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.
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.
6. Calorimetry
We use an insulated device called a
calorimeter to measure this heat transfer.
A typical device is a “coffee cup calorimeter.”
7. Calorimetry
We use an insulated device called a
calorimeter to measure this heat transfer.
A typical device is a “coffee cup calorimeter.”
8. 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
9. 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, ΔTwater.
10. 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, ΔTwater.
We also know that water has a specific heat of
Cwater = 4.18 J/°C-g.
11. 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, ΔTwater.
We also know that water has a specific heat of
Cwater = 4.18 J/°C-g.
We can calculate the heat of reaction by:
12. 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, ΔTwater.
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 = ΔH = −qsurr = -mwater × Cwater × ΔTwater
13. 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 ΔH (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
ΔTwater = Tfinal − Tinitial = 32.0°C − 25.0°C = +7.0°C
Cwater = 4.18 J/°C-g
Calculation: mwater = 50.0 g
ΔH = −1463 J = −1.5×103 J = −1.5 kJ
14. Calorimetry
We can also do calorimetry at a constant
volume rather than at a constant pressure.
This is called “bomb calorimetry.”
15. 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.
16. 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.
17. 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.
18. 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.
19. 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.
20. 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.
21.
22. 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.
23. Summary
Two calorimeters used are ...
the coffee cup calorimeter (for constant pressure
measurements)
the bomb calorimeter (for constant volume
measurements)
