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Instructor: Dr. Marinella Sandros

Nanochemistry NAN 601. Instructor: Dr. Marinella Sandros. Lecture 4: Thermodynamics. Thermodynamics. Based on several fundamental laws that summarize our experience with energy changes. HOW TO KNOW IF A REACTION HAPPENS?.

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Instructor: Dr. Marinella Sandros

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  1. Nanochemistry NAN 601 Instructor: Dr. MarinellaSandros Lecture 4:Thermodynamics

  2. Thermodynamics • Based on several fundamental laws that summarize our experience with energy changes.

  3. HOW TO KNOW IF A REACTION HAPPENS? First, one of the important consequences of thermodynamics is the ability to explain whether a reaction occurs or not.

  4. First Law of Thermodynamics • States that energy is conserved • Energy is neither created nor destroyed in any process. Examples: • Falling of a brick • Melting of an ice cube • Reaction of chemicals

  5. Energy • Energy flows from one part of nature to another or is converted from one form to another, but the total remains constant.  E = q + w • E= change in energy of a system • q= is the heat absorbed by the system from its surroundings • w= work is done on the system by its surroundings

  6. Second Law of Thermodynamics • There is an inherent direction in which any system moves if it is not at a equilibrium. For example: A shiny nail left outdoors eventually rusts. • This process occurs without outside intervention , such processes are considered spontaneous.

  7. Second Law of Thermodynamics • Note that for every spontaneous reaction, there is a reverse non-spontaneous reaction and it would be nice to look at a reaction and tell whether it is going to happen as written or as the reverse.

  8. Free Energy The state function that determines spontaneity is free energy, G: Rxn is spontaneous - Free energy = G Rxn is not spontaneous + So when you look at a reaction, if you know it occurs from experience, you know it has G= -

  9. EXAMPLE AgNO3 + NaCl AgCl + Na+ + NO3- Solubility rules state that AgCl is insoluble and the reaction shifts to the right, so you know that for this reaction: -  G is

  10. Sign Conventions

  11. Example Example: A Fire Burns Down a House—Is the process exothermic or endothermic. Is work done on this system or the surroundings?

  12. Answer Exothermic: (-), heat evolves, the wood of the house gets cold, i.e. the strong wood bonds become weak bonds CO2 + H2O bonds. Work Done on the Surroundings: (-), gas evolves, wood becomes CO2 + H2O. A bomb is formed as the volume of the house expands.  E = q + w

  13. Temperature Dependence Of Free Energy • Case 1: (Always spontaneous) Conditions for G always • G= H - TS spontaneous • if and • Case 2: (Never spontaneous) Conditions for G always • G= H - TS non-spontaneous • if and - - + + + - ***T is always + so a positive S makes -TS

  14. Temperature dependence of free energy • Case 3: (Temp dependent) • G= H - TS • if and and T large • if and and T small • Example of temperature dependent spontaneity • Ice Melting: H20 (s)  H20 (l) - + + - - - Is it spontaneous? It is at high T, but not as in a freezer. So it must be the high T spontaneous case.

  15. Energy

  16. Enthalpy • H= enthalpy can be or • = heat given off to surroundings= exothermic • Example: CaO + H2O  Ca(OH)2 + heat • Cooks an egg, makes a mess and  H is - + -

  17. Enthalpy + = heat absorbed so surroundings cold= endothermic + Example: Ba(OH)2 + NH4NO3 NH3 and other stuff + cold H is • There are 3 ways you will be asked to determine H • Calorimetry mC(T) calculation • Heat of formation calculation • Bond energy calculation

  18. Entropy: disorder + = entropy increases = entropy decreases - As you see from G= H-T S, you want S positive for spontaneity And this makes sense, reactions happen because something is made easier, and increasing disorder, like creating a messy room, is easier than creating order (cleaning up a room).

  19. Can we predict the sign of entropy? 1. Increased temperature increases S. Why? When it gets hotter, kinetic energy goes up, velocity goes up, molecules separate more. 2. Increased volume increases S. Why? If molecules that bounced around in a cup will be more disordered bouncing around in a gallon jug.

  20. Can we predict the sign of entropy? Solid  Liquid  gas increases S. Why? Increased n of reaction increases S. Why? More molecules, more mess.

  21. Predicting Entropy So we can look at a chemical reaction and predict S: H2O (l) H2O(g) S is . Why? + reasons: Evaporation of a liquid is accompanied by a large increase in volume. Because the molecules are distributed throughout a much larger volume in the gaseous state than in the liquid state, an increase in disorder accompanies vaporization.

