Let a system undergoes a change of state reversibly at a constant temperature T. The heat absorbed by the system from the suuroundings may be represented by qrev . The increase in entropy of the system (ΔSsystem) will then be given by
ΔSsystem = qrev/T …(i)
Since the surroundings have lost heat equal to qrev , reversibly at the same temperature T , the entropy change of surroundings ΔSsurroundingswill given by
ΔSsurroundings = – qrev/T …(ii)
(since the surroundings have lost heat , therefore ,it is given a negative sign)
Therefore , total change in entropy of the system and its surroundings during reversible process is given by
ΔSsystem + ΔSsurroundings= qrev/T – qrev/T = 0
It may, therefore , be concluded that in a reversible process, the net entropy change of the systemand the surroundings is zero.In other words , there is no net entropy change in a reversible process.
The physical significance of entropy can be well understood by an examinatiion of various processes which take place with an increase of entropy. On studying these processes, it has been founded that those processes which are accompained by anet increase of entropy also show an incresed randomness.
Consider, for example, the melting of a solid which is accompained by a net increase of entropy . In this case , at the same time there is an increase in th erandom ness of the atoms, ions or molecules .This is because, in the solids, the ions, atoms or molecules are supposed to have fixed positions and in the molten state, they become more free to move.
The evaporation of aliquid alsoi involves in entropoy as well as disorder
Similarly all spontaneous process like flow of heat from a hot end to a cold end of a conductor , transference of molecules of a solute from a more concentrated to a less concentrated solution, flow of electricity from a poin tat a higher potential to a point at a lower potential are all accompained by a net increased of entropy as well as an increase of randomness or disorder.it
follows from above examples that an increase in the entropy is also accompained by an increase in the disorder of the system.
Therefore entropoy is regarded as a measure of the disorder of the system.
Analogy from daily life The concept of entropy and randomness can be better understood by considering the following exmples in our daily life.
Consider the working of a college, when the different classes are being held in different rooms . The randomness or disorder is minimum. But when the bells goes, the students change their own classrooms and get mixed up for 2 – 3 minutes. For these 2 3 minutes, the randomness (or disorder) and hence, the entropy of the college increases .After 2 or 3 minutes, when the students again occupying different rooms, the randomness (or disorder ) and hence, the entropy of the college again increases
(ii) Before the start of any match between the two teams th edisorder is m,inimum as both the teams are on their respective sides but as the game starts players start running and get mixed up in such a situation , the randomness and hence, the entropy increases.
1. Calculate the amount of heat supplied to carnot cycle working between 368K and 268K, if the maximum work obtained is 890 joules.
2. A carnot engin eworking between 373K and 273K takes up 840 joules from the high temperature reservoir. Calculate the work done , the heat rejected and the efficiency of the heat engine.
3. Calculate th efficiency of a steam engine which operates between 373K and 573K. What is the minimum heat which must be withdrawn from the reservoir to obtain 171.52 joule of work ?
4. Two steam engines , one red and one black , take steams at 277ᵒC, while red engine rejects it at 27ᵒC , the black one reject it at 47ᵒC. compare the efficiencies of the engines.
5. Calculate the efficiencies of a carnot engine working between 200K and 1000K.
6. Heat supplied to a carnot engine is 453.6 kcal. How much useful work can be done by the engine which work between 0ᵒC and 100ᵒC ?
7. A carnot engine converts one sixth of heat input into work . When the temperature of the sink is lowered to 335K, its efficiency is doubled. Calculate the temperature of source and the sink.
8. If the efficiency of a carnot engine is the same (i) between 100 K and 400 K and (ii) between T K and 800 K, calculate the temperature T of the sink.
9. Calculate the maximum efficiency of a steam engine operating between 110ᵒC and 250ᵒC. What would be the efficiency of engine if the boiler temperature is raised to 140ᵒC, the temperature of the sink remaining same?
10. What per cent T₁ and T₂ for a heat engine whose efficiency is 10% ?
It is a well known fact that most of the phyiscal changes and chemical changes are accompained by energy changes. these energy changes may take place in the form of heat, light, work, electical changes, etc. All these forms of energy are convertible into one another and hence are related to each other quantitatively.
“The branch of science which deals with th estudy of different forms of energy and the quantitative relation ship between them is known as thermodynamics.
The name thermodynamics is given by the mechanicals engineers in the beginning whon were intrested in only in the conversion of heat into mechanical work. Thermo means heat and dynamics means motion resulting into mechanical work.
NEED FOR THE LAW AND DIFFERENT STATEMENTS OF THE LAW .
According to the first law of thermodynamics ‘ one form of energy can be converted into another form and the total amount of the energy can be conserved ‘ the 1st law gave us the two important state functions E and H , still have some limitations
Due to this limitations , it becomes very necessory to introduce an another law of thermodynamics to overcome those limitations .
LIMITATIONS OF 1ST LAW OF THERMODYNAMICS:
As discussed earlier the 1st law of thermodynamics tell us about the conservation of the energy during any chemical or physical process. but this law doesn’t tell about the feasiblity of a process, i.e, whether the process under the given conditions is feasible or not.
For example , if we burned a papper of a piece in the presence of oxygen , the reverse can not be happen, from ashes paper can not form.
similarly, a bottle of a perfume is opend in a room, it’s vapour spreads in the whole room but the reverse process, i.e, vapours of perfume can not collect themselves in the bottle.
similarly , a gas can expand into vaccum and the process doesn’t violate first law of thermodynamics but the reverse can not be happened.
you have a question in a mind that why all the process above explained are uni-directional in nature ,i.e, occurs only in one direction from products —–> reactants. and cannot be reversed under the similar set of conditions. answers of such questions could not answered by 1st law of thermodynamics.
The answers of these questions was given by 2nd law of thermodynamics . the law states that ‘all the natural and spontaneous phenomenons/processes are unidirectional and irreversible in nature .
For example , the processes like flow of water from up hill to down hill , flow of haet from hot end to cold end, and diffusion of gas from high pressure to a l ow pressure. etc….. all these processes are unidirectional and thermodynamiocally irreversible in nature.
Here , in the above statement of 2nd law of thermodynamics , a term ‘SPONTANEOUS PROCESS.’ is used.
The term spontaneuos means — themselves, and spontaneous process means that the processes may occur themselves , with out any help of external stimulai.
IMPORTANT POINTS ABOUT SPONTANEOUS ;
(a) The spontaneous processes do not proceed in the reverse direction themselves. For example, water will not go uphill by itself. however the spontaneous processes can be reversed with the help of any external factors
(b) The term ‘spontaneous’ doesn’t give any idea about the rate at which the process occurs.
SECOND LAW OF THERMODYNAMICS.
Now lets consider the conversion of heat into work . According to the 1st law of thermodymaics there is an eqivalence between heat absorbed and the work obtained but the heat absorbed can not be completely converted into work without leaving some changes in the system or surroundings. for example heat produced in a steam engine can not completely converted into mechanical work because a part of heat wasted in overcoming the friction. according to this second law of thermodynamics may be stated as :
it is impossible to convert heat into equivalent amount of work with out leaving some changes on the system or surroumdings .
The second law of thermodynamics can also be stated in a number of other forms which we will discuss at various stages in this article . but there are two more common and important forms of second law of thermodynamics are:
1. heat can not flow from a cold body to a hot body with out use of an extrernal agency and,
2. it is impossible to construct a machine , functioning in cycles, which can convert heat completely into an equuivalent amount of work without producing some changes elsewhere in the system.
thank you dear learners that’s all for today see you in next article
In the next article we will discuss about cyclic processes.
InoOP Chem 
