Enthalpy Changes
Measuring and Expressing ∆H
Calorimetry
• When you complete this presentation, you will be able to
• Define enthalpy
• Define calorimetry
• Describe how to use a “coffee cup” calorimeter to
determine the change in enthalpy of a reaction and the
specific heat of a material.
• Describe the parts of a bomb calorimeter
Objectives
• 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.
Introduction
• 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.
• A typical device is a “coffee cup” calorimeter.
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.
• 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
Calorimetry
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 ρ = 1.00 g/mL.
Vfinal = VHCl + VNaOH = (25.0 + 25.0) mL = 50.0 mL
ρwater = 1.00 g/mL
∆Twater = Tfinal − Tinitial = 32.0°C − 25.0°C = +7.0°C
Cwater = 4.18 J/°C-g
mwater = Vfinal × ρwater = (50.0 mL)(1.00 g/mL) = 50.0 g
∆H = −m × C × ∆T = −(50.0 g)(4.18 J/°C-g)(+7.0°C)
∆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.
• This is called “bomb” calorimetry.
• A sample is placed in the crucible.
• Oxygen is introduced into the
chamber.
• The lid is tightened and the chamber
is placed in a water bath.
• The ignition coil ignites the sample.
• The heat generated in the chamber is
transferred to the water.
• The change in temperature is then
measured on the thermometer.
Calorimetry
• 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)
Summary