1 / 46

Energy and thermochemistry

Energy and thermochemistry. Thermochemistry: study of transfer of energy as heat occurring during chemical and physical changes Temperature (T): average kinetic energy of the particles in a sample as kinetic energy increases, so does temperature. Heat and Enthalpy.

morey
Download Presentation

Energy and thermochemistry

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Energy and thermochemistry

  2. Thermochemistry: study of transfer of energy as heat occurring during chemical and physical changes • Temperature(T): average kinetic energy of the particles in a sample • as kinetic energy increases, so does temperature

  3. Heat and Enthalpy • Heat - thermal energy that is transferred from a hot object to a cold object • Enthalpychange (DH) - amount of heat given off or absorbed in a reaction; unit kJ • H rxn = HP - HR Final Initial

  4. Reactions can be either … Exothermic= heat is released Endothermic= heat is absorbed What temperature do each feel?

  5. Examples of Thermochemicalequations 4Fe(s) + 3O2(g)  2Fe2O3(s) DH= -1625 kJ OR 4Fe(s) + 3O2(g)  2Fe2O3(s) + 1625 kJ NH4NO3(s)  NH4+(aq) + NO3-(aq) DH= 27 kJ OR 27 kJ + NH4NO3(s)  NH4+(aq) + NO3-(aq) What do you think the – and + indicate? What is the unit kJ?

  6. Summary of DH and Reaction Pathways - +

  7. Remember: A chemical system is usually more stableif products have less energythan the reactants

  8. To measure Heat Transfer Endothermic or exothermic?

  9. Heat • measured using a calorimeter (reaction container that is surrounded by water) • measure the energy given off during a rxn and gained by water • is equal to the energy absorbed by the water Q is Heat

  10. Two Types of Calorimeters Bomb calorimeter (Coffee cup) calorimeter

  11. Amount of energy transferred during a temperature change depends on: • type of material • mass of material • size of Temperature change

  12. Heat- q q= cp m DT

  13. specific heat: (cp) the amount of energy required to raise the Temperature of one gram of substance by one degree celsius (measured under constant pressure) Note units

  14. Example A 4.0 g sample of glass was heated from 274 K to 314 K and was found to have absorbed 32 J of energy. • Find the specific heat of the glass sample.

  15. Enthalpy and heat • Heat - thermal energy that is transferred from a hot object to a cold object • Enthalpy change (DH) - • H rxn = HP - HR Final Initial

  16. Many chem rxns involve a change in heat such as phase changes:

  17. Using heat of fusion and heat of vaporization DH = mHfusion (used for freezing and melting) DH = mHvaporization (used for boiling and condensing)

  18. Molar Heat of Fusion • amount of heat required to melt one mole of solid at its melting point

  19. Heating Curve-Looking at Fusion & Vaporization plateau from s to l is shorter than from l to g because it required much less energy: heat of vaporization > heat of fusion G L S

  20. Activation energy Enthalpy change, DH Reaction Pathway Diagram Activated complex Reactants Energy DH is negative Exothermic Products Reaction Pathway (time) go

  21. Ea • Activation Energy • depends on reactants • low Ea = fast rxn rate • This is exothermic-bond formation Activated complex Hump to make transition R  P Reactants Products

  22. Activationenergy Enthalpy change, DH Reaction Pathway Diagram DH = Hfinal - Hinitial Activated complex Products DH is positive Energy Reactants Endothermicrxn Reaction Pathway (time) go

  23. Activation Energy • Activation Energy (Ea) • minimum energy required for a reaction to occur (energy to get a rxn going) • This is endothermic-breaking bonds Activatedcomplex Products PE of R Reactants PE of P go

  24. Hess’s Law • If you add 2 or more thermochemical equations to produce a final equation, THEN the sum of the H for individual rxns is the H change for final rxn

  25. 2 S + 3 O2 2 SO3 H? Use A- S + O2 SO2 H= - 297 kJ B-2 SO3 2SO2 + O2 H= 198kJ

  26. Use A- S + O2 SO2 H= - 297 kJB- 2 SO3 2SO2 + O2 H= 198kJ 2 S + 3 O2 2 SO3 H? • 2S + 2O2 2 SO2 -594kJ • Add coefficients to make the products the same • 2SO2 + O22 SO3 -198kJ • Reverse this eq to make the cmpds on the same side Now cancel out the appropriate cmpds. Are you left with the beginning equation? Add the two units of heat. ANSWER:_______

  27. 2 H2O2 2 H2O + O2 • Given: 2 H2 + O2  2 H2O H2 + O2  H2O2 H= -572kJ H= -188kJ Get the same cmpds on the same side of the equation. Change coef if you need to. Also change your H.

  28. On your notes try: • 4 Al + 3 MnO2 2Al2O3 + 3Mn H=? • Given: • 4 Al + 3O2  2 Al2O3 + 3Mn H= -3352kJ • Mn + O2  MnO 2 H= -521kJ

  29. Spontaneous Rxns • Chemical reactions that occur with no outside intervention: heating, cooling, stirring, etc. • Spontaneity has nothing to do with the speed of the reaction. Some rxns are very slow.

  30. Nature tends to move toward disorder

  31. Solid particles are in a regular array. Little disorder, low entropy. Gases move randomly Disorder is high, entropy is high.

  32. Increasing “disorder” in molecules Increasing entropy Entropy(S)is a measure of the disorder or randomness in a system. Solids Liquids Gases

  33. Gas and solid less entropy Two gases more entropy The following reaction is endothermic (heat is absorbed): H2O(g) + C(s)  CO(g) + H2(g) Yet the reaction is spontaneous. (Occurs with no outside intervention)

  34. Second law of thermodynamics Nature is always proceeding to a state of higher entropy

  35. Two gases more entropy Gas and solid less entropy Increasing entropy H2O(g) + C(s)  CO(g) + H2(g) The increase in entropy (disorder) helps drive the rxn to the right, towards the products, even if it is endothermic.

  36. Gibb’s Free Energy • Gsystem • A way to determine if a rxn is spontaneous( combines enthalpy and entropy) • Constants: P and T • Free energy: can do work (it’s free) • Gsystem = H system - (T Ssystem)

  37. Spontaenity • -disorder; spontaneous • H – S = • H - S = + No disorder; not spontaneous Problem next

  38. You are given H = 145 KJ and S = 322 J/K. Is the process spontaneous at 382K? • Gsystem = H system - (T Ssystem) Answer next slide

  39. Gsystem = H system - (T Ssystem) = 145 000J - (382 K x 322J/K) 22,000J + means nonspontaneous

  40. The occurrence of a reaction depends on two factors: Enthalpy and Entropy Even if a reaction is endothermic, the reaction may be spontaneous because entropy increases. (Disorder/randomness increases.)

  41. 5 Factors Affecting Rxn Rates • Chapter 17

  42. Factors Affecting Rxn Rate • 1-Surface Area • high SA = fast rxn rate • more opportunities for collisions

  43. Factors Affecting Rxn Rate • 2-Concentration • high conc = fast rxn rate • more opportunities for collisions

  44. Factors Affecting Rxn Rate • 3-Temperature • high temp = fast rxn rate • high KE • fast-moving particles • more likely to reach activation energy

  45. Factors Affecting Rxn Rate • 4-Catalyst • substance that increases rxn rate without being consumed in the rxn • lowers the activation energy

  46. Factors Affecting Rxn Rate • 5-Enzyme Catalysis (Biology connection)

More Related