Entropy and Spontaneity

Entropy The Second Law of Thermodynamics states that in spontaneous changes, the universe tends toward a state of greater disorder Spontaneous changes occur without the need to do work. A spontaneous reaction occurs without adding energy. For example, if a bottle of a carbonated drink is left open, you would expect to find it “flat” after a couple of days. The carbon dioxide escapes from solution and diffuses or spreads out in the surroundings. You would not expect all the carbon dioxide to return to the bottle. In a similar way, a hot cup of coffee will cool down and lose some heat to the surroundings. The heat will not return. These are both examples of spontaneous change; they occur naturally without the need to do work. Both of these examples illustrate a general principle: energy and matter tend to disperse and the universe becomes more disordered. Entropy (S) refers to the distribution of available energy among the particles. The more ways the energy can be distributed, the higher the entropy. So, entropy is a measure of disorder. The greater the state of disorder, the higher the entropy. In a spontaneous process, the change in entropy of the universe is greater than zero.

Predicting Entropy Changes

Predicting Entropy Changes

Predicting Entropy Changes

Predicting Entropy Changes

Entropy values are positive Entropy values are positive. A perfectly ordered solid at absolute zero has an entropy of zero. All other states, which are more disordered, have positive entropy values.

Spontaneity So far, we have focused on the entropy of the substances present in the system. However, to consider the total entropy change of a reaction, we must also consider the accompanying entropy change in the surroundings. For example: How does this reaction increase the total entropy of the universe? The heat produced by the reaction is dispersed into the surroundings and as a result, the total entropy is increased.