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MHT-CET : Chemistry Entrance Exam

MHT - CET : Chemistry - Chemical Thermodynamics and Energetics Page 2

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11.

First Law of Thermodynamics

 

It can be stated in any one of the following forms:

  1. Energy can neither be created nor destroyed, however, it may be converted from one form into another.
  2. Whenever energy in one form disappears, an equivalent amount of energy in another form appears.
  3. It is not possible for any machine to produce work without consuming energy. Such a machine is called a perpetual motion machine.
  4. The total energy of the universe remains constant.
  5. The total mass and energy of an isolated system remains constant.

 

12.

Corollary of the First Law of Thermodynamics

 

 

The total amount of work or mechanical energy absorbed by a system is converted into heat energy and vice-versa.

 

13.

Expression for First Law of Thermodynamics

 

 

Q = DE + W or Q = DE + PDV
where Q = Amount of heat absorbed
DE = Change in the internal energy
W = Work done

  1. For isothermal process, temperature (T) remains constant
    \ DE = 0
    \ Q = PDV
  2. For isochoric process, volume (V) remains constant
    \ DV = 0
    \ Q = DE
  3. For isobaric process, pressure (P) remains constant
    \ Q = DE + PDV
  4. For adiabatic process, heat is neither absorbed not lost
    \ Q = 0
    \ DE = PDV

 

14.

Internal Energy or Intrinsic Energy (E)

 

 

The sum of all forms of energy associated with the matter present in a system is called internal energy or intrinsic energy of the system. The absolute or actual value of the internal energy cannot be determined but the change in the internal energy (DE) can be measured.
D
E = E2 - E1
The change in the internal energy of in a process is independent of the path taken and depends only on the initial and final states of the system. Hence, it is a state function.

 

15.

Bond Energy

 

 

  1. Intramole Energy (Intramolecular Energy)
    The energy required to break a molecule into its constituent atoms is called intramole energy.
    Example: H2O(g)
    H2(g) O2(g)
    DH = - 57.8 kcal.
  2. Intermole Energy
    The energy required to separate the molecules of a substance from each other in a liquid or solid state is called intermole energy.
    Example: H2O(l) H2O(g);
    DH = -10.5 kcal.

 

16.

Enthalpy of a System (H)

 

 

The total heat content of a system is called enthalpy of the system.
H = E + PV
Expression for change in enthalpy (
DH):
DH = DE + PDV

  1. For an isochoric process, volume remains constant.
    \ DV = 0
    \ DHv = DE
  2. For an isobaric process, pressure remains constant.
    \ DHp = DE + PDV

 

17.

Thremochemistry

 

 

The branch of chemistry, which deals with the quantitative study of the heat changes associated with chemical reactions is called thermochemistry.

 

18.

Heat of Reaction (DH)

 

 

The quantity of heat absorbed or evolved (enthalpy change) during the complete transformation of the reactants into the products as shown in the corresponding thermochemical equation, at constant temperature and pressure, is called the heat of reaction.
DH = H(products) - H(reactants)
Example: C(s) + O2(g) CO2(g);
DH = - 393.6 kJ

  1. Relationship Between Heats of Reaction at Constant Pressure (DH) and at Constant Volume (DE)
    DH = DE = PDV or
    DH = DE + DnRT or
    Qp = Qv +
    DnRT
  2. Factors Affecting Heat of Reaction
    (i) Physical state, (ii) Pressure and (iii) Temperature

 

19.

Heat of Formation (DHf)

 

 

The quantity of heat absorbed or evolved when one mole of a compound is formed from its constituent elements, with every substance being in its standard physical state, is called heat of formation.

 

20

Heat of Neutralisation (DHn)

 

 

The quantity of heat liberated when one gram equivalent of an acid is completely neutralised by one gram equivalent of a base in a very dilute solution is called the heat of neutralisation.

 

21

Heat of Combustion (DHc)

 

 

The quantity of heat evolved when one mole of a compound is completely oxidised to its stable oxidation products, with every substance being in its standard physical state, is called heat of combustion.

 

22

Heat of Solution (DHs)

 

 

The quantity of heat absorbed or evolved when one mole of a compound dissolves completely in a large excess of a solvent so that further dilution of the solution produces no heat change, under standard conditions, is called the heat of solution of the compound in that solvent.

Example: KCl(s)

+ water

KCl(aq); DH = + 18.4 kJ

 

(excess)

 

 

23

Molar Heat Capacity

 

 

The quantity of heat required to raise the temperature of one mole of a substance through one degree kelvin is called molar heat capacity of the substance.

  1. Molar Heat Capacity at Constant Pressure (Cp)
    The quantity of heat required to raise the temperature of one mole of a substance through one degree kelvin at constant pressure is called molar heat capacity at constant pressure.

Cp =

dq

p

=

DH

p

dT

DT

  1. Molar Heat Capacity at Constant Volume (Cv)
    The quantity of heat required to raise the temperature of one mole of a gas by one degree kelvin at constant volume is called molar heat capacity at constant volume.

Cv =

dq

v

=

DE

 

dT

DT

  1. Relationship Between Cp and Cv (Mayer's Relationship)
    Cp - Cv = R
    i.e. Cp > Cv

 

24.

Kirchhoff's Equation

 

 

  1. Expression showing the effect of temperature on the heat of reaction at constant pressure:

DH2 - DH1

= DCp

T2 - T1

  1. where,
    DCp = (Cp of products) - (Cp of reactants)
    DH1 = Heat of reaction at temperature T1
    DH2 = Heat of reaction at temperature T2

 

25.

Internal Energy and Change in the Internal energy

 

 

  1. Translational Energy
    The energy associated with the translational motion of molecules in space is called translational energy.
  2. Vibrational Energy
    The energy associated with the vibration of the atoms in a molecule about their equilibrium positions is called vibrational energy.
  3. Rotational Energy
    The energy associated with the rotation of a molecule about an axis passing through its centre of gravity is called rotational energy.

 

26.

Hess's Law of Constant Heat Summation

 

 

The total enthalpy change accompanying a chemical reaction is always constant (at constant pressure or constant volume) and is independent of the number of steps and the path taken to complete the reaction.
i.e.
DH = DH1 + DH2 + DH3 +

 



 

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