Laws of Thermodynamics Lecture -03 Laws of Thermodynamics The basic laws of thermodynamics are: law of conservation of mass I. law of thermodynamics II. law of thermodynamics III. law of thermodynamics

I. law of thermodynamics If a system is doing a work or the surroundings is doing a work on the system, its internal state is changed. E.g. if we compress a gas in a cylinder with a piston the temperature of the gas increases. Similarly, if there is a chemical reaction between the components of the system, its temperature changes. Or, if you consider an iceberg moving on rocky surface, the friction produces heat and the iceberg changes its phase – it melts. The cause producing the change of the state is called energy. Energy can be thus defined as the ability to change given (equilibrium) state of matter.

Principle of energy conservation. Initial experimets indicated an equivalence between heat and mechanical work (the work produces heat and heat can be used to do a work) This studies led to the formulation of Principle of energy conservation. This principle can be formulated in different ways, e.g.: “It is not possible to construct a machine generating energy from nothing. That means it is not possible to produce a perpetuum mobile of the first kind. In a more general formulation: The total energy of isolated system is constant during all processes.”

here q indicates heat accepted by the system from surroundings, w So we can expres the I. law of thermodynamics in this way “The total energy that a system exchanges with surroundings in any process is dependent only on the initial and final state of the system, and not on the way this change was achieved.” This means there is an energetic function, whose difference between initial and final state corresponds to energy exchanged between the system and surroundings. This function is called Internal energy of the system and is labelled as U. ΔU = U2 - U1 = q - w here q indicates heat accepted by the system from surroundings, w is a work done by the system, indexes 1 a 2 indicate initial and final state of the system

E.g. the amount of heat released during reaction: C + O2 = CO2 The I. law of thermodynamics implies that total heat released in a chemical reaction will be the same if the reaction proceeds in one step or in more steps. E.g. the amount of heat released during reaction: C + O2 = CO2 equals the sum of heat produced in the following reactions: C + 1/2O2 = CO CO + 1/2O2 = CO2 This conclusion is known as the Hess law.

Now we can introduce new thermodynamic function Now we can introduce new thermodynamic function. It is called enthalpy, labeled H, and defined by an equation: H = U + PV where P is pressure and V volume of the system Now we can calculate the amount of heat released in the system under constant pressure: qP = H2 - H1 = ΔH This expression says that the change of enthalpy in any process is dependent only on the initial and final state of the system. In the case of chemical reaction it is the state of the reactants at the beginning of the reaction and the state of products in the end of the reaction.

If the reaction proceeds under constant pressure, the reaction Reaction heat is the amount of heat exchanged by the system with surroundings during the chemical reaction. If the heat is released we speak of an exothermic process, if the heat is consumed by the system, it is referred to as endothermic process. If the reaction proceeds under constant volume, the reaction heat corresponds to the change of inner energy of the system. If the reaction proceeds under constant pressure, the reaction heat corresponds to the change of enthalpy.  

II. law of thermodynamics By the beginning of 19th century Carnot studied the efficiency of heat machines. He created a concept of cyclically working heat machine, in which the volume in the cylinder was changed by interaction with two heat exchangers having different temperature. Theoretical work out of this concept led to the formulation of the theorem: All the reversible machines working between the same heat exchangers have the same efficiency in spite of the composition of the exchangers.

II. law of thermodynamics Related formulation was stated by Clausius:   It is not possible to construct an equipment that would do nothing else than transfer heat from the colder body to a warmer body. This implies that it is not possible to create the so called perpetuum mobile of the second kind. These formulations are the expressions of the II. law of thermodynamics

dS = dq/T we get: dS > dq/T The studies of the efficiency of heat engines revealed the existence of a new state function called entropy labeled S   dS = dq/T   According to Carnot theorem the efficiency of reversible machine is maximum. Thus, the irreversible machines have always lower efficiency. For the irreversible process we get: dS > dq/T  

If the system does not exchange heat with surroundings we get for irreversible process: dS > 0 and for reversible process: dS = 0 It means that entropy is growing under irreversible processes and in equilibrium, when only reversible processes can proceed, it does not change. Entropy can be looked upon as a measure of spontaneousness, as it increases during spontaneous processes.

III. law of thermodynamics   The formulation was developing in time. As a definitive version is considered the formulation by Planck from 1912: Entropy of every chemically homogenous condensed phase approaches with decreasing temperature zero.  Another formulation explains it more clearly: It is not possible to cool a physical body to absolute zero in a finite number of steps.