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# Chemical Thermodynamics

## Chemical Thermodynamics PDF Notes, Important Questions and Formulas

Thermochemistry

DEFINITION

Thermodynamics deals with energy interaction b/w two bodies & its effect on the properties of matter.

Scope of thermodynamics:

• Feasibility of a process
• Extent of a process
• Efficiency of a process

TERMS USED IN THERMODYNAMICS

System + Boundary + Surrounding = Universe

SYSTEM: The part of the universe under thermodynamical observation is called system.

SURROUNDINGS: All the part of the universe excepting system is called surroundings.

BOUNDARY: The part which separates system and surroundings is called boundary it may be rigid or flexible. It may be diathermic (Heat can be exchanged) or adiabatic

(Mass and energy both                             (Only energy can be        (Neither mass nor energy

Can be transferred)                                               transferred)             can be transferred)

STATE VARIABLES

State or condition of a system is described by certain measurable properties & these measurable properties are called state variables. e.g. mass, temperature, volume, pressure etc.

PATH FUNCTION

Path function depends on the initial as well as final state of a system & also depends on the path of the process. e. g. heat and work.

THERMODYNAMIC PROCESSES

 PROCESS SPECIAL CONDITION CONSTRAINTS 1) Isochoric Process V=constant, △v=O 2) Isobaric  Process P=constant, △P=O 3) Isothermal process T=constant, △T=O 4) Adiabatic process Q=neither enters nor leaves the system

Basic & Kirchhoff’s law and Hess’s law

Thermochemistry is the branch of physical chemistry which deals with the thermal or heat changes caused by chemical reactions

SPECIFIC HEAT(S)

Amount of energy required to raise the temp by C of 1 gm of a substance.

Unit → J/kg-K

Heat Capacity (ms)

The amount of heat required to raise the temperature by 1℃ or 1K of given amount of a substance.

Units→J/K

Total heat given to increase the temperature by △t.

Q=ms△t

Molar Heat Capacity (Cm)

The amount of heat required to raise the temp by of mole of a substance.

Classification of molar heat Capacity

1. Molar heat capacity at constant pressure(Cp,m)
2. Molar heat capacity at constant volume (Cv,m)

Relation between Cp and Cv

Rules for thermo chemical Equation

1. It is necessary to mention physical state of all rectants and products
2. A→B,△H=HB-HA

If A→B + x kJ/mole

△H<0 ⟹ (Exothermic reaction)

△H=-x kJ/mole

If A + x kJ/mole → B

△H>0 ⟹ (Endothermic reaction)

△H=+x kJ/mole

(3) After reversing a thermo chemical eqthen sign of enthalpy also get changed.

e.g.     A(g)+B(g) →C(g)+D(g), △H=x kJ

C(g)+D(g) →A(g)+B(g),△H= -x kJ

(4) When two reactions are added their enthalpies are also get added with their sign

e.g.

If a thermochemical equation is multiplied by a number then △H is multiplied by the same number.

e.g.

Intensive property

The property which does not depend upon the mass of substance is called intensive property.

e. g. density, refractive index, specific heat, etc.

Extensive property

Mass dependant properties are called extensive properties

e.g. △H, △S, △G, V, U, Resistance, Number of moles etc.

Two extensive property can be added

* Ratio of two extensive properties is Intensive.

* Intensive properties cannot be added directly.

e .g. We cannot add the density of two liquids to get the density of the final mixture of the two.

Kirchoff's Equation

This gives the relation between enthalpy and temperature.

* Physical state is changed at constant temperature.

• Hess’s Law of constant heat summation:
• The heat absorbed or evolved in a given chemical equation is the same whether the process occurs in one step or several steps.
• The chemical equation can be treated as ordinary algebraic expression and can be added or subtracted to yield the required equation. The corresponding enthalpies of reactions are also manipulated in the same way so as to give the enthalpy of reaction for the desired chemical equation.
• Since △, H stands for the change of enthalpy when reactants (substances on the left hand side of the arrow) are converted into products (substance on the right hand side of the arrow) at the same temperature and pressure, it the reaction is reversed (i.e. product are written on the left hand side and reactant on the right hand side), then the numerical value of △H remains the same, but its sign changes.
• The utility of Hess’s law is considerable. In almost all the thermochemical numericals, Hess’s law is used.
• One of the important applications of Hess’s law is to determine enthalpy of reaction which is difficult to determine experimentally. For example, the value △for example, the value △H for the reaction

C (graphite)+O2(g) → CO(g)

Which is difficult to determine experimentally, can be estimated from the following two reactions for which △H can be determined experimentally.

C(graphite)+O2(g) →CO2(g)                △rH1

CO(g)+O2(g) →CO2                         △rH2

Substracting the latter from the former, we get

C(graphite)+O2(g) →CO(g)

Consequently, △rH=△rH1-△rH2

Enthalpy of formation & Enthalpy of combustion and Bomb calorimeter Heat of formation

Enthalpy change during the formation of 1 mole of a compound form its most stable common occurring form (also called reference states) of elements is called heat of formation.

C(S)+O2(g) →CO2(g)

△H=△H(CO2)

Co(g)+1/202(g) →CO2(g)

△H¹△HfCO(g)

(Because CO2 has not been formed from its element in their most stable form )

Similarly

CH2-CHO+H→ C2H5OH

Heat of reaction

 Element Most stable form H H2(gas) O O2(gas) N N2(gas) F F2(gas) Cl Cl2(gas) Br Br2(gas) I I2(solid C C(grapnite) P P(white) S S(rhombic)

All metal except Hg exist in solid form (reference states)