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Spontaneous Rxns

Spontaneous Rxns. Thermodynamics. We’ve already learned that energy is conserved Energy can neither be created nor destroyed In other words the amount of energy lost by the system cannot be more than the energy gained by the surroundings

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Spontaneous Rxns

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  1. Spontaneous Rxns

  2. Thermodynamics • We’ve already learned that energy is conserved • Energy can neither be created nor destroyed • In other words the amount of energy lost by the system cannot be more than the energy gained by the surroundings • Another fundamental truth about energy is that energy of all types changes from being localized (ordered) to being dispersed (disordered)

  3. Thermodynamics • The measure of the dispersal of energy from localized to dispersed is called entropy (S) • Entropy describes the tendency for the system to move toward chaos • Enthalpy and entropy are integral to determining whether or not a reaction or process will occur spontaneously • Spontaneous in chemistry means naturally occurring • Only processes that produce free energy are spontaneous

  4. Spontaneous Rxns • 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 • If a surplus of energy is produced in a chemrxn it is called free energy, or Gibb’s free energy (G) • Free energy is usable energy and is available to do work

  5. Spontaneous Rxns • It’s easy to see how exothermic reactions produce this free energy. • Free energy is the fire and light and heat we observe during an exothermic reaction • The energy used by burning charcoal to cook food on the grill is free energy • Electrical energy that is converted to heat energy in a hotplate is free energy • The temp of the body is maintained at 37C by free energy

  6. Spontaneous Rxns • Only spontaneous processes give off free energy • Most rxns or processes have a direction to them that is considered spontaneous • Consider CO2 • CO2 can decompose into C and O2 • CO2 can be produced from C and O2 • Only one of the two reactions is spontaneous. • Only one of the two reactions is the natural rxn

  7. Spontaneous Rxns • So the world of rxneqns can be divided into 2 groups • Those rxns that actually occur naturally or are spontaneous • Those rxns that donot tend to occur naturally, or at least not efficiently • Every chemical rxn fights to establish chemical equilibrium • the state at which the forward and reverse rxns take place at the same rate • When equilibrium is established the rxn is considered complete

  8. Spontaneous Rxns • In principle every reaction or process is reversible • One direction or the other would be considered natural or spontaneous • The direction that has a higher percentage of substances upon the reaction reaching equilibrium is considered the spontaneous direction • This is considered the favored direction

  9. Cd(NO3)2+Na2S CdS+2NaNO3 • The other direction might still happen, but it wouldn’t be the favored direction • It would be considered nonspontaneous • Nonspontaneousrxns do not produce substantial amnts of products at equili. • Think about the rxn: • The formation of CdS & NaNO3 is the spontan- eous direction, • CdS is insoluble yellow powder

  10. Spontaneous Rxns • CdS isn’t available to revert back into Cd(NO3)2, therefore, backward is the nonspontaneous direction • Spontaneous & nonspontaneous don’t predict the how fast the rxn proceeds to equilibrium • It deals only with whether or not the rxn is naturally occurring • Some rxns that are spontaneous proceed so slowly that they appear to be nonspontaneous

  11. C12H22O11+12O2 <==>12CO2+11H2O  Spontaneous Rxns • The rxn of sugar with oxygen, for exa-mple, produces carbon dioxide & water: • Isn’t a bowl of sugar on a table doing nothing? • Might we assume that the equilibrium between sugar, O2, CO2, & H2O greatly favors sugar & O2? • However, the favored direction is actually the products • It just take thousands of years to come to completion

  12. When you supply energy in the form of heat, the rxn is much faster, only then is it obvious that, at equilibrium, the form-ation of CO2 & H2O is highly favored • Some rxns that are non-spontaneous under one set of conditions may be spontaneous under other conditions • temp or press, for example, adjustments may determine whether or not a rxn will be spontaneous • Photosynthesis is a non-spontaneous rxn and could not be driven to completion w/o the energy supplied by the sun

  13. Sometimes a non-spontaneous rxn can be made to occur if it is coupled to a spontaneous rxn • Coupled rxns are a common feature of the complex biological processes that take place in living organisms • Within cells, a series of spontaneous rxns release the energy stored in glucose • There are molecules (ATP & ADP) in a cell that can capture and transfer free energy to non-spontaneous rxns. • Assists in the the formation of proteins

  14. Entropy • Recall that enthalpy changes or energy changes accompany most chem and phys processes? • Combustion rxns release a large amount of heat energy so they are exothermic and obviously spontaneous • Exothermic rxns which produce free energy are spontaneous, • But what about endothermic rxns? • Endothermic rxns absorb energy rather than produce it so how can they be energetically spontaneous?

