Question
Define entropy and entropy change. What are the units of entropy?
Solution
The degree of randomness or disorderliness is expressed by a thermodynamic function called entropy. It may be defined as the property of a system which measures the degree of randomness or disorderliness in the system. It is denoted by S. It is a state function. It depends only on the initial and final state of the system and not on the path followed.
If SA and SB are the entropies at state A and B, then entropy change ∆S is given by![<pre>uncaught exception: <b>Http Error #404</b><br /><br />in file: /home/config_admin/public/felixventures.in/public/application/css/plugins/tiny_mce_wiris/integration/lib/com/wiris/plugin/impl/HttpImpl.class.php line 61<br />#0 [internal function]: com_wiris_plugin_impl_HttpImpl_0(Object(com_wiris_plugin_impl_HttpImpl), NULL, 'http://www.wiri...', 'Http Error #404')
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For a chemical reaction, 
For a reversible process at equilibrium, the change in entropy may be expressed as,
Where qrev represents the total heat absorbed reversibly and isothermally at temperature T.
A small change in entropy can be represented by dS since entropy is a state function. Thus, we may write
Units of entropy: The entropy change is expressed in terms of heat divided by absolute temperature. Thus entropy is expressed in terms of calories per degree i.e. cal deg–1 . This unit is called entropy unit i.e. e.u. In S.I. units, it is expressed in Joules per degree Kelvin i.e. JK–1 . ∆S is an extensive property and therefore its value depends on upon the amount of the substance involved. Therefore, units of entropy will be JK–1 mol–1.
Units of entropy: The entropy change is expressed in terms of heat divided by absolute temperature. Thus entropy is expressed in terms of calories per degree i.e. cal deg–1 . This unit is called entropy unit i.e. e.u. In S.I. units, it is expressed in Joules per degree Kelvin i.e. JK–1 . ∆S is an extensive property and therefore its value depends on upon the amount of the substance involved. Therefore, units of entropy will be JK–1 mol–1.
