Chapter 17 Thermodynamics

The Second Law of Thermodynamics It is based on human experience. It doesn’t come from complicated theory and equations. So, think of these experiences that you have had: A rock will fall if you lift it up and then let go. Hot frying pans cool down when taken off the stove. Air in a high-pressure tire shoots out from even a small hole in its side to the lower pressure atmosphere. Ice cubes melt in a warm room. Iron exposed to oxygen and water will form rust.

What’s happening in each of those processes? Energy of some kind is changing from being localized and concentrated to becoming more spread out and transformed. Energy spontaneously disperses from being localized to becoming spread out if it is not hindered from doing so. Entropy measures the spontaneous dispersal of energy: how much energy is spread out in a process, or how widely spread out it becomes ,at a specific temperature.

Entropy (S) is a measure of the randomness or disorder of a system. If the change from initial to final results in an increase in randomness ΔS > 0 For any substance, the solid state is more ordered than the liquid state and the liquid state is more ordered than gas state Ssolid < Sliquid << Sgas H2O (s) H2O (l) ΔS > 0

Which substance has the greater entropy? Solid CO2 or Gaseous CO2 Predicting Entropy Changes: Which substance has the greater entropy? Solid CO2 or Gaseous CO2 H2 gas at 1 atm or H2 gas at 1.0 x 10 -2 atm What will happen to the sign of the entropy change ? Solid sugar is added to water to make a sugar solution. Water vapor condenses on a cold surface to form water droplets.

Entropy Changes in the System (ΔSsys) When gases are produced (or consumed) If a reaction produces more gas molecules than it consumes, ΔS0 > 0. If the total number of gas molecules diminishes, ΔS0 < 0. If there is no net change in the total number of gas molecules, then ΔS0 may be positive or negative BUT ΔS0 will be a small number. What is the sign of the entropy change for the following reaction? 2Zn (s) + O2 (g) 2ZnO (s) The total number of gas molecules goes down, ΔS is negative.

Spontaneous vs Nonspontaneous

Second Law of Thermodynamics To predict if a given process will be spontaneous, you must know the sign of ΔSuniv Spontaneous process: ΔSuniv = ΔSsys + ΔSsurr > 0 A Positive number means that the entropy of the universe increases. A negative number means that the process is spontaneous but in the opposite direction. Equilibrium process: ΔSuniv = ΔSsys + ΔSsurr = 0 The process has no tendency to occur.

For a constant-temperature process: Gibbs free energy (G) ΔG = ΔHsys -TΔSsys ΔG < 0 The reaction is spontaneous in the forward direction. ΔG > 0 The reaction is nonspontaneous as written. The reaction is spontaneous in the reverse direction. ΔG = 0 The reaction is at equilibrium.

ΔG = ΔH - TΔS

Recap: Signs of Thermodynamic Values Negative Positive Enthalpy (ΔH) Exothermic Endothermic Entropy (ΔS) Less disorder More disorder Gibbs Free Energy (ΔG) Spontaneous Not spontaneous