Enthalpy Changes • Measuring and Expressing ∆H • ☾ Calorimetry ☽
Introduction • We have been introduced to heat producing (exothermic) reactions and heat using (endothermic) reactions.
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.
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.
Calorimetry • We measure the transfer of heat (at a constant pressure) by a technique called calorimetry.
Calorimetry • We measure the transfer of heat (at a constant pressure) by a technique called calorimetry. • In calorimetry ...
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.
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.
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.
Calorimetry • We use an insulated device called a calorimeter to measure this heattransfer.
Calorimetry • We use an insulated device called a calorimeter to measure this heattransfer. • A typical device is a “coffeecup calorimeter.”
Calorimetry • We use an insulated device called a calorimeter to measure this heattransfer. • A typical device is a “coffeecup calorimeter.”
Calorimetry • To measure ΔH for a reaction ...
Calorimetry • To measure ΔH for a reaction ... • dissolve the reacting chemicals in known volumes of water
Calorimetry • To measure ΔH for a reaction ... • dissolve the reacting chemicals in known volumes of water • measure the initial temperatures of the solutions
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
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
Calorimetry • The heat generated by the reactants is absorbed by the water.
Calorimetry • The heat generated by the reactants is absorbed by the water. • We know the mass of the water, mwater.
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.
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.
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:
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
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.
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:
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:
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
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
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)
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
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
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
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
Calorimetry • We can also do calorimetry at a constant volume rather than at a constant pressure.
Calorimetry • We can also do calorimetry at a constant volume rather than at a constant pressure. • This is called “bomb calorimetry.”
Calorimetry • We can also do calorimetry at a constant volume rather than at a constant pressure. • This is called “bomb calorimetry.”
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.
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.
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.
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.
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.
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.
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.
Summary • Two calorimeters used are ... • the coffee cup calorimeter (for constant pressure measurements) • the bomb calorimeter (for constant volume measurements)
