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