Standard Entropy of Reaction

Where n and m are the stoichiometric coefficients of each product or reactant.

If a reaction is carried out in a series of steps, ∆S for the overall reaction will be equal to the sum of the ∆S’s for the individual steps.

For example, we want to determine the change in entropy for the following reaction.

• A + 2B → C + 2D

The steps needed to achieve this reaction are as follows:

• 2E + A → C
  • ∆S1
• B → D + E
  • ∆S2

In order to get the overall reaction, we would need to do step 2 twice. This will lead to an overall change in entropy of ∆S = ∆S1 + 2∆S2.

Calculating Entropy

The ∆S_sys, alone, is not an indicator of spontaneity. However, the ∆S of the universe (system + surroundings) is enough to determine spontaneity.

The 2nd Law of Thermodynamics states that the entropy of the universe is increasing during a spontaneous process. In other words:

• ∆S of universe > 0 means spontaneous.
• ∆S of universe < 0 means non-spontaneous.

Exothermic processes increase the entropy of the surroundings (∆S_surr > 0).

Endothermic processes decrease the entropy of the surroundings (∆S_surr < 0).

Calculating ∆S_surr (entropy of surroundings)

The heat that flows into or out of the system changes the entropy of the surroundings. For an isothermal process (const temperature):

• ∆S_surr = - ( q_sys ) / T

At constant pressure, q_sys is simply ∆H° for the system:

• ∆S_surr = - ( H_sys ) / T