  22. Predicting Entropy • Ag+(aq) + Cl-(aq)  AgCl (s) S is . Why? - In this process the ions that are free to move about in the larger volume of the solution from a solid in which the ions are confined to highly ordered positions. Thus, there is a decrease in disorder.

  23. Crystallization • The nature of crystallization process is governed by both thermodynamic and kinetic factors, which can make it highly variable and difficult to control. Factors such as impurity level, mixing regime, vessel design, and cooling profile can have a major impact on the size, number, and shape of crystals produced.

  24. Molecular Self-Assembly Process by which molecules adopt a defined arrangement without guidance or management from an outside source.

  25. Nature

  26. Cohesion Energy • Cohesion energy represents how strongly atoms or molecules stick together to hold the material. • It was shown that at nanoscale, by decreasing the size of particle, its cohesion energy decrease.

  27. Cohesion Energy • Binding energy of atoms determines the ability of particle for reaction in atomic-scale. • Thus, the decrease of cohesion energy represents the instability or in other words, the higher reaction tendency.

  28. Entropic Forces Example from classical thermodynamics: Gas molecules distribute evenly in the available volume (likely distribution)It is highly unlikely that all gas molecules are found in one half of the volumeIt takes an external force to create such an unlikely distribution

  29. Entropic Forces in Polymers Polymers are long chain molecules Chemical bonds are hard to stretch, but can often rotate freely Likeliest conformation: Random walk Unlikely conformation: Stretched or bent A force is required to stretch a polymer!

  30. All biochemical and cellular processes obey the laws of chemistry and physics • Biochemistry is not a special case. Therefore, in studying biochemistry, it is necessary to consider the relevant laws that control possible reactions.

  31. ATP • Energy is released from ATP through the loss of phosphate groups • Catabolic reaction resulting from hydrolysis producing ADP + Pi (inorganic Phosphate) + energy (G = -7.3Kcal/mol in the lab, -13Kcal/mol in the cell)

  32. How ATP works • Hydrolysis of ATP produces inorganic phosphate that is attached to a molecule involved in an endergonic process • Phosphorylation is the process of ATP transferring phosphate to a molecule • Results in a phosphorylated intermediate that can complete the intended reaction

  33. Staying alive: metabolic disequilibrium. Because chemical systems at equilibrium have a G = 0, they can do no work. A cell that reached equilibrium would be dead; therefore many processes are driven forward by having them out of equilibrium, meaning they are forced in one direction only by the concentrations of reactants and products or by having the products be immediately used in a subsequent reaction.

  34. A cell does three main kinds of work: • (1) Mechanical work, such as beating of cilia, muscle contraction • (2) Transport work, moving substances across membranes • (3) Chemical work, enabling non-spontaneous reactions to occur spontaneously, such as protein synthesis.

  35. ATP performs work in the cell by linking up (or chemically coupling) ATP hydrolysis to otherwise energetically unfavorable cellular reactions. • Glycolysis which cells use to derive energy. The first reaction of glycolysis is energetically unfavorable:

  36. glucose + Pi -----> glucose-6-phosphate ΔG = +3.3 kcal/mol • When coupled to ATP hydrolysis, the reaction becomes energetically favorable: • glucose + Pi -----> glucose-6-phosphate ΔG = +3.3 kcal/mol • ATP-----> ADP + Pi ΔG = -7.3 kcal/mol • overall:glucose + ATP -----> glucose-6-phosphate + ADPΔG = -4.0 kcal/mol • Transfer of a phosphate group from ATP causes this energetically unfavorable reaction to occur spontaneously.

  37. Homework!!! • Predict whether the entropy change of the system in each of the following reactions is positive or negative and state why?? • CaCO3(s)  CaO(s) + CO2(g) • N2(g) +3H2(g)  2NH3 (g) • N2(g) +O2(g)  2NO (g) • 4Fe(s) +3O2(g)  2Fe2O3 (s)

  38. Answers • CaCO3(s)  CaO(s) + CO2(g) A: entropy is +because a solid is converted into a gas. • N2(g) +3H2(g)  2NH3 (g) A: entropy is negative because there are fewer moles of gas in the product than in the reactants. • N2(g) +O2(g)  2NO (g) A: entropy will be small because the same number of moles of gas is involved in the reactants and products • 4Fe(s) +3O2(g)  2Fe2O3 (s) A: entropy will be negative because g  s

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