be
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much smaller than 1 and the reaction is unlikely to form much product
52. In case of exothermic reactions enthalpy of reaction ( r H ) is
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negative and r G0 is also large and negative. In such cases
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equilibrium constant (K) is larger than 1 and reactions go to
completion
53. Effect of temperature on Spontaneity of Reactions
ch
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kM
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w.
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