Enthalpy ChangesMeasuring 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 heat transfer.

Calorimetry•We use an insulated device called a

calorimeter to measure this heat transfer.

•A typical device is a “coffee cup calorimeter.”

Calorimetry•We use an insulated device called a

calorimeter to measure this heat transfer.

•A typical device is a “coffee cup calorimeter.”

Calorimetry•To measure ΔH for a reaction ...

Calorimetry•To measure ΔH for a reaction

...

1.dissolve the reacting chemicals in known volumes of water

Calorimetry•To measure ΔH for a reaction

...

1.dissolve the reacting chemicals in known volumes of water

2.measure the initial temperatures of the solutions

Calorimetry•To measure ΔH for a reaction

...

1.dissolve the reacting chemicals in known volumes of water

2.measure the initial temperatures of the solutions

3.mix the solutions

Calorimetry•To measure ΔH for a reaction ...

1.dissolve the reacting chemicals in known volumes of water

2.measure the initial temperatures of the solutions

3.mix the solutions

4.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

ExampleWhen 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.

ExampleWhen 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:

ExampleWhen 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

ExampleWhen 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 = Vfinal × Dwater = (50.0 mL)(1.00 g/mL)

ExampleWhen 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

ExampleWhen 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

ExampleWhen 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)

ExampleWhen 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

ExampleWhen 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

ExampleWhen 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

ExampleWhen 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)