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To Do. 5 th HW assignment due Friday by 10 pm. Begin reading Chapter 7 and doing the homework: we will start Chapter 7 on Tuesday. Determining  H rxn. Since  H = q p , determine the heat transferred at constant pressure. This is termed calorimetry.

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To Do

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  1. To Do • 5th HW assignment due Friday by 10 pm. • Begin reading Chapter 7 and doing the homework: we will start Chapter 7 on Tuesday.

  2. Determining Hrxn • Since H = qp, determine the heat transferred at constant pressure. This is termed calorimetry. • Since energy is conserved, we can often determine Hrxn for one reaction given values of Hrxn for others. Uses the principle in Hess’s Law. • Use standard enthalpies of formations for reactants and products (we have tables of these).

  3. A Coffee-Cup Calorimeter Made of Two Styrofoam Cups

  4. Clicker Question You drop 50.0 g of 100.0C metal into 100.0 g of 22.0C water. The final temperature of the water is 29.5C. Identify the metal. c(water) = 4.18 J/g C a) aluminum (c=0.89 J/gC) b) iron (c=0.45 J/gC) c) copper (c=0.39 J/gC) d) lead (c=0.13 J/gC) e) I do not know how to do this.

  5. Constant-Pressure Calorimetry • Can also determine enthalpy change for chemical reactions that occur in solution. See Example 6.5 on page 256 of your text. • In Chemistry 103 you will also determine enthalpy change of a salt dissolving (Lab 7), enthalpy changes for phase changes (Lab 8), and explore Hess’s law (Lab 9).

  6. Bomb Calorimeter: Constant V

  7. Hess’s Law • Due to first law and H being a state function. • If a reaction is reversed, the sign of H is also reversed • The magnitude of ΔH is directly proportional to the quantities of reactants and products in a reaction. • That is, if the coefficients in a balanced equation are multiplied by an integer, the value of H is multiplied by the same integer.

  8. Hess’s Law

  9. Clicker Question Given the following data 2NH3(g)  N2(g) + 3H2(g) H = 92kJ 2H2(g) + O2(g)  2H2O(g) H = -484kJ Determine ΔH for 2N2(g) + 6H2O(g)  3O2(g) + 4NH3(g) a) -1268 kJ b) 1268 kJ c) 1636 kJ d) -1636 kJ e) I did not/cannot get any of these answers.

  10. Determining Hrxn • Since H = qp, determine the heat transferred at constant pressure. This is termed calorimetry. • Since energy is conserved, we can often determine Hrxn for one reaction given values of Hrxn for others. Uses the principle in Hess’s Law. • Use standard enthalpies of formations for reactants and products (we have tables of these).

  11. Standard Enthalpy of Formation: Hf° Change in enthalpy that accompanies the formation of 1 mole of a compound from its elements with all substances in their standard states. Example: H2(g) + (1/2)O2(g)  H2O(g) Hf° = -242kJ

  12. Pathway for the Combustion of Methane

  13. Schematic Diagram of Energy Changes

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