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1 CHEM112 LRSVDS Thermodynamics part 1 ! reactants 2 1 products CHEM112 LRSVDS Thermodynamics part 1 ! !E 2 THERMOCHEMISTRY LAWS OF THERMODYNAMICS 1st Law: The total energy in the universe is constant. !Euniverse = 0 !Euniverse = ! Esystem + ! Esurroundings !Esystem = - ! Esurroundings Heat and work: !Esystem = q + w and for PV work at const. pressure, !Esystem = q – P!V CHEM112 LRSVDS Thermodynamics part 1 ! 3 CHEM112 LRSVDS Thermodynamics part 1 ! 4 Example Problem: What is the Heat of Formation of gaseous mercury? CHEM112 LRSVDS Thermodynamics part 1 ! 5 What are the Standard States of the Elements? (at 298K and 1 atm) 1. Metals: all are ___________ (except ____) 2. Metalloids: all are _________ 3. Nonmetals A: Noble gases (Group 8): _________! B: Diatomics: (there are 7)_____________ which are gases? _____________ which is a liquid? ____ which is a solid? ____ C: Other non-metals are __________ C (graphite), S, P, Se CHEM112 LRSVDS Thermodynamics part 1 ! 6 SPONTANEOUS REACTIONS Define spontaneous reaction: Note: spontaneity has nothing to do with the rate at which a reaction will occur. (A spontaneous reaction may be fast or slow.) Bunsen Burner: Do I have to keep lighting it? CH4(g) + 2 O2(g) " CO2(g) + 2 H2O(g) Spontaneous or Non-Spontaneous? Reverse of Rusting? 2 Fe2O3(s) " 4 Fe(s) + 3 O2(g) Spontaneous or Non-Spontaneous? NaCl(s) " Na(s) + " Cl2(g) Spontaneous or Non-Spontaneous? CHEM112 LRSVDS Thermodynamics part 1 ! 7 SPONTANEITY AND WORK Is this process spontaneous? Can this process do work? What is the reverse of this process? Is the reverse process spontaneous? Which process can do work? 2 H2O " 2 H2 + O2 2 H2 + O2 " 2 H2O CHEM112 LRSVDS Thermodynamics part 1 ! 8 Spontaneity and Equilibrium At -10°C At + 10°C H2O(l) " H2O(s) Is it Spontaneous? H2O(l) " H2O(s) At 0°C? Is it Spontaneous? H2O(s) H2O(l) When a chemical system is at equilibrium, reactants and products can interconvert reversibly. In a spontaneous process, the path between reactants and products is irreversible. 9 CHEM112 LRSVDS Thermodynamics part 1 ! EXAMPLES H2O(l) " H2O(s) At -10°C !H = ? 2NaCl(s) " 2Na(s) + Cl2(g) !H=? 4Fe(s) + 3O2(g) " 2 Fe2O3(s) !H=? N2(g) " 2N(g) !H=? 2H2(g) + O2(g) " 2H2O(g) !H=? CHEM112 LRSVDS Thermodynamics part 1 ! 10 If !H is negative, does that mean the reaction is spontaneous? Examples: H2O(s) " H2O(l) At +10°C !H = ? Ba(OH)2•8H2O(s) + 2NH4SCN(s) "Ba(SCN)2(aq) + 2NH3(aq) + 10H2O(l) !!H = ? NH4Cl(s) + H2O(l) " NH4Cl(aq) !H=? CHEM112 LRSVDS Thermodynamics part 1 ! 11 SPONTANEITY Nature tends to move spontaneously from a state of lower probability to one of higher probability »!G.N. Lewis (Nobel Laureate) !H alone doesn’t determine spontaneity; what also affects spontaneity? CHEM112 LRSVDS Thermodynamics part 1 ! 12 ENTROPY! A thermodynamic parameter (S) that is a measure of the disorder or randomness in a system. The more disordered a system, the greater its entropy. Entropy is a state function -- its value depends only upon the state of the system (not how it got there). We are usually concerned with the change in entropy (!S ) during a process such as a chemical reaction. !S = S final - S initial The Second Law of Thermodynamics For spontaneous processes !S >0 !Suniv = (!Ssystem + !Ssurroundings) > 0 CHEM112 LRSVDS Thermodynamics part 1 ! 