Enthalpy Changes

1. Enthalpy Changes • Measuring and Expressing ∆H • ☾ Calorimetry ☽

2. Introduction • We have been introduced to heat producing (exothermic) reactions and heat using (endothermic) reactions.

3. Introduction • We have been introduced to heat producing (exothermic) reactions and heat using (endothermic) reactions. • Heat is a measure of the transfer of energy from a system to the surroundings and from the surroundings to a system.

4. Introduction • We have been introduced to heat producing (exothermic) reactions and heat using (endothermic) reactions. • Heat is a measure of the transfer of energy from a system to the surroundings and from the surroundings to a system. • The change in heat of a system is called the change in enthalpy (ΔH) when the pressure of the system in kept constant.

5. Calorimetry • We measure the transfer of heat (at a constant pressure) by a technique called calorimetry.

6. Calorimetry • We measure the transfer of heat (at a constant pressure) by a technique called calorimetry. • In calorimetry ...

7. Calorimetry • We measure the transfer of heat (at a constant pressure) by a technique called calorimetry. • In calorimetry ... • the heat released by the system is equal to the heat absorbed by its surroundings.

8. Calorimetry • We measure the transfer of heat (at a constant pressure) by a technique called calorimetry. • In calorimetry ... • the heat released by the system is equal to the heat absorbed by its surroundings. • the heat absorbed by the system is equal to the heat released by its surroundings.

9. Calorimetry • We measure the transfer of heat (at a constant pressure) by a technique called calorimetry. • In calorimetry ... • the heat released by the system is equal to the heat absorbed by its surroundings. • the heat absorbed by the system is equal to the heat released by its surroundings. • The total heat of the system and the surroundings remains constant.

10. Calorimetry • We use an insulated device called a calorimeter to measure this heattransfer.

11. Calorimetry • We use an insulated device called a calorimeter to measure this heattransfer. • A typical device is a “coffeecup calorimeter.”

12. Calorimetry • We use an insulated device called a calorimeter to measure this heattransfer. • A typical device is a “coffeecup calorimeter.”

13. Calorimetry • To measure ΔH for a reaction ...

14. Calorimetry • To measure ΔH for a reaction ... • dissolve the reacting chemicals in known volumes of water

15. Calorimetry • To measure ΔH for a reaction ... • dissolve the reacting chemicals in known volumes of water • measure the initial temperatures of the solutions

16. Calorimetry • To measure ΔH for a reaction ... • dissolve the reacting chemicals in known volumes of water • measure the initial temperatures of the solutions • mix the solutions

17. Calorimetry • To measure ΔH for a reaction ... • dissolve the reacting chemicals in known volumes of water • measure the initial temperatures of the solutions • mix the solutions • measure the final temperature of the mixed solution

18. Calorimetry • The heat generated by the reactants is absorbed by the water.

19. Calorimetry • The heat generated by the reactants is absorbed by the water. • We know the mass of the water, mwater.

20. Calorimetry • The heat generated by the reactants is absorbed by the water. • We know the mass of the water, mwater. • We know the change in temperature, ∆Twater.

21. Calorimetry • The heat generated by the reactants is absorbed by the water. • We know the mass of the water, mwater. • We know the change in temperature, ∆Twater. • We also know that water has a specific heat of Cwater = 4.18 J/°C-g.

22. Calorimetry • The heat generated by the reactants is absorbed by the water. • We know the mass of the water, mwater. • We know the change in temperature, ∆Twater. • We also know that water has a specific heat of Cwater = 4.18 J/°C-g. • We can calculate the heat of reaction by:

23. Calorimetry • The heat generated by the reactants is absorbed by the water. • We know the mass of the water, mwater. • We know the change in temperature, ∆Twater. • We also know that water has a specific heat of Cwater = 4.18 J/°C-g. • We can calculate the heat of reaction by: • qsys = ∆H = −qsurr = -mwater × Cwater × ∆Twater

24. Example When 25.0 mL of water containing 0.025 mol of HCl at 25.0°C is added to 25.0 mL of water containing 0.025 mol of NaOH at 25.0°C in a coffee cup calorimeter, a reaction occurs. Calculate ∆H (in kJ) during this reaction if the highest temperature observed is 32.0°C. Assume the densities of the solutions are 1.00 g/mL.

