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Thermodynamics

Thermodynamics. Thermodynamics. We know from experience that some processes occur by themselves, without requiring us to do anything. Ice melts Air escapes from a balloon and the balloon deflates Wood burns These are all examples of spontaneous processes

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Thermodynamics

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  1. Thermodynamics

  2. Thermodynamics • We know from experience that some processes occur by themselves, without requiring us to do anything. • Ice melts • Air escapes from a balloon and the balloon deflates • Wood burns • These are all examples of spontaneous processes • A spontaneous process is one that can occur in a system left to itself • No action outside the system is necessary to bring it about

  3. Thermodynamics • We know from experience that some processes don’t occur without help. • Ice does not freeze at room temp • air does not rush into a balloon to inflate it • Carbon dioxide and oxygen don’t react and rain paper on us • These are all examples of nonspontaneous processes • A nonspontaneous process is one that does not occur in a system left to itself • No action outside the system is necessary to bring it about

  4. Thermodynamics • So reactions and processes tend to have a natural direction that works and a direction that doesn’t work. • Wouldn’t it be nice to be able to predict if a process will occur naturally? • There are actually only two variables that we need to interpret to decide if a reaction is spontaneous • Enthalpy (ΔH) so the whether energy is absorbed or given off • Entropy (ΔS) whether the disorder is increasing or decreasing

  5. What is Entropy? • The measure of the disorder inherent in a substance is its entropy (S) • For example gases tend to have higher entropies than do liquids due to have greater disorder inherent in the molecules • Entropy is a fundamental law of the universe that states that the universe constantly moves to increased chaos • So reactions that tend to increase that chaos tend to be spontaneous

  6. What is Entropy? • Entropy is statistics, which is statistically more likely to occur if you drop a pile of marbles? • Option 1: they fall into a nice neat pile of marbles • Option 2: they fall into a scattered mess • Since there is more disorder present in a scattered mess, that is the most likely to occur, because it is an increase in entropy.

  7. What is Entropy? • When it comes to a chemical rxn, we can calculate the change in entropy of the rxn if we know the absolute entropy of each component in the rxn • The change in the disorder of a system is known aschange in entropy (Ssys) • Ssys= ∑nSproducts- ∑nSreactants • It’s an energy measurement (not heat) with units of J/mol•K • Rxns that tend to increase in disorder or lead to the expansion of energy tend to be thermodynamically favored

  8. What is Entropy? • Let’s examine what the entropy equation creates in sign convention • If the products have more entropy than the reactants we get a +ΔS (entropy is increasing) • If the products have less entropy than the reactants we get a –ΔS (entropy is decreasing) • We can calculate the ΔS of a system if we have the data, but we can also predict if the entropy is increasing or decreasing with some common sense.

  9. Calculating Entropy Example : Calculate the change in entropy that occurs during the synthesis of water: 2H2+ O2 2H2O

  10. ClassWork 1: What is the change in entropy for the following reaction and is there an increase or decrease in entropy overall? 2Al + 6H+ +6Cl- 2AlCl3 + 3H2 The S of the following decom-position is 361.1 J/molK. The entropy of H2 and N2 is 130.7 J/mol•Kand 111.3 J/mol•Krespectively. What is the entropy of NH3? 2NH3 N2 + 3H2

  11. Entropy • We also need to be able to look at a rxn or process and tell qualitatively if the rxn is increasing or decreasing in entropy (no calculation) • We can look for an increased disorder to indicate a positive change in entropy • Changing from solid to liquid to gas (+ΔS) • rxns in which solid reactants form liquid or gaseous products and liquid reactants form gases (+ΔS)

  12. Entropy increases when a substance is particulated • Grinding, chipping, tearing, ripping, smashing, or dissolving, etc.  

  13. Entropy • Entropy increases in a chemical rxn in which the products are more numerous than the reactants • Decomposition rxns are spontaneous in part because of their movement toward increasing less concentrated or localized energy. 2H2O(g)  2H2(g) + O2(g)  2 particles  3 particles 2H2(g) + O2(g)  2H2O(g)  3 particles  2 particles

  14. Entropy • Entropy tends to increase as temperature increases • As the temp increases, the molecules move faster and faster, which increases the disorder of the system

  15. ClassWork 2: increasing or decreasing 2Al(s) + 3Cl2(g)  2AlCl3(s) Sanding wood 2CH4(g)  C2H6(g) + H2(g) Repairing a copy machine Mg(OH)2(s) + 2HCl(g) MgCl2(s)+ 2H2O(l) Steam condensing on the mirror 2CH3OH(g) + 3O2(g)  2CO2(g) + 4H2O(g) Ice melting

  16. Free Energy : G • Many chemical and physical processes release the kind of energy (free) that can be used to do work • Such as driving the pistons of an internal-combustion engine. • The energy our bodies receive from the conversion of glucose into ATP • Energy that is available to do work is called free energy, or Gibb’s free energy (G) • All spontaneous rxns generate free energy & can be symbolized with a -ΔG

  17. Calculations with DG and DS DG =DH - TDS • One way to calculate the change in free energy is the following equation. • ΔH is change in enthalpy • ΔS is change in entropy • T is temperature in Kelvin • To be spontaneous, ΔG must be negative • Some reactions spontaneous at 1 temp could be nonspontaneous at a different temp

  18. Heat, Entropy, & Free Energy • So that makes ±ΔH, ±ΔS, & ±ΔG

  19. Calculations with DG and DS • For example in this rxn at 298 K: C2H4(g) + H2(g) C2H6(g) • DS° = - 0.1207kJ/molK • DH° = - 136.9kJ/mol • What is DG°? And is the rxn spontaneous?

  20. For the rxn NH4Cl(s) NH3(g) + HCl(g), at 298.15K, DH°= 176 kJ/mol & DS°= .285kJ/molK, Calculate DG°, & tell whether this rxn is spontaneous at 298.15 K. Positive free energy means that at this temp this rxndoes not occur naturally.

  21. ClassWork 3: spontaneous or not Calculate ΔG°rxn at 25°C for this rxn: 2NaHCO3(s)Na2CO3(s)+CO2(g)+ H2O(g). Given: ΔH°rxn = 128.9 kJ/mol and ΔS°rxn= 321 J/molK A certain reaction has ΔH°= -35.4kJ and ΔS°= -85.5 J/K. Is the rxn exothermic or endothermic? Does the rxn lead to an increase or decrease in disorder? Calculate the ΔG° for the rxn at 298K. Is the rxn spontaneous at 298K?

  22. Calculations with DG • A theoretical value of ΔG can also be calculated using the following equation: • For example Hydrogen peroxide decomposes into water and oxygen gas. Calculate the ΔG°for this decomposition.

  23. ClassWork 3: ΔG calculations Calculate the ΔH°, ΔS°, and ΔG° for the following reactions: 2SO2(g) + O2(g)  2SO3(g) CaCO3(s)  CaO(s) + CO2(g)

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