Chemical energy stored by molecules can be released as heat
during chemical reactions when a fuel like methane, cooking gas or coal burns in air. The chemical energy may also be used to do mechanical work when a fuel burns in an engine or to provide electrical energy through a galvanic cell like dry cell. Thus, various forms of energy are interrelated and under certain conditions, these may be transformed from one form into another. The study of these energy transformations forms the subject matter of thermodynamics. The laws of thermodynamics deal with energy changes of macroscopic systems involving a large number of molecules rather than microscopic systems containing a few molecules. Thermodynamics is not concerned about how and at what rate these energy transformations are carried out, but is based on initial and final states of a system undergoing the change. Laws of thermodynamics apply only when a system is in equilibrium or moves from one equilibrium state to another equilibrium state. Macroscopic properties like pressure and temperature do not change with time for a system in equilibrium state.

in this unit.we would like to answer some of the important questions through thermodynamics, like:

how do we determine the energy changes involved in a chemical reaction / process?

to what extent do the chemical reactions proceed?


We are interested in chemical reactions and the energy changes accompanying them for this we need to know certain thermodynamic terms. these are discussed below.


SYSTEM in thermodynamics refers to that part of universe in which observation are made and remaining universe constitutes the surroundings. the surroundings include everything other than the system. system and the surroundings constitute the universe.

the universe=the system+the surroundings however,the entire universe other than the system is not affected by the changes taking place in the system.therefore, for all practical purposes.the surroundings are the portion of the remaining universe which can interact with the system. usually, the region of space in the neighborhood of the system constitutes its surroundings.

for example,if we are studying the reaction between two substances A and B kept in a beaker, the beaker containing the reaction mixture is the system and the room where the beaker is kept is the surroundings.

Note that the system may be defined by physical boundaries, like be beaker or test tube, or the system may simply be defined by a set of cartesian coordinates specifying a particular volume in space. it is necessary to think of the system as separated from the surroundings by some sort of wall which may be real or imaginary. the wall that separates the system from the surroundings is called boundary. this is designed to allow us to control and keep track of all movements of matter and energy in or out of the system.


We further, classify the systems according to the movements of matter and energy in or out of the system.


In an Open system. there is exchange of energy and matter between system and surroundings. the presence of reactants is an open beaker is an example of an open system. here are the boundary is an imaginary surface enclosing the beaker and reactants.


In an closed system, thee is no exchange of matter,but exchange of energy is possible between system and the surroundings the presence of reactants in a closed vessel made of conducting material e.g, copper or steel is an example of a closed system.


In an isolated system, there is no exchange of energy or matter between the system and the surroundings. the presence of reactants in a thermos flask or any other closed insulated vessel is an example of an isolated system.

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