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 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 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 2. measure the initial temperatures of the solutions

Calorimetry To measure ΔH for a reaction 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 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, m water.

Calorimetry The heat generated by the reactants is absorbed by the water. We know the mass of the water, m water. We know the change in temperature, ∆T water.

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

Calorimetry The heat generated by the reactants is absorbed by the water. We know the mass of the water, m water. We know the change in temperature, ∆T water. We also know that water has a specific heat of C water = 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, m water. We know the change in temperature, ∆T water. We also know that water has a specific heat of C water = 4.18 J/°C-g. We can calculate the heat of reaction by: q sys = ∆H = −q surr = -m water × C water × ∆T water

Example When 25.0 mL of water containing mol of HCl at 25.0°C is added to 25.0 mL of water containing 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 mol of HCl at 25.0°C is added to 25.0 mL of water containing 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 mol of HCl at 25.0°C is added to 25.0 mL of water containing 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:V final = V HCl + V NaOH = ( ) mL = 50.0 mL D water = 1.00 g/mL ∆T water = T final − T initial = 32.0°C − 25.0°C = +7.0°C C water = 4.18 J/°C-g

Example When 25.0 mL of water containing mol of HCl at 25.0°C is added to 25.0 mL of water containing 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:V final = V HCl + V NaOH = ( ) mL = 50.0 mL D water = 1.00 g/mL ∆T water = T final − T initial = 32.0°C − 25.0°C = +7.0°C C water = 4.18 J/°C-g Calculation:m water = V final × D water = (50.0 mL)(1.00 g/mL)

Example When 25.0 mL of water containing mol of HCl at 25.0°C is added to 25.0 mL of water containing 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:V final = V HCl + V NaOH = ( ) mL = 50.0 mL D water = 1.00 g/mL ∆T water = T final − T initial = 32.0°C − 25.0°C = +7.0°C C water = 4.18 J/°C-g Calculation:m water = 50.0 g

Example When 25.0 mL of water containing mol of HCl at 25.0°C is added to 25.0 mL of water containing 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:V final = V HCl + V NaOH = ( ) mL = 50.0 mL D water = 1.00 g/mL ∆T water = T final − T initial = 32.0°C − 25.0°C = +7.0°C C water = 4.18 J/°C-g Calculation:m water = 50.0 g ∆H = −m × C × ∆T

Example When 25.0 mL of water containing mol of HCl at 25.0°C is added to 25.0 mL of water containing 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:V final = V HCl + V NaOH = ( ) mL = 50.0 mL D water = 1.00 g/mL ∆T water = T final − T initial = 32.0°C − 25.0°C = +7.0°C C water = 4.18 J/°C-g Calculation:m water = 50.0 g ∆H = −(50.0 g)(4.18 J/°C-g)(7.0°C)

Example When 25.0 mL of water containing mol of HCl at 25.0°C is added to 25.0 mL of water containing 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:V final = V HCl + V NaOH = ( ) mL = 50.0 mL D water = 1.00 g/mL ∆T water = T final − T initial = 32.0°C − 25.0°C = +7.0°C C water = 4.18 J/°C-g Calculation:m water = 50.0 g ∆H = −1463 J

Example When 25.0 mL of water containing mol of HCl at 25.0°C is added to 25.0 mL of water containing 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:V final = V HCl + V NaOH = ( ) mL = 50.0 mL D water = 1.00 g/mL ∆T water = T final − T initial = 32.0°C − 25.0°C = +7.0°C C water = 4.18 J/°C-g Calculation:m water = 50.0 g ∆H = −1463 J = −1.5×10 3 J

Example When 25.0 mL of water containing mol of HCl at 25.0°C is added to 25.0 mL of water containing 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:V final = V HCl + V NaOH = ( ) mL = 50.0 mL D water = 1.00 g/mL ∆T water = T final − T initial = 32.0°C − 25.0°C = +7.0°C C water = 4.18 J/°C-g Calculation:m water = 50.0 g ∆H = −1463 J = −1.5×10 3 J = −1.5 kJ

Example When 25.0 mL of water containing mol of HCl at 25.0°C is added to 25.0 mL of water containing 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:V final = V HCl + V NaOH = ( ) mL = 50.0 mL D water = 1.00 g/mL ∆T water = T final − T initial = 32.0°C − 25.0°C = +7.0°C C water = 4.18 J/°C-g Calculation:m water = 50.0 g ∆H = −1463 J = −1.5×10 3 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)