Thermodynamics Laws

Sajjad Ahmed Memon S.S./ Health Physicist NIMRA

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Thermodynamics Laws

Sajjad Ahmed Memon

Senior Scientist

NIMRA


Thermodynamics is the study of the relationship between heat, work and the associated flow of energy.

Any system, physical or chemical or even biological, if it utilizes energy, it can be considered as a thermodynamical system and the laws of thermodynamics can be applied to it.

Thermodynamics

Thermodynamics Laws

After many decades of experience with heat phenomena, scientists formulated the fundamental laws of thermodynamics.


Zeroth LawIf two systems are each in thermal

equilibrium with a third, they are also in thermal equilibrium with each other.

This statement implies that thermal equilibrium is a similar relation on the set of thermodynamic systems under consideration. Systems are said to be in thermal equilibrium with each other if spontaneous molecular thermal energy exchanges between them do not lead to a net exchange of energy.


First LawA change in the internal energy of a closed

thermodynamic system is equal to the difference between the heat supplied to the system and the amount of work done by the system on its surroundings.


The first law allows a given internal energy of a system to be reached by the combination of heat and work. It is important that internal energy is a variable of state of the system whereas heat and work are variables that describe processes or changes of the state of systems.

The first law observes that the internal energy of an isolated system obeys the principle of conservation of energy, which states that energy can be transformed (changed from one form to another), but cannot be created or destroyed.


Let us consider a system absorbs Q amount of heat energy. Addition of heat energy increases the internal energy of system from U1 to U2 and some useful work is also performed by the system.


If the work done is W and the increase in internal energy is U

then according to the first law of thermodynamicsQ = U+ W

Where U = U1 – U2

Q will be positive if heat is added to a system

Q will be negative if heat is released from a systemW will be positive if work is done by the systemW will be negative if work is done on the system


First Law StatementsFirst law can be expressed as

"During any process total energy of a system and its surroundings is constant.“

OR"It is impossible to construct a machine which performs work continuously with taking energy from an external source.”

OR"Energy can neither be created nor destroyed but it can be converted from one form of energy to another form of energy."


Second LawHeat cannot spontaneously flow from a

colder location to a hotter location.

The second law of thermodynamics is an expression of the universal principle of dissolution of kinetic and potential energy observable in nature. The second law is an observation of the fact that over time, differences in temperature, pressure and chemical potential tend to even out in a physical system that is isolated from the outside world.


Entropy is a measure of how much this process has progressed. The entropy of an isolated system which is not in equilibrium will tend to increase over time, approaching a maximum value at equilibrium.

The second law is a basic postulate applicable to any system involving heat energy transfer or is a consequence of the assumed randomness of molecular disorder.


Second Law can also be expressed as follows:

It is inherit tendency of heat that it always flows from hot reservoir to cold reservoir. Rather than to flow in both the directions with equal probability. On the basis of this tendency of heat Second Law was proposed.


Third Law

As a system approaches absolute zero, all processes cease and the entropy of the system approaches a minimum value.

The third law is a statistical law of nature regarding entropy and the impossibility of reaching absolute zero temperature. This law provides an absolute reference point for the determination of entropy.


Third Law Statements“The entropy of all systems and of all states

of a system is smallest at absolute zero”

OR

“It is impossible to reach the absolute zero of temperature by any finite number of processes”

Absolute zero, at which all activity (with the exception of that caused by zero point energy) would stop is −273.15 °C (degrees Celsius), or −459.67 °F (degrees Fahrenheit) or 0 K (kelvin).





Entropy increases when

Gases are formed from liquids and solids.

Liquids or solutions are formed from solids.

The number of gas molecules increases.


Gases have higher entropies than liquids.

Liquids have higher entropies than solids.

Entropy is greater for larger atoms.

Entropy is greater for molecules with more atoms.


Solids have a much more regular structure than liquids. Liquids are therefore more disordered than solids.

The particles in a gas are in a state of constant, random motion. Gases are therefore more disordered than the corresponding liquids.

Any process that increases the number of particles in the system increases the amount of disorder


S > 0 implies that the system becomes more disordered during the reaction.

S < 0 implies that the system becomes less disordered during the reaction.


T

QS

Examples

Argon(Ar) has higher entropy than Neon(Ne) as Ar molecules are larger.

Octane (C8H18(L))has higher entropy than Pentane(C5H12(L)) as complex molecules have higher entropy than simple ones.

Bromine in gaseous form has higher entropy than liquid form as gases have higher entropies than liquids since gases have more ways of being arranged.



Compound Entropy “S” (J/mol-K)

SO3(s) 70.7

SO3(l) 113.8

SO3(g) 256.76



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Thermodynamics Laws