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Calculation OF Enthalpy changes

Calculation OF Enthalpy changes. Molar Enthalpy. the enthalpy change for 1 mole of a substance associated with a chemical, physical or nuclear change the units will J or kJ per mole. Example.

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Calculation OF Enthalpy changes

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  1. Calculation OF Enthalpy changes

  2. Molar Enthalpy • the enthalpy change for 1 mole of a substance associated with a chemical, physical or nuclear change • the units will J or kJ per mole

  3. Example A common refrigerant , Freon-12, has a molar mass of 120.91 g/mol. The molar enthalpy to vaporize Freon is 34.99 kJ/mol. If 500 g of Freon is vaporized, what is the enthalpy change? On Board

  4. Specific Heat Capacity • the quantity of heat needed to change the temperature of 1 g of substance by 1˚C • the symbol is C • the units are J/g˚C or Jg-1˚C-1 EX. C water = 4.18 J/g˚C

  5. Specific Heat Capacity • the quantity of heat needed to change the temperature of 1 g of substance by 1˚C • the symbol is C • the units are J/g˚C or Jg-1˚C-1 EX. C water = 4.18 J/g˚C Specific heat capacity is a measure of how ‘easily’ a substance changes temperature. A substance with a high heat capacity can absorb more energy without changing temperature than a substance with low specific heat.

  6. Heat Transfer Problems • ΔH= mCΔT Change in Temperature Tf –Ti (°C) Specific Heat Capacity (Jg-1°C-1 ) Enthalpy Change (J) Mass of the substance changing temperature (g)

  7. Heat Transfer Problems • When Tf is greater than Ti : -energy is absorbed - the process is ENDOthermic -ΔHis positive • ΔH= mCΔT Change in Temperature Tf –Ti (°C) Specific Heat Capacity (Jg-1°C-1 ) Enthalpy Change (J) Mass of the substance changing temperature (g)

  8. Heat Transfer Problems • When Tf is less than Ti : -energy is released - the process is EXOthermic -ΔHis negative • ΔH= mCΔT Change in Temperature Tf –Ti (°C) Specific Heat Capacity (Jg-1°C-1 ) Enthalpy Change (J) Mass of the substance changing temperature (g)

  9. Examples 1) A 150 g sample of Pb at 100.0 °C is plunged into a beaker containing 50 g of water at 22.0 °C. The final temperature of the mixture is 28.8 °C. a) What is the energy change for the water? b) How much energy is lost by the Pb? c) Calculate the CPb On Board

  10. Examples 2) While running an adult expends 5.0 x 105J/mile. If this energy could be transferred as heat to a beaker of water, how many grams of water could be heated from 25.0 °C to 100.0 °C? On Board

  11. CALORIMETRY Calorimeter – an insulated device used to determine temperature changes during a chemical or physical process. • the calorimeter is part of the surroundings Two Kinds: 1) Simple 2) Bomb

  12. Simple Calorimetry • the process occurs at constant pressure • RECALL that ΔH is + for an ENDOTHERMIC change and – for an EXOTHERMIC ΔHobserved = mCΔT = - ΔH rxn

  13. Assumptions • for a dilute aqueous solution, use Cwater = 4.18 J/g°C as the heat capacity for the solution • 1 mL of solution = 1 g of solution • no heat is transferred to the outside of the calorimeter( i.e., the calorimeter is perfectly insulated)

  14. Diagram insulated lid thermometer insulated cup reaction mixture

  15. Example 1) 5.00 mL of 1.0 mol/L NaOH and 5.00 mL of 1.0 mol/L HCl are mixed in a simple calorimeter. The initial temperature of each substance is 23.0 °C. After mixing, the temperature rose to 27.5 °C. a) What is the enthalpy change for this rxn? b) Calculate the energy change per mole of HCl. On Board

  16. Example 2) 1.50 g of NH4NO3 is added to 35.0 g of water in a simple calorimeter. The initial temperature of the water is 22.7°C and the final temperature of the mixture is 19.4 °C. Calculate the enthalpy change per mole of salt.

  17. Bomb Calorimetry

  18. Bomb Calorimetry Not Really !

  19. Bomb Calorimetry • the process occurs at constant volume • used to measure the heat released in a combustion reaction

  20. Diagram

  21. Bomb Calorimetry • the process occurs at constant volume • used to measure the heat released in a combustion reaction • NOTE: q is used in place of ΔH for an energy change at constant volume q observed = C’ΔT = - q combustion J °C -1

  22. Example A food manufacturer wishes to know the number of calories per serving of a new dessert. They hire a chemist at the Dep’t of Consumer Affairs to help them out. a) The chemist places a 1.00 g sample of the dessert in a bomb calorimeter. The heat capacity of the calorimeter is 8.151 kJK-1. The temperature increases by 4.937 °C. Calculate the number of food calories (kcal) in the sample of dessert. b) A single serving of the dessert weighs 30.00 g. How many food calories are in the dessert? On Board

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