25. Example When 25.0 mL of water containing 0.025 mol of HCl at 25.0°C is added to 25.0 mL of water containing 0.025 mol of NaOH at 25.0°C in a coffee cup calorimeter, a reaction occurs. Calculate ∆H (in kJ) during this reaction if the highest temperature observed is 32.0°C. Assume the densities of the solutions are 1.00 g/mL. Knowns:

26. Example When 25.0 mL of water containing 0.025 mol of HCl at 25.0°C is added to 25.0 mL of water containing 0.025 mol of NaOH at 25.0°C in a coffee cup calorimeter, a reaction occurs. Calculate ∆H (in kJ) during this reaction if the highest temperature observed is 32.0°C. Assume the densities of the solutions are 1.00 g/mL. Knowns: Vfinal = VHCl + VNaOH = (25.0 + 25.0) mL = 50.0 mL Dwater = 1.00 g/mL ∆Twater = Tfinal − Tinitial = 32.0°C − 25.0°C = +7.0°C Cwater = 4.18 J/°C-g Calculation:

27. Example When 25.0 mL of water containing 0.025 mol of HCl at 25.0°C is added to 25.0 mL of water containing 0.025 mol of NaOH at 25.0°C in a coffee cup calorimeter, a reaction occurs. Calculate ∆H (in kJ) during this reaction if the highest temperature observed is 32.0°C. Assume the densities of the solutions are 1.00 g/mL. Knowns: Vfinal = VHCl + VNaOH = (25.0 + 25.0) mL = 50.0 mL Dwater = 1.00 g/mL ∆Twater = Tfinal − Tinitial = 32.0°C − 25.0°C = +7.0°C Cwater = 4.18 J/°C-g Calculation: mwater = 50.0 g

28. Example When 25.0 mL of water containing 0.025 mol of HCl at 25.0°C is added to 25.0 mL of water containing 0.025 mol of NaOH at 25.0°C in a coffee cup calorimeter, a reaction occurs. Calculate ∆H (in kJ) during this reaction if the highest temperature observed is 32.0°C. Assume the densities of the solutions are 1.00 g/mL. Knowns: Vfinal = VHCl + VNaOH = (25.0 + 25.0) mL = 50.0 mL Dwater = 1.00 g/mL ∆Twater = Tfinal − Tinitial = 32.0°C − 25.0°C = +7.0°C Cwater = 4.18 J/°C-g Calculation: mwater = 50.0 g ∆H = −m × C × ∆T

29. Example When 25.0 mL of water containing 0.025 mol of HCl at 25.0°C is added to 25.0 mL of water containing 0.025 mol of NaOH at 25.0°C in a coffee cup calorimeter, a reaction occurs. Calculate ∆H (in kJ) during this reaction if the highest temperature observed is 32.0°C. Assume the densities of the solutions are 1.00 g/mL. Knowns: Vfinal = VHCl + VNaOH = (25.0 + 25.0) mL = 50.0 mL Dwater = 1.00 g/mL ∆Twater = Tfinal − Tinitial = 32.0°C − 25.0°C = +7.0°C Cwater = 4.18 J/°C-g Calculation: mwater = 50.0 g ∆H = −(50.0 g)(4.18 J/°C-g)(7.0°C)

30. Example When 25.0 mL of water containing 0.025 mol of HCl at 25.0°C is added to 25.0 mL of water containing 0.025 mol of NaOH at 25.0°C in a coffee cup calorimeter, a reaction occurs. Calculate ∆H (in kJ) during this reaction if the highest temperature observed is 32.0°C. Assume the densities of the solutions are 1.00 g/mL. Knowns: Vfinal = VHCl + VNaOH = (25.0 + 25.0) mL = 50.0 mL Dwater = 1.00 g/mL ∆Twater = Tfinal − Tinitial = 32.0°C − 25.0°C = +7.0°C Cwater = 4.18 J/°C-g Calculation: mwater = 50.0 g ∆H = −1463 J

