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Introduction |
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Physical and chemical changes are accompanied by energy changes. The branch of science, which deals with the energy changes associated with physical and chemical reactions is called Thermodynamics. |
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Some Basic Concepts |
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A system, which can exchange matter as well as energy with the surroundings is called an open system. For e.g., hot coffee in an open flask because it can gain or loose matter and energy with the surroundings. |
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State of the System and State Functions |
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State of the system implies the conditions of its existence. In order to understand it, let H2O be considered as a chemical system. H2O can exist in three physical states; ice, water and steam depending upon the conditions of temperature and pressure. |
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Modes of Transference of Energy |
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Chemical reactions are accompanied by the energy changes. The exchange of energy between the system and surroundings can occur in several ways. |
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Internal Energy and Change in Internal Energy |
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Since chemical changes are always accompanied by energy changes, it indicates that reactants and products must be having certain amounts of energy. A fixed quantity of any substance is associated with a definite amount of energy, which depends upon chemical nature of the substance and its state of existence. This energy is called internal energy (E) or intrinsic energy of the substance. Internal energy is a state function, which means its value depends upon state of the substance but does not depend upon how that state is achieved. For e.g., CO2 can be obtained by various methods such as by heating calcium carbonate or by burning coal. However, one mole of CO2 at N.T.P. is associated with a definite amount of internal energy which does not depend upon the source from which it is obtained. |
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Law of Conservation of Energy |
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It is a common experience that energy cannot be generated without consuming energy of some other kind. If a certain amount of one kind of energy is produced, an equal amount of some other kind of energy disappears so that the total energy of the Universe remains constant. This observation forms the basis of law of conservation of energy (also known as First law of thermodynamics), which states that. |
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Enthalpy and Enthalpy Change |
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It has been seen that energy change occurring during the reaction at constant temperature and constant volume is given by internal energy change. However, most of the reactions in the laboratory are carried out in open beakers or test tubes, etc. In such cases, the reacting system is open to atmosphere. Since atmospheric pressure is almost constant, therefore, such reactions may involve the changes in volume. The energy change occurring during such reactions may not be equal to the internal energy change. In order to understand this, let us assume a chemical reaction involving gaseous substances which proceeds with the evolution of heat. When the reaction is carried out at constant pressure, two possibilities arise. |
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Limitations of First Law of Thermodynamics: Need of Second Law |
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The first law of thermodynamics is concerned with the conversion of energy from one form to another. It helps us to understand energy transformation in different chemical reactions. The basic point of the first law is that all physical and chemical processes take place in such a manner that the total energy of the universe (for example the energy of the system and the energy of the surroundings) is constant. However, it is a common observation that all processes have a natural direction - a direction in which they take place on their own. Naturally, a curious question that comes up is "Why do changes take place in a particular-direction?" The first law of thermodynamics does not determine the feasibility or spontaneity of a process. |
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Randomness and Spontaneity |
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In order to find out some other factor that may be responsible for the feasibility of a process, let us examine endothermic reactions. The simplest process occurring without any energy change (where DH is almost zero) may be illustrated with the help of following experiment. |
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Entropy |
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The extent of disorder or randomness in a system may be expressed by a property known as entropy. Entropy may be termed as the property of a system which measures the degree of disorder or randomness in the system. It is generally expressed by the symbol, S. Entropy like internal energy and enthalpy, is a state function and change in entropy therefore, depends only on the initial and final states of the system. The change in entropy during the process when a system undergoes a change from one state to another is represented by DS. Thus, DS = S(final state) - S(initial state) and for chemical reactions, DS = S(products) - S(reactants). |
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Second Law of Thermodynamics |
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The second law of thermodynamics introduces the concept of entropy and its relation with spontaneous processes. In an isolated system such as mixing of gases, there is no exchange of energy or matter between the system and the surroundings. But due to increase in randomness, there is increase in entropy. Thus, it can be stated that for a spontaneous process in an isolated system, the change in entropy is positive or DS > 0. However, if the system is not isolated, the entropy changes of the system and the surroundings are also to be taken into account. Then, the total entropy change (DStotal) will be equal to the sum of the change in entropy of the system (DSsystem) and the change in entropy of the surroundings (DSsurroundings). |
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Free Energy and Free Energy Change |
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Free energy of a system is defined as the maximum amount of energy available to a system during a process that can be converted into useful work. In other words it is a measure of capacity of a system to do useful work. |
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Standard Free Energy Change |
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The standard free energy change is defined as the free energy change for a process at 298 K and 1 atmospheric pressure in which the reactants in their standard states are converted to the products in their standard states. Like the standard enthalpy of formation of an element, the standard free energy of formation of an element in its standard state is assumed to be zero. |
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Gibbs Free Energy Change and Work |
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We have learnt that of DG is a measure of the spontaneity of a chemical reaction. It can be shown that the free energy of a process is equal to maximum possible work that can be derived from the process. |
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Absolute Entropies and Third Law of Thermodynamics |
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The third law of thermodynamics deals with the entropies of the perfect crystalline substances at absolute zero of temperature. |
