Chapter 19 Chemical Thermodynamics

Spontaneous vs Nonspontaneous Change Spontaneous process – any process, once started, that proceeds without the external input of energy Nonspontaneous process – any process which requires the continual, external input of energy to keep the process going Any process that is spontaneous in one direction will be nonspontaneous in the reverse direction!

Nonspontaneous Change Occurs only with outside assistance Never occurs by itself: Room gets straightened up Pile of bricks turns into a brick wall Decomposition of H2O by electrolysis Continues only as long as outside assistance occurs: Person does work to clean up room Bricklayer layers mortar and bricks Electric current passed through H2O

Spontaneous Change What factors influence spontaneity? Occurs by itself Without outside assistance until finished e.g. Water flowing over waterfall Melting of ice cubes in glass on warm day

Spontaneous processes tend to favor: 1. Decrease in Energy 2. Increase in Disorder

Disorder is driven by statistical probability Statistical probability – refers to the number of possible arrangements of a system A disordered state is more probable because it can be achieved in more statistical ways The larger the number of different possible combinations, the greater the probability of getting a disordered state

Entropy (Symbol S ) Thermodynamic quantity Describes number of equivalent ways that energy can be distributed Can be thought of as a measure of the randomness or disorder of a system The greater the statistical probability of a particular state the greater the entropy! Larger S, means more possible ways to distribute energy and that it is a more probable result

There are 2,598,960 possible five-card poker hands

more disorder S = larger # Entropy, S less disorder S = smaller # Ludwig Boltzmann 1844 – 1906 more disorder S = larger #

Gases have greater entropy than liquids which, in turn, have greater entropy than solids.

Order Disorder ΔS = + # Disorder Order ΔS = - # Order Disorder is favored to occur spontaneously Order Disorder ΔS = + # Disorder Order ΔS = - # 2nd Law of Thermodynamics – the entropy of the universe is increasing over time

Effect of Volume on Entropy For gases, entropy increases as volume increases Gas separated from vacuum by partition Partition removed, more ways to distribute energy Gas expands to achieve more probable particle distribution More random, higher probability, more positive S

Effect of Temperature on Entropy 3rd Law of Thermodynamics – entropy decreases as temperature decreases (and vice-versa)

Effect of Physical State on Entropy Crystalline solid very low entropy Liquid higher entropy, molecules can move freely More ways to distribute KE among them Gas highest entropy, particles randomly distributed throughout container Many, many ways to distribute KE

Entropy Affected by Number of Particles Adding particles to system Increase number of ways energy can be distributed in system So all other things being equal Reaction that produces more particles will have positive S

= [(sum S products) – (sum S reactants)] Standard Entropy of Reaction – the ΔS (entropy, or disorder) that accompanies ANY reaction under standard conditions (units are J/K) = [(sum S products) – (sum S reactants)] 1. write and balance the reaction 2. use table of thermodynamic data of S to calculate Figure: 19-13 Standard Entropy, S – disorder of a substance at standard conditions (units are J/mole·K)

3 4 = – 93.0 J/K Determine the Standard Entropy of Reaction, 2 N2O (g) + O2 (g) NO2 (g) 3 4 = – 93.0 J/K = – #, suggests the reaction is nonspontaneous

Recap: 2 forces in nature which drive processes to occur spontaneously ΔHrxn only slightly varies with temperature ΔSrxn is highly dependent on temperature Recap: 2 forces in nature which drive processes to occur spontaneously 1. Decrease in energy ΔHrxn = – # (exo) 2. Increase in disorder ΔSrxn = + # (disorder)

ΔGrxn = ΔHrxn – T ΔSrxn Willard Gibbs 1839 – 1903 ΔHrxn is the heat of reaction T is the temperature in Kelvin Willard Gibbs 1839 – 1903 ΔSrxn is the entropy of reaction ΔGrxn is the Gibbs Free Energy

ΔGrxn or Gibbs Free Energy, is the ultimate, final deciding factor as to whether a reaction will occur spontaneously, anywhere in the universe Bottom Line: When ΔGrxn = – #, the reaction is spontaneous. When ΔGrxn = + #, the reaction is nonspontaneous. When ΔGrxn = 0, the reaction is at equilibrium (has no tendency to go one way or the other)

Is the following reaction spontaneous at 175 °C? 4 2 N2O (g) + O2 (g) NO2 (g) 3 = – 28.0 kJ favors spontaneous = – 93.0 J/K favors nonspontaneous Who wins ?? ΔGrxn = ΔHrxn – T ΔSrxn ΔGrxn = +13.7 kJ ΔGrxn = + #, the reaction is nonspontaneous

Figure: 19-T04