It Is Reasonable to Assume That a Process That Will Bring About Greater Randomness Will

Entropy, S

It is reasonable to assume that a process that will bring about greater randomness will be more likely than a process that will bring about more order. Disorder is increasing in the universe because of this logic. The term that we have for randomness is called entropy.

We have learned that exothermic processes are spontaneous, however there are many everyday examples where endothermic reactions occur spontaneous. One such example is melting ice or evaporating water. Each of the processes are endothermic, however they occur readily without inputting energy. Another such example is dissolving NaI in water. The solution becomes cool as the process is endothermic, however the dissolving process occurs readily. The reason for this is that entropy is also a factor in spontaneity.

There are several analogies to help us understand this idea. Suppose a perfectly ordered deck of cards is sitting on a table, by knocking them of there are many opportunities for the deck to become disordered. The probability however that you could knock a randomly assorted deck of cards off a table and achieve a perfectly organized deck of cards is astronomical. Similarly, a solid has a sturdy and definite arrangement of molecules. The liquid state however is much more disordered and has a greater probability of occurring if we were to ‘drop’ molecules into a beaker.

Entropy describes the randomness of a system. The greater the entropy value, the greater the randomness of the system. We use the symbol S to indicate entropy and can look at the change in entropy as :

DS = Sfinal – Sinitial and DS = Sproducts – Sreactants

If S final is greater than S initial, then the value of DS will be positive. A positive DS will indicate that the reaction will be spontaneous.

Any event that is accompanied by an increase in entropy tends to be spontaneous.

Predicting the DS for physical and chemical changes.

1)  A change of state from solid to liquid or liquid to gas is an increase in disorder, thus the DS is postitive.

2)  An increase in the number of moles of a gas for a reaction creates a situation with more disorder, thus the DS is positive.

For the following, predict the sign of the DS value.

1)  condensation of steam in to water

2)  water freezing into ice

3)  sublimation of dry ice into CO2 gas

4)  2SO2 (g) + O2 (g) à 2SO3 (g)

5)  2NaHCO3 (g) à Na2CO3 (s) + CO2 (g) + H2O (g)

6)  Raindrops form in a cloud

7)  air is pumped into a tire

8)  sugar dissolves in coffee

9)  PCl3 (g) + Cl2 (g) à PCl5 (g)

10)  I2 (s) à I2 (g)