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Thermodynamics

Thermodynamics explores the nature of processes that occur naturally without external assistance, known as spontaneous processes, such as melting ice and burning wood. Conversely, non-spontaneous processes require external action to occur, like ice freezing at room temperature. By examining two key variables—enthalpy (ΔH) and entropy (ΔS)—we can predict the spontaneity of reactions. Entropy measures disorder, with systems favoring increased chaos being more likely to occur. This overview highlights the principles of spontaneity, entropy changes, and the implications for chemical reactions.

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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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