7.7: Standard Molar Entropies Standard Molar Entropy = Entropy of 1 mole of material at K and 1 bar When we consider the standard molar entropies of molecules, we apply the following rule: Standard Molar Entropies Increase as the Complexity of a Substance Increases and The Standard Molar Entropies of gases are higher than those of liquids or solids at the same temperature

What Does this Mean? 

7.8: Standard Reaction Entropies Entropy and the tendency for spontaneous processes to increase the entropy of a system can be used to predict if/how reactions will occur We know from S º m that one set of molecules will have lower entropy than another –Taking the same logic a step further will allow us to predict the change in entropy of a reaction

Standard Reaction Entropies Remember: 1.Molar entropy of a gas is much higher than that of a liquid or solid Increase in the number of gas molecules means (usually) a higher entropy after a reaction Vice versa for a decrease in the number of gas molecules 2.Breaking larger molecules down into smaller molecules usually results in an increase in entropy More molecules means more disorder 3.Dissolving a solid into a liquid usually increases the entropy 

Calculating the Entropy of a Reaction Sometimes we can’t always use our judgment and we need to calculate the entropy In order to do this, we need the standard molar entropies of the products and the reactants as well as the number of moles of each 

Global Changes in Entropy We know now about the role of entropy in chemical reactions and spontaneous processes How can we use entropy to explain processes that spontaneously happen, but appear to go against the 2nd Law? –Water freezing to ice –Cold packs becoming cold in the summer –Cells forming from the primordial seas The contradiction is just a matter of scale…

Isolated Systems The system may be doing something that seems to be contradictory to the 2nd Law, but when the surroundings are included, we form an Isolated System Reactions will occur spontaneously if the TOTAL entropy change is positive

The Surroundings If  S Tot is positive, the reaction is spontaneous If the  S system is negative, the reaction will still be spontaneous if  S Surr is that much more positive  S Tot =  S System +  S Surr

 S and Enthalpy We can better understand the role of the surroundings by looking at the boiling of water… 

Summary of System and Surroundings Exothermic vs Endothermic Reactions 