Tips and Tricks, IIT JEE Chemistry, Thermodynamics

Niranjan G.   (On Spanedea since October 08, 2013)

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B.Tech. from IIT Madras in , 2009, M.Tech. from Politecnico Di Milano, Italy in , 2016

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Posted on 30 June 2016

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Thermodynamics is a study of interrelation of energy accompanying physical and chemical changes. We study thermodynamics to interrelate the various energy changes during physical or chemical transformation, to predict the feasibility of a given change, to deduce various laws thermodynamically and to determine the conditions favoring the equilibrium. Unfortunately thermodynamics can’t predict the rate of the reaction, is unable to tell about equilibrium concentrations, moreover thermodynamics laws are valid for bulk of matter and doesn’t provide any information about an individual atom.

 

Our subject of interest may vary, for ex, a reaction taking place in chamber or a volume of air which will enter a balloon, we define our subject to be in an imaginary space called as the system. The system is an imaginary space and of any shape which contains our subject of interest. Everything other than the system is called as surroundings and the line separating them is called as boundary. In chemistry we would be dealing with reactions involving gases and liquids mostly which occur in a system. If the system holds a substance in one phase only then it is said to be a homogeneous system otherwise a system holding a mix of gases or if they have more than one phase then it is said to be a heterogeneous system. Our interest is to extract or give heat to these systems. Technically, Energy in the form of work & heat cross the system boundary which is extracting or feeding energy to the substances in a system. There can also a possibility of transfer of mass from the system boundary, as in the example of a fully blown up balloon, our system is the inside surface of the balloon which contains the air and it can lose mass too, but this is always not demanded. The systems which allow transfer of energy and mass across its boundary are called as open systems, when they only allow transfer of energy they are called as closed systems and when they neither allow energy nor mass to cross its boundary are called as isolated systems. The gas in a cylinder piston arrangement is an example of closed system and fluid in a thermos flask is assumed to be an isolated system.

 

Apart from defining the system theoretically, it can also be defined with measurable properties, technically which define the state of a system known as state variables eg, P, V, T etc. At any given time these parameters define a system. These properties can dependent or independent of mass of the system. Those that depend on mass are called as intensive properties like, Temperature, Pressure, Viscosity, Density, Surface Tension etc. The properties that depend on mass are called as extensive properties for eg: mass, volume, energy, enthalpy, entropy. We know that the system would change from time to time and hence these properties too would change. This change in system from one state to another is called a process. There are many thermodynamic processes like, Isothermal process, ΔT=0. In this process the temperature of the system remains a constant throughout. Also the change in internal energy is 0. In an adiabatic process the system does not exchange any heat. Hence ΔQ=0. This can be achieved by insulating the boundary. In an isobaric process, the pressure of the system remains a constant throughout, ΔP =0.In an isochoric process the volume remains a constant throughout, ΔV =0. The system can start at a state and after multiple states can return to its original state. Such a process is called a cyclic process, ΔE=0, ΔH = 0. In all these processes there are energy interactions and can be calculated.

 

When all the changes occurring at any part of the process are exactly reversed, when it is carried out in the opposite direction, such a process is called as reversible process. It involves a slow change during investigations, the driving force is infinitesimally greater than the opposing force and maximum work is obtained. A process whose direction cannot be reversed by small changes is called as irreversible process. It involves a fast change during investigation, a unidirectional process and the net work is lesser than the maximum. Eg: combustion.

 

The energy which crosses the boundary can be divided broadly into Heat Q, Work W and internal energy ΔU. The Q and W depend on the path whereas the internal energy depends only on the state and hence it is a state function and denoted with Δ, a change. For eg: You sweat more when you take a longer route to your school from home if you go by walk. You do work in the form of walking and lose Q in the form of sweat, but your internal energy is calculated from the starting and the ending point. Our body is taken as the system. So work can be done on or by the system. To ease our calculations, a sign convection is used which is voluntary. The work done by the system is taken as –ve and the work done on the system is taken as +ve. In the similar way Q given to the system is +ve and Q given by the system is –ve. The calculation of work is done by the set of equations from classical mechanics.

 

In a bank account, you can deposit money and can withdraw money for your expenditures and a net balance is always maintained which is greater than zero after multiple transactions. Taking forward this analogy to thermodynamics, heat or work can be given to the system or taken from it, which has an effect on the internal energy. If we impart energy to the system in the form of work, some part of it will be converted to heat and some would be stored as internal energy. Just like when you deposit money, some part of it would be in the balance and some would be spent. Mathematically it is denoted as Q + W = ΔU. This is called as the First law of thermodynamics. Here work is done on the system hence +ve, some heat is released by the system hence +ve and the net balance is stored as the internal energy. This law can be applied to different processes which we discussed earlier. For an isothermal process where ΔT =0 hence ΔU = 0, Q = -W, it means that when the system does work (-ve) heat (+ve) is given to the system. In a cylinder piston arrangement, when you heat the system, the volume expands and pushes the piston hence work is done by the system on the piston. For an adiabatic process, Q = 0 hence W = ΔU. For eg, when we charge our mobile phones, the battery is designed to be adiabatic. So the work done on the system, the battery, by the electrons is stored as internal energy. For isochoric process, ΔV = 0, Q = ΔU. If you give heat to a container containing full with water and assuming no heat is taken by the walls of the container, all the heat is used to increase the internal energy of water and hence in turn increases the temperature of water. In an isobaric process, ΔP = 0, the classical definition of work can be taken to calculate the work done, W = -PΔV. After calculations, Q= ΔH, ie. Heat given to a system under constant pressure P is used up in increasing heat enthalpy of the system. Using the classical mechanics equation of work the work done for irreversible isothermal process, isothermal reversible process and adiabatic reversible process can be calculated.

 

Using our analogy of bank interactions, you can never withdraw money more than your balance. Whereas using the first law of thermodynamics, you can do work on the system and the system can take heat too. Logically it is not possible. Hence first law is only quantitative not qualitative. So the energy interactions have to be defined qualitatively. Hence second law of thermodynamics comes into picture which talks about the quality of the energy which is defined in several ways. R.Classius defined as, heat cannot pass itself from a cooler body to a hotter body without the intervention of external energy. According to Kelvin, it is impossible to obtain work by cooling a body below its lowest temperature. According to Ostwald, It is impossible to construct a machine functioning in a cycle which can convert heat completely into equivalent amount of work without producing changes elsewhere, ie perpetual motion machines are not allowed. Carnot said that it is impossible to take heat from a hot reservoir and convert it completely into work by a cyclic process without transferring a part of it to cold reservoir. These laws govern the systems available worldwide and give a right direction for using the first law correctly.

 

Another important state variable which defines a system is entropy which talks about the measure of degree of disorder or randomness of system. These configurations are possible due to numerous movements of the gas molecules. Greater the disorder of the system greater is the entropy. It is defined as ΔS = .

 

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