31. Example When 25.0 mL of water containing 0.025 mol of HCl at 25.0°C is added to 25.0 mL of water containing 0.025 mol of NaOH at 25.0°C in a coffee cup calorimeter, a reaction occurs. Calculate ∆H (in kJ) during this reaction if the highest temperature observed is 32.0°C. Assume the densities of the solutions are 1.00 g/mL. Knowns: Vfinal = VHCl + VNaOH = (25.0 + 25.0) mL = 50.0 mL Dwater = 1.00 g/mL ∆Twater = Tfinal − Tinitial = 32.0°C − 25.0°C = +7.0°C Cwater = 4.18 J/°C-g Calculation: mwater = 50.0 g ∆H = −1463 J = −1.5×103 J

32. Example When 25.0 mL of water containing 0.025 mol of HCl at 25.0°C is added to 25.0 mL of water containing 0.025 mol of NaOH at 25.0°C in a coffee cup calorimeter, a reaction occurs. Calculate ∆H (in kJ) during this reaction if the highest temperature observed is 32.0°C. Assume the densities of the solutions are 1.00 g/mL. Knowns: Vfinal = VHCl + VNaOH = (25.0 + 25.0) mL = 50.0 mL Dwater = 1.00 g/mL ∆Twater = Tfinal − Tinitial = 32.0°C − 25.0°C = +7.0°C Cwater = 4.18 J/°C-g Calculation: mwater = 50.0 g ∆H = −1463 J = −1.5×103 J = −1.5 kJ

33. Example When 25.0 mL of water containing 0.025 mol of HCl at 25.0°C is added to 25.0 mL of water containing 0.025 mol of NaOH at 25.0°C in a coffee cup calorimeter, a reaction occurs. Calculate ∆H (in kJ) during this reaction if the highest temperature observed is 32.0°C. Assume the densities of the solutions are 1.00 g/mL. Knowns: Vfinal = VHCl + VNaOH = (25.0 + 25.0) mL = 50.0 mL Dwater = 1.00 g/mL ∆Twater = Tfinal − Tinitial = 32.0°C − 25.0°C = +7.0°C Cwater = 4.18 J/°C-g Calculation: mwater = 50.0 g ∆H = −1463 J = −1.5×103 J = −1.5 kJ

34. Calorimetry • We can also do calorimetry at a constant volume rather than at a constant pressure.

35. Calorimetry • We can also do calorimetry at a constant volume rather than at a constant pressure. • This is called “bomb calorimetry.”

36. Calorimetry • We can also do calorimetry at a constant volume rather than at a constant pressure. • This is called “bomb calorimetry.”

37. Calorimetry • We can also do calorimetry at a constant volume rather than at a constant pressure. • This is called “bomb calorimetry.” • A sample is placed in the crucible.

38. Calorimetry • We can also do calorimetry at a constant volume rather than at a constant pressure. • This is called “bomb calorimetry.” • Oxygen is introduced into the chamber.

39. Calorimetry • We can also do calorimetry at a constant volume rather than at a constant pressure. • This is called “bomb calorimetry.” • The lid is tightened and the chamber is placed in a water bath.

40. Calorimetry • We can also do calorimetry at a constant volume rather than at a constant pressure. • This is called “bomb calorimetry.” • The ignition coil ignites the sample.

41. Calorimetry • We can also do calorimetry at a constant volume rather than at a constant pressure. • This is called “bomb calorimetry.” • The heat generated in the chamber is transferred to the water.

42. Calorimetry • We can also do calorimetry at a constant volume rather than at a constant pressure. • This is called “bomb calorimetry.” • The change in temperature is then measured on the thermometer.

43. Summary • Heat is a measure of the transfer of energy from a system to the surroundings and from the surroundings to a system. • The change in heat of a system is called the change in enthalpy (ΔH) when the pressure of the system in kept constant. • We measure the transfer of heat (at a constant pressure) by a technique called calorimetry. • We use an insulated device called a calorimeter to measure this heat transfer.

44. Summary • Two calorimeters used are ... • the coffee cup calorimeter (for constant pressure measurements) • the bomb calorimeter (for constant volume measurements)