Thermodynamics vs. Kinetics Domain of Kinetics Rate of a reaction depends on the pathway from reactants to products. Thermodynamics tells us whether a reaction is spontaneous based only on the properties of reactants and products.

Entropy (S) A measure of the randomness or disorder The driving force for a spontaneous process is an increase in the entropy of the universe Entropy is a thermodynamic function describing the number of arrangements that are available to a system Nature proceeds toward the states that have the highest probabilities of existing

Positional Entropy The probability of occurrence of a particular state depends on the number of ways (microstates) in which that arrangement can be achieved Ssolid < Sliquid << Sgas

The Expansion of An Ideal Gas Into an Evacuated Bulb

The Microstates That Give a Particular Arrangement (State)

The Microstates That Give a Particular Arrangement (State)

Entropy is a Measure of Disorder Generally, the more complex the molecule, the higher the standard entropy value. Entropies of ionic solids become larger as the attractions among ions become weaker. Entropy usually increases when a pure solid or pure liquid dissolves in a solvent.

Concept Check Predict the sign of S for each of the following, and explain: The evaporation of alcohol The freezing of water Compressing an ideal gas at constant temperature Heating an ideal gas at constant pressure Dissolving NaCl in water + – a) + (a liquid is turning into a gas) b) - (more order in a solid than a liquid) c) - (the volume of the container is decreasing) d) + (the volume of the container is increasing) e) + (there is less order as the salt dissociates and spreads throughout the water)

Second Law of Thermodynamics "In any spontaneous process there is always an increase in the entropy of the universe" "The entropy of the universe is increasing" For a given change to be spontaneous, Suniverse must be positive Suniv = Ssys + Ssurr

Calculating Entropy Change in a Reaction (System) Entropy is an extensive property (a function of the number of moles)

Calculating Entropy Change for Surroundings ∆Ssurr. = qsurr. - ∆Hsystem T

ΔSsurr Copyright © Cengage Learning. All rights reserved

ΔSsurr Copyright © Cengage Learning. All rights reserved

Interplay of Ssys and Ssurr in Determining the Sign of Suniv