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AP Chemistry Unit 7 – Thermodynamics

AP Chemistry Unit 7 – Thermodynamics. Lesson 3 – Calorimetry Book Sections: 5.5. Calorimetry. Since we cannot know the exact enthalpy of the reactants and products, we measure  H through calorimetry , the measurement of heat flow. Heat Capacity and Specific Heat.

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AP Chemistry Unit 7 – Thermodynamics

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  1. AP ChemistryUnit 7 – Thermodynamics Lesson 3 – Calorimetry Book Sections: 5.5

  2. Calorimetry Since we cannot know the exact enthalpy of the reactants and products, we measure H through calorimetry, the measurement of heat flow.

  3. Heat Capacity and Specific Heat The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity.

  4. Heat Capacity and Specific Heat We define specific heat capacity(or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K. VIDEO: Making Cheesecake

  5. Heat Capacity and Specific Heat Specific heat (Cp), then, is used mathematically by the following equation:

  6. Constant Pressure Calorimetry By carrying out a reaction in aqueous solution in a simple calorimeter such as this one, one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter.

  7. Constant Pressure Calorimetry Because the specific heat for water is well known (4.184 J/g-K), we can measure H for the reaction with q = mCpT. (The specific heat of water is also 1.000 cal/g-K)

  8. Constant Pressure Calorimetry • How much heat is needed to warm 250 g of water (about 1 cup) from 22 ºC (about room temperature) to near its boiling point, 98 ºC? The specific heat of water is 4.18 J/g-K.

  9. Constant Pressure Calorimetry • How much heat is needed to warm 250 g of water (about 1 cup) from 22 ºC (about room temperature) to near its boiling point, 98 ºC? The specific heat of water is 4.18 J/g-K. • 7.9 x 104 J

  10. Constant Pressure Calorimetry • If the mass specific heat of water is 4.18 J/g-K, what is the molar specific heat of water? (J/mol-K)?

  11. Constant Pressure Calorimetry • If the mass specific heat of water is 4.18 J/g-K, what is the molar specific heat of water? (J/mol-K)? • 75.2 J/mol-K

  12. Bomb Calorimetry • Reactions can be carried out in a sealed “bomb” such as this one. • The heat absorbed (or released) by the water is a very good approximation of the enthalpy change for the reaction.

  13. Bomb Calorimetry • Bomb calorimeter specific heats, because the mass is constant, do not have mass as a component. • qrxn = Ccal ΔT

  14. Bomb Calorimetry • When a student mixed 50 mL of 1.0 M HCl and 50 mL of 1.0 M NaOH in a coffee-cup calorimeter, the temperature of the resultant solution increases from 21.0 ºC to 27.5 ºC. Calculate the enthalpy for the reaction in kJ/mol HCl, assuming that the calorimeter loses only a negligible quantity of heat, that the total volume of the solution is 100 mL, that its density is 1.0 g/mL, and that its specific heat is 4.18 J/g-K.

  15. Bomb Calorimetry • When a student mixed 50 mL of 1.0 M HCl and 50 mL of 1.0 M NaOH in a coffee-cup calorimeter, the temperature of the resultant solution increases from 21.0 ºC to 27.5 ºC. Calculate the enthalpy for the reaction in kJ/mol HCl, assuming that the calorimeter loses only a negligible quantity of heat, that the total volume of the solution is 100 mL, that its density is 1.0 g/mL, and that its specific heat is 4.18 J/g-K. • -54 kJ/mol

  16. Bomb Calorimetry • Methylhydrazine (CH6N2) is used as a liquid rocket fuel. The combustion of methylhydrazine with oxygen produces N2(g), CO2(g), and H2O(l): • 2 CH6N2(l) + 5 O2(g)  2 N2(g) + 2 CO2(g) + 6 H2O(l) • When 4.00 g of methylhydrazine is combusted in a bomb calorimeter, the temperature of the calorimeter increases from 25.00 ºC to 39.50 ºC. In a separate experiment the heat capacity of the calorimeter is measured to be 7.794 kJ/ºC. Calculate the heat of reaction for the combustion of a mole of CH6N2.

  17. Bomb Calorimetry • Methylhydrazine (CH6N2) is used as a liquid rocket fuel. The combustion of methylhydrazine with oxygen produces N2(g), CO2(g), and H2O(l): • 2 CH6N2(l) + 5 O2(g)  2 N2(g) + 2 CO2(g) + 6 H2O(l) • When 4.00 g of methylhydrazine is combusted in a bomb calorimeter, the temperature of the calorimeter increases from 25.00 ºC to 39.50 ºC. In a separate experiment the heat capacity of the calorimeter is measured to be 7.794 kJ/ºC. Calculate the heat of reaction for the combustion of a mole of CH6N2. • -1.30 x 103 kJ/mol CH6N2

  18. HW: 5.48, 5.50, 5.52, 5.54, 5.56 (Read 5.6) • This week: • Tuesday: Hess’s Law (5.6) • Wednesday: Heats of Formation (5.7) • Thursday: FLEX (No 6th Pd) • Friday: Titration 1 – Standardization of NaOH • Ionic Equations Due Jan. 11 • Midterm Exam (Units 1-6): Jan. 14

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