Lecture 3. Entropy. Professor Hicks General Chemistry (CHE132) Entropy (S) - imaginary substance - associated with disorder

Transcription

1 Lecture 3 Professor Hicks General Chemistry (CHE132) Entropy Entropy (S) - imaginary substance - associated with disorder matter or energy spreads out entropy increases Standard Entropy Change ( S o ) Ludwig Boltzmann Second law (for a scientist) In any spontaneous process the entropy of the universe increases explains why processes tend to not reverse themselves (even if heat is not released) q (heat) HOT COLD BOTH SAME TEMPERATURE heat will not flow between objects at same temperature 1

2 entropy (disorder) In a chemical reaction the side that has more moles of gas has more entropy A (solid) B(gas) Entropy increases S positive The side that has a material dissolved in a solvent has more entropy A (solid) A (aqueous) entropy increases so S positive Is S positive or negative? 2C 8 H 18 (g) +25O 2 (g) 16CO 2 (g) + 18H 2 O (g) 27 moles gas 34 moles gas (less moles gas) (more moles gas) (less entropy) (more entropy) Under constant pressure and temperature more moles gas will have to occupy larger volume And is therefore more spread out V = nrt V P n Change in entropy S = S final S initial more less + positive + Entropy is created when heat flows from hotter to cooler S = q/t T must be in Kelvins Estimate the entropy created per second by a 60 kg man sitting in a room at 22 C 2

3 Gibbs Free Energy two reasons a process can be spontaneous: 1) heat released ( H negative) 2) matter spreads out ( S positive) -q system + S system Josiah Willard Gibbs Both should be visualized as products forming that escape preventing reaction from reversing G H S Gibbs Free Energy ( G) Gibbs free energy expresses both factors that affect if a reaction will happen H and S G = H-T S G is negative = thermodynamically spontaneous accounts for heat and entropy G is the maximum amount of work that could be obtained 3

4 G Why is a negative G spontaneous? negative sign = spontaneous picked to be like gravity G like height so G negative like falling to lower height graph of G is always shaped like a bowl if G= negative the system sits on the walls of a bowl on reactant side if G = positive the system sits on the walls of a bowl on product side reaction quotient (Q) thermodynamically spontaneous does NOT mean happens fast or slow if it is thermodynamically spontaneous then it CAN HAPPEN and it will tend to not reverse itself if it can happen, kinetics tells how FAST/SLOW Standard Gibbs Free Energy ( G o ) If measured under standard conditions G o = H o -T S o H o and S o can be calculated from standard formation values G o calculated at any temperature H o and S o do not change much with temperature 4

5 Why is entropy multiplied by temperature? G o = H o -T S o spreading out happens by random motion - T = average kinetic energy lower T matter spreads out less extreme case 0 K matter is not moving! -cannot spread out at all so entropy does not matter! Example: If a reaction has H o = kj/mol and S o = 4.0 kj/mol*k at 300 K. a) Calculate G o at 1.0 K. b) Calculate G o at 300 K. c) How high must the temperature be raised in order for the reaction to become thermodynamically spontaneous by 1 kj/mol? a) G o = H o -T S o G o = * 4.0 (kj/mol) K * (kj/mol*k) = (1255 4) kj/mol = kj/mol positive = not spontaneous c) G o = H o -T S o -1 = T * 4.0 (kj/mol) K * (kj/mol*k) T*4.0 = T = 1256/4 = 314 K b) G o = H o -T S o G o = * 4.0 (kj/mol) K * (kj/mol*k) = ( ) kj/mol = +55 kj/mol positive = not spontaneous G o Temp Dependent (an example) metal oxidized 3Fe (s) + 2 O 2 (g) Fe 3 O 4 (s) + heat a) Is S o positive or negative? negative b) Is H o positive or negative? negative c) At high temperature is G o positive positive or negative? high temp reverse reaction becomes spontaneous as the forward reaction become non-spontaneous Fe 2 O 3 (rust) O 2 O 2 O 2 O 2 Fe (metal) heat 5

6 energy use and thermodynamics fuels products + heat (if we do it sloppily) if more work is done then less heat is released all heat The theoretical maximum amount of work we can get out is not the heat but the G for the reaction That is the reason it is called Free Energy it means the energy that is available or free that can be obtained for work Hess law tells us that to reverse the reaction to reform the fuels we must add at least as much energy as was released less heat some work G=maximum work S Example: Estimate the entropy created when 1000 Joules of heat flows from a Bunsen burner flame at 600 C into a boiling water bath at 100 C. 6

7 Example: Estimate the entropy created every second that a rabbit sits in a 20 C room? This rabbit consumes about 200 dietary calories (~6 carrots) a day. Example: Can more work be obtained from burning a gallon of octane if it is burned at constant pressure or constant volume? Example: If the H for the reaction A + B C is +500 kj/mol and S is +300 J/mol*K calculate G at 25 C Does C form spontaneously from A+B? Is there a temperature G where =0? 7

8 8