  15. Entropy • Consider this endothermic process • ice absorbs energy in order to change phase from solid to liquid • Considering only heat changes, the energy of the water is higher than the energy of the ice • Yet ice obviously does melt • So what’s the deal? • Rxns can also be spontaneous if the result of the rxn leans toward energy dispersal

  16. Entropy • If you leave a piece of steel out in the air it will rust (you almost can’t prevent it) • If you punch a tiny hole in a balloon the air will escape through the hole. • A hot object will eventually cool down. • If you drop a rock it will fall down with a bang. • If you smack a rock hard enough with a hammer it will shatter. • Even the most advanced machine will eventually break down with repeated use

  17. Entropy • All of those illustrations lead to a profound generalization: in all everyday or exotic spontaneous physical or chemical happenings, energy flows from being localized or concentrated to becoming more spread out or dispersed. • The measure of the dispersal of energy in a system is known asentropy (S) • S = ∑Sproducts - ∑Sreactants • Energy flows in the direction of hotter to cooler because that direction results in an increase of entropy.

  18. Entropy is one of the most powerful laws in the universe • In the past entropy has been described as a tendency to increasing chaos or increasing disorder • S (J/mol•K) of a chemical rxn can be calculated 2H2 + O2 2H2O S = 130.7 205.1 188.3 Srxn = 2(188.3) – [2(130.7)+205.1] Srxn = -89.9 J/mol•K (-) indicatesan overall decrease in entropy

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

  20. Entropy • Rxns or processes that move toward increased disorder or increased disper-sion of energy are favored entropically. • Positive changes in entropy are favored • We can look for an increased disorder to indicate a positive change in entropy • entropy increases from solid to liquid to gas • entropy increases in rxns in which solid reactants form liquid or gaseous products and liquid reactants to form gases • Etc.

  21. Entropy Increasing Entropy Entropy • Entropy increases when a substance is particulated • Grinding, chipping, tearing, ripping, smashing, etc.

  22. 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  2H2 + O2  2 particles  3 particles 2H2 + O2 2H2O  3 particles  2 particles

  23. Entropy increases in chemical reactions in which the products are more numerous than the reactants.  

  24. Increasing Entropy Entropy • Entropy tends to increase as temperature increases • As the temp increases, the molecules move faster and faster, which decreases the localization of the energy of the system

  25. Entropy increases as a substance is dissolved. Even a highly ordered crystal can be pulled made more random when pulled apart by water.  

  26. Heat, Entropy, & Free Energy • Determining the spontaneity of a rxn is accomplished by examining both energy & entropy • An exothermic rxn combined by an increase in entropy or disorder, is identified as spontaneous since both factors are favorable • For example: in the combustion of carbon • A combustion is exothermic so the energy is favorable

  27. Heat, Entropy, & Free Energy • C(s) is converted to CO2(g) so there is a favorable change in entropy • Since both energy and entropy are favorable, the rxn is considered spontaneous • The reverse rxn: CO2  C+ O2 is non-spontaneous • In this case neither the energy nor entropy is proceeding favorably • A rxn may also be spontaneous if a decrease in entropy is offset by a large release of heat

  28. Heat, Entropy, & Free Energy • Or an endothermic rxn (unfavorable) may be spontaneous if an entropy increase offsets the heat absorption • i.e. energy change & entropy change work in opposition when ice melts • Ice melting absorbs heat energy, which is endothermic, but is favorable entropy so the overall rxn is considered spontaneous • Either of the 2 variables, but not both, may be unfavorable & remain spontaneous

  29. Calculations with DG and DS • The change in free energy of a system can be calculated by finding the difference between the change in enthalpy, and the product of the Kelvin temperature and the change in entropy. DG° =DH° - TDS° • This expression is for substances in their standard states. • Each of the variables can have positive our negative values, which leads to four different combinations of terms.

  30. Calculations with DG and DS • If DH is negative and DS is positive, then both terms on the right in the free energy equation are negative • Both heat energy and entropy are contributing to the process being spontaneous • If DH is positive (endothermic) and DS is negative (decrease in randomness) • process is never negative so therefore the reaction can never be spontaneous

  31. Calculations with DG and DS • DG° must be negative in order for the rxn or process to be spontaneous • For example in this rxn at room temp: C2H4(g) + H2(g) C2H6(g) SPONTANEOUS • If… • DS° = -.1207kJ/molK (decrease in entropy) • DH° = -136.9kJ/mol (exothermic) • What is DG°? DG° = DH° - TDS° DG°=(-136.9kJ/mol)–(298K)(-.1207kJ/molK) DG° = -101.1 kJ/mol

  32. For the rxn NH4Cl(s) NH3(g) + HCl(g), at 298.15K, DH°= 176 kJ/mol and DS°= .285kJ/molK, Calculate DG°, and tell whether this rxn can proceed in the forward direction at 298.15 K. NOT SPONTANEOUS DG° = DH° - TDS° DG°=(176 kJ/mol)–(298.15K)(.285kJ/molK) DG° = 91.0 kJ/mol Positive free energy means that at this temperature this reaction does not occur naturally.

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