Thermodynamic and First Law of Thermodynamics

Thermodynamic -Thermodynamic is the study of heat and work. -Thermodynamics is the name we give to the study of processes in which energy is transferred as heat and as work

Heat is the transfer of energy across the boundary of a system resulting from a temperature difference between the system and its surroundings. * Note that heat is a transfer of energy due to a difference of temperature; work is a transfer of energy by mechanical means.

System: Is any object or set of objects that we wish to consider.

*The Calorie: is the amount of energy necessary to raise the temperature of 1 g of water from 14.5C 0 to 15.5C 0. * The mechanical equivalent of heat. Joule found that: the loss in mechanical energy is proportional to the increase in water temperature ΔT, the constant of proportionality= J/g.

Hence: J of mechanical energy raises temperature of 1g of water by 1 C 0 1cal = 4.186J

*The heat capacity: [C] The amount of energy needed to raise the temperature of the sample by 1C 0 Q= CΔT C=Q/ΔT ( J/C 0 or Cal/ C 0 )

*The specific heat [c]: The heat capacity per unit mass or the energy Q required to change the temperature of a mass m of a substance by an amount ΔT Q= mcΔT c=Q/mΔT ( J/Kg.C 0 or Cal/ g.C 0 )

The Latent heat of the substance [L]: The energy required to change the phase of a pure substance of mass m Q= ± mL L= ±Q/m (J/Kg) (+) if energy enters the system. (-) if energy leaves the system. *Latent heat of fusion L f : when the phase change from solid to liquid. *Latent heat of vaporization L V : when the phase change from liquid to gas.

*The Internal energy: is the total energy that is associate with the system's microscopic component include kinetic energy of random translation, rotation and vibrational of molecule. Potential energy within molecules, and potential energy between molecules.

*Extensive and Intensive quantity: Intensive property is that does not depend on the size of the system ( e.g. pressure, temperature,density, and partial molar ) Extensive property or quantity is that depends on the size of the system e.g. mass, volume, energy[ but not energy per unit volume or mass], heat capacity[ but not specific heat capacity].

*Type of systems in thermodynamics: 1- Isolated system: matter and energy may not cross the boundary. 2- Adiabatic system: heat may not cross the boundary. 3- Closed system: matter may not cross the boundary. 4- Open system: heat, work, and matter may cross the boundary.

In thermodynamic we describe the state of a system by using variables, such as ( pressure, volume, temperature, and internal energy), as a result, these quantities called state variables.

*Work in thermodynamics: Consider a gas contained in a cylinder fitted with a movable piston, at equilibrium the gas occupies a volume V and exerts a uniform pressure P on the cylinder's walls and on the piston. The force exerted by the gas on the piston is F=PA

The work done on the gas if the piston push inward and the gas compressed dw= F.dy dw= P A dy dw= P dV, ( dw is the work done on the gas)

If the gas is compressed dV(-) and the work done on the gas (-), that means the system is doing the work on the gas If the gas expands dV(+) the work done on the gas is (+) If V remains constant, the work done on the gas zero.

On a PV diagram to show that, take case (1) and case (2) In case (1) volume reduced from V i to V f at constant P i, then P i increase from P i to P f at constant V f by heating. The work done on the gas a long this path w=P i ( V f - V i ) i f pPfpPf PiPi V i vVfVf

In case (2) pressure increase from P i to P f at constant V i, then the V i decrease from V i to V f at constant P f. The work done on the gas a long this path w=P f ( V f - V i ) i f PPfPPf PiPi V i v VfVf

Not that from (1) and (2) Work in case (2) > Work in case (1) The work done on the gas from initial state to final state depends on the path between these states.

Vacuum membrane Gas at T i Insulating Walls A gas expand rapidly into an evacuated region after a membrane is broken

5- Isobaric process: A process that occurs at constant pressure, this occurs by allowing the piston to move freely so that, there is equilibrium between the net force from the gas pushing up ward and the weight of the piston plus the force due to atmospheric pressure pushing down ward

Q

6- Isovolumetric = Isochoric process A process takes place at constant volume in the figures clamping the piston at a fixed position ( V constant) W=PdV→dV=0→PdV=W=0→ ∆U=Q This means if energy is added by heat to a system at constant volume, then all of the transferred energy remains in the system as an increase in its internal energy.

Note that the energy entering the gas by heat leaves by work. So the temperature can remain constant.

From (1) and (2)

ProcessWhat is constantThe first law predicts IsothermalT=constantΔT=0, ΔU=0,W=Q IsobaricP=constantQ=ΔU+W=ΔU+PΔV IsovolumetricV=constant ΔV=0 makes, W=0,Q=ΔU adiabaticQ=0ΔU=-W

dP=0 because P is constant

H=enthalpyU=internal energy DefinitionH=U+PVdQ=dU+dW Differentiati on form dH=dU+PdV+VdP dQ dU=dQ-dW Change w.r.t temperature dH=dQ P, at P constant dU=dQ V,at V constant In adiabatic process In ideal gas