Entropy
Entropy maybe explained as a measure of number of specific ways in which a thermodynamic system may be arranged. It is commonly understood as the measure of disorder of a system. And according to Second Law of Thermodynamics entropy of a system must increase.
Second law of thermodynamics:
In any cyclic process, the entropy will either increase or remain the same.
Note: Systems that are not isolated may decrease in entropy provided that they increase the entropy of their entropy by at least the same amount.
Entropy=klnW
where k=Boltzman Constant=1.38x10^-23
and W=number of ways a change can occur. It is governed by laws of chance and probability.
The total entropy can be given as the sum of entropy of the system and the entropy of the surroundings.
ΔS(surroundings)(total)=ΔS(system)+ΔS(surroundings)
Factors affecting Entropy:
1. Pressure:
The entropy of a system decreases with an increase in pressure. If we increase the pressure on the system, the volume decreases. The energies of the particles are in a smaller space, so they are less spread out. The entropy decreases. The effect is greatest for gases because a change in pressure causes a large change in volume. The volume change for a liquid is small and even smaller for a solid. So, pressure changes have lesser effects on liquids and solids.
2. Temperature:
As temperature increases, entropy increases. The warmer a body is, the more intensely and randomly the atoms oscillate, spin and swirl, the greater the agitation, the worse the atomic disorder. It should be noted here that randomness of type, orientation and motion cause the total disorder.
3. Energy:
Entropy increases with energy. The more energy or quanta there are to share, the greater the number of ways of sharing them. This can be also related to the temperature as increasing the temperature increases the number of energy quanta available and so increases the entropy.
4. Number of molecules:
The greater the number of molecules involved, the greater the ways of sharing them and this increases the W and so increases the entropy.
Some important points to remember:
1. Entropy increases in the order solid, liquid and gas.
2. Ions and molecules in solution generally have higher entropies than solids.
3. Substances with larger, complex molecules have higher entropies than those with smaller ones,
4. If a large molecule breaks down into smaller ones, entropy increases since there are more ways of
arranging several small molecules than few large molecules i,e W increases increasing the entropy.
5. For a reaction to occur the total entropy must be positive. If it is negative the reaction will not
occur. For example, conversion of CO2 to C and O2.
Feasibility of reactions:
A reaction will only occur if there is a total increase in the entropy. Reactions with total negative entropy do not occur. However, by changing some conditions the reactions can be made to happen but they do not happen spontaneously. But the total entropy should increase. There are reactions in which the entropy tends to decrease, but there is an increase in the entropy of the surroundings leading to total increase in entropy.
ΔS(total)=ΔS(system)+ΔS(surroundings)
So, positive ΔS(surroundings) outweighs the negative entropy of the system and thus, Second law of Thermodynamics still holds.ΔS(surroundings) can be given by: -ΔH/T
where ΔH is the enthalpy change of the reaction
and T is the temperature of the body.
But there are reactions in which kinetic stability plays its role and they need high activation energy to occur. These reactions once started can proceed spontaneously but need high activation energy in the first place.
The total entropy of a reaction may be found with the help of the formula given below because it is a state function.
ΔS=ΣS(products)-ΣS(reactions)
Effects of increasing Entropy:
Increase in entropy will cause:
1. bodies to become warmer
2. thermal expansion
3. substance will finally melt.
Entropy is involved in every reaction that occurs and plays a major part in each and there are many interesting phenomenons related to entropy. This was only a short review of the topic. Feel free to search more on it internet and in books. You might also be interested in the HEAT DEATH theory of the end of the universe.
-Psycho Killer
