ENTROPY SPONTANEITY OF REACTIONS

Reaction Spontaneity Entropy (S) is a measure of the disorder or randomness of the particles that make up a system. Spontaneous processes always result in an increase in the entropy of the universe. The change in the entropy of a system is given by the following equation.The change in the entropy of a system is given by the following equation.

Whether ∆S system is positive or negative can be predicted in some cases by examining the reaction or process. Several factors affect the change in entropy of a system. Changes of stateChanges of state Entropy increases when a solid changes to a liquid and when a liquid changes to a gas because these changes of state result in freer movement of the particles.

Dissolving of a gas in a solvent When a gas is dissolved in a liquid or solid solvent, the motion and randomness of the particles are limited and the entropy of the gas decreases. Change in the number of gaseous particles When the number of gaseous particles increases, the entropy of the system usually increases because more random arrangements are possible.

Dissolving of a solid or liquid to form a solutionDissolving of a solid or liquid to form a solution When solute particles become dispersed in a solvent, the disorder of the particles and the entropy of the system usually increase. Change in temperatureChange in temperature A temperature increase results in increased disorder of the particles and an increase in entropy. A temperature increase results in increased disorder of the particles and an increase in entropy.

Free Energy For a reaction or process occurring at constant temperature and pressure, the energy that is available to do work is the free energy (G).For a reaction or process occurring at constant temperature and pressure, the energy that is available to do work is the free energy (G). The change in free energy is related to the change in enthalpy and the change in entropy by the following equation.The change in free energy is related to the change in enthalpy and the change in entropy by the following equation.

In this equation, T is the Kelvin temperature.In this equation, T is the Kelvin temperature. If ∆G system is negative, the reaction or process is spontaneous; if ∆G system is positive, the reaction or process is nonspontaneous.If ∆G system is negative, the reaction or process is spontaneous; if ∆G system is positive, the reaction or process is nonspontaneous.

Determining Reaction Spontaneity For a chemical reaction, ∆H system = – 81 kJ and ∆S system = –215 J/K.For a chemical reaction, ∆H system = – 81 kJ and ∆S system = –215 J/K. Is the reaction spontaneous at 50°C?Is the reaction spontaneous at 50°C? Convert the temperature to kelvins.Convert the temperature to kelvins.

Find ∆G systemFind ∆G system. Because ∆G system is negative, Because ∆G system is negative, the reaction is spontaneous. the reaction is spontaneous.

Question 1 Predict the sign of ∆S system for

Negative Negative Answer

Predict the sign of ∆S system for Question 2

positive Answer

Predict the sign of ∆S system for Question 3

positive Answer