Efficiency of Heat Engine
The efficiency may be defined as “The fraction of heat absorbed by an engine which it can convert into work”. Thus, Since is always less than 1, hence the efficiency of a heat engine is always less than one. In fact, No heat engine has yet been constructed which may have and efficiency equal to unity”. Example : Calculate the entropy change when 1 mole of an ideal gas expands reversibly from an initial volume of 2 litres to an final volume of 20 litres at . Solution: We know...
read moreGibb’s Free Energy [Gibb's Function]
In order to define this term, let us consider a process taking place isothermally and reversibly at constant pressure. There will be a volume change say . The maximum work obtained by it will not be amount of energy available for doing useful work. From the total amount of work, some part of the work is used to perform the mechanical work or pressure volume work of expansion or contraction against the atmospheric pressure. This work will be equal to . Hence the whole of work given by the process will be made up of two parts: (a) The...
read moreMeasurement of Entropy Change
For a reversible change at constant temperature the change in entropy is equal to heat absorbed or evolved divided by the constant temperature in Kelvin. Thus, The unit of entropy in . The value of AS is positive if heat is absorbed and negative if heat is evolved. The entropy change in melting a solid can be calculated if enthalpy of fusion is known. Similarly, the entropy change for vaporisation of a liquid into its vapour at its boiling point can be calculated if enthalpy of vapourisation is known. Similarly the entropy...
read morePhysical significance of Entropy
The entropy of a substance is real physical quantity and is a definite function of the state of the body like pressure, temperature, volume of internal energy. It is difficult to form a tangible conception of this quantity because it can not be felt like temperature or pressure. We can, however, readily infer it from the following aspects: 1. Entropy and unavailable energy The second law of thermodynamics tells us that whole amount of internal energy of any substance is not convertible into useful work. A portion of this energy which is...
read moreSecond Law of Thermodynamics
The second law of thermodynamics states that, “Whenever a spontaneous process takes place, it is accompanied by an increase in the total energy of the universe”. More specifically the term universe means the system and the surroundings, thus, The second law as stated above tells us that when an irreversible spontaneous process occurs, the entropy of the system and the surrounding increases. In other words, when reversible process occurs, the entropy of the system remains constant. . Since the entire universe is undergoing...
read moreSpontaneity, Randomness & Nature of driving force
Spontaneity and Randomness Careful examination shows that in each of the processes E. g. melting of ice and evaporation of rain water, there is an increase in randomness or disorder of the system. The water molecules in ice are arranged in a highly organised crystal pattern which permits little movement. As the ice melts, the water molecules become disorganised and can move more freely. The movement of molecules becomes free still when the water evaporates into space as now they can roam about throughout the entire atmosphere. In other words,...
read moreCriteria of Spontaneity
Some important criteria of spontaneous processes are as follows: (i) A spontaneous change is one way or unidirectional. For reverse change to occur, work has to be done. (ii) For a spontaneous change to occur, time is no factor. A spontaneous reaction may take place rapidly or very slowly. (iii) If the system is not in equilibrium state, a spontaneous change is inevitable. The change will continue till the system attains the state of equilibrium. (iv) Once a system is in equilibrium state, it does not undergo any...
read moreSpontaneous Process
A process which proceeds of its own accord without any outside assistance is termed as spontaneous or natural process. The reverse process which does not proceed on its own is referred to as a non-spontaneous or unnatural process. In general, the tendency of a process to occur naturally is called the spontaneity. Illustrations: (i) Rolling ball A ball rolls down hill spontaneously but it will not roll uphill unless work is done on it. (ii) Heat flow When two balls of metal, one hot and one cold are connected, heat flows...
read moreHeat Content or Enthalpy
When the change of state of a system is brought about at constant pressure, there will be a change in volume. The heat transferred such a process is known as heat content or enthalpy and is denoted by H. It may be defined as: H = E + PV Where E is an internal energy. P and V are the pressure and volume of the system respectively. Heat change at constant pressure may be expressed as: Where, = Heat of reaction at constant pressure = Change in internal energy = No. of moles of gaseous reactants R = Gas constant T = Absolute...
read moreApplications of Bond Energies
Applications of Bond Energies are listed below:- 1. Determination of enthalpy of reaction The bond energies can be used for determining enthalpies of reactions for example, we want to determine the enthalpy of the following reaction: In this reaction a triple bond breaks in acetylene and two H — H bonds break in 4H. In turn, one C C- bond and four C — H bonds are formed in. Therefore 2. Determination of enthalpies of formation of compounds The bond energies can be used for determining enthalpies of formation of...
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