10.4 Second Law of Thermodynamics
A chemical reaction or a physical change, is said to be spontaneous if it has the potential to proceed of its own accord under the conditions specified. The first law of thermodynamics provides no basis for determining whether or not a proposed change will occur spontaneously.
The second law of thermodynamics established criteria for making such predictions.
The Second law of Thermodynamics states
that every spontaneous change is accompanied by an increase in entropy
which is a measure of the randomness or disorder of a system.
In a solid system molecules or ions are quite closely held. When the system is heated by applying external energy, molecules or ions will start moving from each other subject to electrostatic repulsion or attraction. As a result, in the liquid state molecules or ions have looser arrangements or display less order than in the solid state. Hence entropy of a substance in the liquid state is always higher than that of one in the solid state.
A gas has an even higher disorder than a liquid as its molecules or ions have
free movements and are not constrained to be adjacent to each other.
Hence the gaseous state has higher disorder and entropy. The second
law of thermodynamics states that changes in an isolated system
are toward higher entropy.
When entropies are measured at standard conditions (250C and 1 atm ) they are called as standard entropies and symbolized by S0.
The symbol for entropy is S, hence the difference change or difference in entropy can be represented as DS.
DS = S products
- S reactions
Example : If standard entropies for S(s) , O2
(g) & SO2 (g) is 7.6, 49.0 and 59.4 Cal deg
-1 mol -1 respectively, the entropy of reaction
S (s) + O2 (g) ® SO2(g)
DS = 59.4 - [ 49.0 + 7.6 ]
= 2.8 Cal deg -1mol -1
The positive entropy change means that there is greater disorder in the products than in the reactants.