KEY2CHEM

Entropy

Entropy ($$S$$) is often defined as “chaos” or ”randomness” but is perhaps more helpfully thought of as “possible ways to distribute the energy of a substance”. Entropy increases with increased freedom of motion or ways to distribute a substance’s energy. For example, the particles in solid water (ice) are close together and not moving very much. Upon melting, the resulting liquid water particles have more freedom of motion. This process (melting of solid water to liquid water) results in an increase in entropy ($$\Delta S > 0$$).

Example 1.

For which process is $$\Delta S > 0$$?

A. $$\require{mhchem}\ce{H2O (g) -> H2O (l)}$$

B. $$\require{mhchem}\ce{CO2(g) -> CO2(aq)}$$

C. $$\require{mhchem}\ce{2 H2O(l) -> 2 H2(g) + O2(g)}$$

Solution

C. $$\require{mhchem}\ce{2 H2O(l) -> 2 H2(g) + O2(g)}$$

In the gas phase, particles have more freedom of motion compared to the liquid state, so the entropy increases. In dissolving a gas into water, there is less freedom of motion of the gas particles, since the water molecules are organized around them.

Example 2.

During which process does entropy decrease?

A.

B.

C.

Solution

A.

A decrease in entropy results in a decrease in the freedom of motion of the particles (decreasing the possible ways to distribute the energy of the system). An example of a process that decreases the entropy of the system is converting a gas to a solid.

Example 3.

Increasing temperature ________ the entropy of a system.

A. does not change

B. decreases

C. increases

Solution

C. increases

Increasing temperature increases average kinetic energy. This energy is spread more broadly over more states, which results in an increase in entropy.