EFFECT OF PRESSURE AND VOLUME ON THERMODYNAMICS LAW

Before discuss about effect of pressure and volume on thermodynamics law see definition of pressure and volume.

PRESSURE VOLUME WORK:-
Suppose a cylinder with area of cross section A.It is filled with a gas having initial volume V1 and fraction less piston is fitted on it.Gas give pressure P on piston and piston moves upword up to a distance dl and a volume of gas changes to V2. The system does work W going upword.
Pressure on the piston is     P=force/arra. Thus force on the piston F=PA (1)
Work done by the piston is         W=force x distanceor W= f.dl(2)
Then w=PAxdl but as DV=Axdl so  w=PDV(3)
Put the value of W from equation (3) in equation of first law of thermodynamics DE = q-w
DE =q-PDV or DE+PDV=q or q=DE+PDV(4).This is the equation of pressure volume and work.

PROCESS OF THERMODYNAMICS:-
Process of thermodynamic take place at constant volume and at constant pressure.

1)PROCESS AT CONSTANT VOLUME:-
Suppose at constant volume heat given to the system is qv then the equation of pressure volume and work becomes.
qv=DE+PDV as volume is constant so the change in volume is 0.
qv = DE + P x0 or qv = DE thus heat given to a system at constant volume is equal to the change of energy of the system.

2)Process at Constant Pressure:-
Suppose at constant pressure heat given to the system is qp then equation of pressure volume work becomes.
qp=DE + PDV as DE=E2-E1 and DV=V2-V1 this qp= E2-E1+P(V2-V1
Or qp = E2 - E1 + PV2-PV1 by rearranging qp=E2+PV2-E1-PV1
Or qp = E2+PV2-(E1+PV1) as initial enthalpy H1=E1+PV1 and final enthalpy H2=E2+PV1 thus     qp=H2-H1 as DH= H2-H1 so qp=DH thus heat given to system at constant pressure is equal to the change in enthalpy of the system.

ENTHALPY (H):-
Total heat content of a system at constant pressure is called its enthalpy.Enthalpy is a sum of energy "E" and product of pressure volume PV of the system.Enthalpy is a state function of a system and depend on the amount of system.Initial enthalpy of system is H1 and final enthalpy of system is H2 and chane in enthalpy of system is DH.

MATHEMAICAL EXPRESSION OF ENTHALPY:-
H = E + PV. Initial enthalpy
H1 = E1+ PV1 and final enthalpy.
H2 = E2 + PV2.
while change in enthalpy
DH = H2 - H1.

SIGN OF CHANGE OF ENTHALPY:-
Its sign is negative if the heat releasd means that if reaction is exothemic and its sign is positive if heat absorbe by the system means that reaction is endothermic. INTRODUCTION TO HESS's LAW
Hess's Law has given by a Russian Chemist and Dr Germain Hess. Hess helped to formulate the basic principles of thermochemistry. Hess's very famous paper, that was published in 1840, and his law on thermochemistry also published in 1840. Hess's law is due to enthalpy being a state function, that allow us to calculate the all changes in enthalpy by simply summing up the changes for each step of the way, until product is formed, all steps have to proceed at the same temperature and the equations for the individual steps must balance out. The principle underlying Hess's law does not just apply to Enthalpy and can be used to calculate other state functions like changes in energies.

HESS's LAW OF CONSTANT HEAT OF SUMMATION:-
This law is given by G.N Hess.A reaction take place in one or more steps, total change of heat is always same.

EXPLANATION:-
According to Hess's law total change of heat in a process depend on the initial state and final state of system, but don't depend on the middle states or on the manner by which reaction take place. A reaction take place one step or more than one steps then the total change of heat is sum of heat changes in the steps:
DH = DH1 + DH2 + DH3........
Where DH is total change of heat is (Enthalpy) and DH1 , DH2 , DH3 are the heat changes in different steps.
Exapmle:
Direct Method: A----->D DH=-40kj Indirect Method:A--->B DH1=-20kj
B---->C DH2=-15kj C--->D DH3=-5kj
Total heat change=-20-15-5=40kj

APPLICATION OF HESS's LAW:-
Hess's law is applied to calculate the heat of formation, Heat of reaction, Heat of solution, Heat of atomization etc.
 To determine heat of formation of substances that can't be measured experimentally, substances like benzene, carbion dioxide and benzene can't be prepared by uniting their elements, So it is impossible to measure the heat of formation of certain compound directly.
To determine the heat of transition. Heat of transition from one allotropic form to another may also be calculated with the help of this law.
To determine of heat of several reactions. Using this law we can calculate enthalpies(heat) of most of reactions that can't be measured directly. Like as the enthalpy of dimerization of nitrogen dioxide.

HEAT OF FORMATION:(DHF);-
Change in enthalpy when one mole of a compound is formed from it's elements is called heat of formation.

STANDARD HEAT OF FORMATION:-
It is denoted by DHF^0. Heat of formation at 25 degree calcius is called standard heat of formation.

CALCULATION OF HEAT OF FORMATION:-
Heat of formation can be calculated by multiplying, adding and subtracting the equations.