13 ENTROPY! Which processes have !S > 0? S "S Unopened deck of cards " Cards spread out on table Unassembled car parts " Assembled car Seed + CO2 + H2O + Minerals " Tree 1 mole gas in 1 liter flask " 1 mole gas in 2 liter flask NH4Cl(s) " NH4+(aq) + Cl-(aq) CHEM112 LRSVDS Thermodynamics part 1 ! 14 Probability and Locations of Gas Molecules Before Stopcock is opened Possibilities After Stopcock is opened Increase in number of microstates # CHEM112 LRSVDS Thermodynamics part 1 ! 15 Molecular Basis of Entropy !!As # of atoms in a molecule increases, S increases. !S > 0 !SAr < !SHCl < !SCH4 CHEM112 LRSVDS Thermodynamics part 1 ! 16 Molecular Basis of Entropy !! It is an extensive property (more moles, more entropy) As # of molecules increases !S > 0 Al2O3(s) + 3H2(g) " 2 Al(s) + 3 H2O(g) !S = Example: Is !S for this reaction positive or negative? 2NO(g) + O2(g) 2NO2(g) CHEM112 LRSVDS Thermodynamics part 1 ! 17 Molecular Basis of Entropy Dissolution of KCl(s): is !S for this process positive or negative? CHEM112 LRSVDS Thermodynamics part 1 ! 18 Molecular Basis of Entropy !! Which has more Entropy: Gas, Liquid or Solid State? !S for gases liquids solids Trends: sol " gas; sol " liq; liq " gas !S = ? gas " liq; gas " sol; liq " sol !S = ? Structure of ice What happens when T increases within the SAME phase? CHEM112 LRSVDS Thermodynamics part 1 ! 19 Molecular Basis of Entropy !! Adding heat increases entropy CHEM112 LRSVDS Thermodynamics part 1 ! 20 ENTROPY Examples: Which has the greater entropy? S(1 mol HCl(g)) S(1 mole NaCl(s)) S(2 mol HCl(g)) S(1 mole HCl(g)) S(1 mol HCl(g)) S(1 mole Ar(g)) S(1 mol N2(g)) @ 300K S(1 mole N2(g)) @ 200K More Examples !S (+ or - ?) Effect H2O (l, 25oC) " H2O(g) CaCO3 (s) " CaO(s) + CO2(g) N2(g) + 3 H2(g) " 2 NH3(g) Ag+(aq) + Cl-(aq) " AgCl(s) Au(s) at 298K " Au(s) at 1000K CHEM112 LRSVDS Thermodynamics part 1 ! 21 Phase Changes !! During phase changes what happens to temperature? !! What happens to the amount of disorder during phase changes? !!!S is always _________ for melting or boiling. Entropy and heat at a fixed temperature: !Stransition = qrev = !Htransition T T For melting ice, !Sfus = Example: Calculate entropy change when 1 mole of liquid water evaporates at a constant temperature of 100oC. !Hvap = 40.6 kJ/mol CHEM112 LRSVDS Thermodynamics part 1 ! 22 LAWS OF THERMODYNAMICS 1st Law: The total energy in the universe is constant. !Euniverse = 0 !Euniverse = !Esystem + !Esurroundings !Esystem = - !Esurroundings 2nd Law: The total entropy in the universe is increasing. ! Suniverse > 0 ! Suniverse = !Ssystem + !Ssurroundings > 0 3rd Law: The entropy of every pure substance at 0K (absolute zero temperature) is zero. S = 0 at 0K. CHEM112 LRSVDS Thermodynamics part 1 ! 23 3rd Law of Thermodynamics! S = 0 at T = 0 K This means we can measure absolute entropy S (not just !S) Define Absolute Entropy: S° (Standard Molar Entropy) S° is always positive (>0) for pure substances CHEM112 LRSVDS Thermodynamics part 1 ! 24 Entropy Changes for REACTIONS !S°(rxn) = $nS°(products) - $ mS° (reactants) What is !S° for the following reaction? N2(g) + 3 H2(g) " 2 NH3(g) !S > 0 when: "!Gases form from either liquids or solids "!Liquids or solutions form from solids "!The number of molecules of gas increase during a chemical reaction. CHEM112 LRSVDS Thermodynamics part 1 ! 25