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Thermodynamics

Thermodynamics. Measuring Heat. Thermodynamics = the study of changes in energy that accompany chemical or physical changes Exothermic Endothermic When water freezes When ice melts. Heat Transfer. Heat = a form of energy Symbol = q Always flows from a warmer object to a cooler object

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Thermodynamics

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  1. Thermodynamics

  2. Measuring Heat • Thermodynamics = the study of changes in energy that accompany chemical or physical changes • Exothermic • Endothermic • When water freezes • When ice melts

  3. Heat Transfer • Heat = a form of energy • Symbol = q • Always flows from a warmer object to a cooler object • Temperature – measure of average kinetic energy

  4. Specific Heat • Amount of heat needed to raise the temperature of 1 g of a substance by 1 oC • Symbol = C • Unit = J/g oC • High specific heat = absorbs a lot of energy w/small changes in temperature

  5. Calorimetry • Measurement of heat transfer • q = (m)(C)( T) • (+)q = heat absorbed • (-)q = heat released • Actual amount of energy transferred is never a negative number

  6. What amount of heat is needed to increase the temperature of 10.00 mol of mercury by 7.50 oC? (CHg = 0.139 J/goC)

  7. 1st law of thermodynamics • In any physical or chemical process, energy is neither created or destroyed • Energy is not lost or gained, just transferred

  8. System • Specific part of the universe on which you focus your attention (reactants!) • When heat transferred from one substance to another they are both part of the system • Heat absorbed by substance 1 = heat lost by substance 2 • The 2 substances will continue to absorb/lose heat until they are the same temperature

  9. q1 = -q2 • (m1)(C1)( T1) = - (m2)(C2)( T2)

  10. Steps for solving problems • List all known variables • Solve for q1 • Plug in opposite of q1 for q2 and solve for variable

  11. A 118 g piece of tin at 85 oC is dropped into 100 g of water at 35 oC. The final temperature of the mixture is 38 oC. C of water is 4.18 J/goC. What amount of heat is absorbed by the water? What amount is released by the tin? What is the specific heat of tin?

  12. A 125 g sample of iron at 93.5 oC is dropped into an unknown mass of water at 25.0 oC. The final temperature of the mixture is 32.0 oC. The C of iron is 0.451 J/goC, the C of water is 4.18 J/goC. What is the mass of the water?

  13. Enthalpy and Heat of Reaction • Enthalpy = amount of heat a sample has at a certain pressure and temperature • Only changes in enthalpy can be measured • Symbol = H

  14. At the start of a reaction, each reactant has its own enthalpy • As reaction progresses heat is either released or absorbed • Change = heat of reaction, Hrxn • Hrxn = H of products – H of reactants

  15. Exothermic or Endothermic? • Hrxn is positive • Hrxn is negative

  16. Enthalpy and Heat of Formation • Heat of formation = enthalpy change when 1 mol of a compound is formed from its elements. • Hf

  17. For any chemical reaction, enthalpy change uses the equation • Hrxn = (sum of Hfof products) – (sum of Hf of reactants) • Hf of an element = 0

  18. Find the Hrxn for the following balanced equation: 2Fe + 3CO2(g)  Fe2O3(s) + 3CO(g)

  19. Find the Hrxn for the following balanced equation: 4NH3(g) + 7O2(g)  4NO2(g) + 6H2O(g)

  20. Phase Change Diagram Csteam = 1.70 J/goC Liquid  steam: 40.7 kJ/mol CWater = 4.18 J/goC solid  liquid: 6.01 kJ/mol Cice= 2.10 J/goC

  21. The heat absorbed by one mole of a substance in melting from a solid to a liquid at a constant temperature is the molar heat of fusion

  22. How much heat is gained when 57.6 g of ice is melted completely? H2O (s)  H2O (l)

  23. How much heat is released when 50.0 g of steam cools to 40oC?

  24. Challenge!!! • A 39.0g sample of ice at -125 oC changes into steam at 125 oC. How much energy is absorbed during this process?

  25. Entropy • Changes tend to occur so the lowest possible energy of a system is reached • A system at a state of low energy is more stable than a system at a state of high energy • Entropy = measure of the disorder or randomness of a system

  26. What has more entropy? • Ice or liquid water? • Liquid water or steam?

  27. 2nd law of thermodynamics • Entropy of the universe is always increasing • Systems tend to move toward lower energy, they also tend to move toward higher entropy

  28. 3rd law of thermodynamics • Entropy of an ideal solid at 0 K is zero

  29. Changes that result in increased entropy • Solid  liquid (or part of a solution) • Liquid  gas • Temperature increase • In reactions, when the number of particles increases • CaCO3  CaO + CO2

  30. Spontaneity • Spontaneous changes occur naturally • Ice melting • Rust forming: 4Fe + 3O2 2Fe2O3 • Nonspontaneous changes do not occur without the addition of energy • Rust decomposing: 2Fe2O3 4Fe + 3O2

  31. Determining Spontaneity • Enthalpy change – exothermic more likely spontaneous than endothermic • Entropy change – change that results in more entropy more likely spontaneous than change in which entropy decreases • Temperature – determines spontaneity when the first two don’t

  32. Change that is exothermic & results in increased entropy = always spontaneous • C3H8 + 5O2 3CO2 + 4H20 • Moles of reactants and products? • What does this mean? • Hrxn = -2220000 J/mol • What does this mean?

  33. Exothermic changes that have decreased entropy are sometimes spontaneous • Determining factor = temperature • If temp is low enough change is spontaneous • H20 (g)  H2O (l) • H rxn = -40.7 kJ/mol • Entropy increase or decrease? • Temp @ which reaction spontaneous?

  34. Endothermic change that increases entropy is sometimes spontaneous • If temperature is high enough reaction is spontaneous • H2O (s)  H2O (l) • H rxn = 6.01 kJ/mol • Entropy increase or decrease? • Temp @ which reaction is spontaneous?

  35. Endothermic changes with decreased entropy will NEVER be spontaneous

  36. N2 + 3H2 2NH3 Hrxn = -46 kJ/mol • Exothermic or endothermic? • Entropy increase or decrease? • Spontaneous?

  37. Review Problem 1 • A 225 g sample of iron at 98.5 oC is placed into 72.4 g of water at 22.0 oC. The final temperature of the mixture is 41.2 oC. The specific heat of water is 4.18 J/g oC, what is the specific heat of iron?

  38. Review problem 2 • What is the Hrxn for the reaction: • P4(s) + 6H2O(l)  4H3PO4(l)

  39. Review problem 3 • What amount of heat is released when 252 g of tin at 112 oC cools to 37.5 oC. The specific heat of tin is 0.226 J/g oC

  40. Review problem 4 • How much energy is released when 57 g of steam condenses to liquid water? The heat of vaporization of water is 40.7 kJ/mol.

  41. Specific heat • Thermodynamics • 1st, 2nd, 3rd law of thermodynamics • Entropy • Heat • Temperature • Endo/exothermic (- or +)

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