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Calorimetry & Enthalpy Changes
Learning Goals:
I will be able to perform calorimetry and calculate the thermal energy
transfers (q)
I will learn to calculate enthalpy (H)
I will understand the terms calorimetry, enthalpy and bond energy
Do you remember….
• What is heat?
• What is the symbol for heat?
• What are the three types of energy changes?
Measuring Energy Changes:
Calorimetry Calculations
Specific Heat Capacity
• The amount of energy required to raise the temperature of
1g of a substance by 1°C or 1K
• Symbol is ‘c’
• The units are J/g°C
• A substance with a HIGH specific heat capacity
requires a lot of energy to heat it
– Eg. water
(see Table 1, pg.301)
Calorimetry & Thermal Energy Transfer
• Calorimetry – the process of measuring
thermal energy changes in a physical/chemical
change using a calorimeter.
– Any change in the thermal energy of a system is
detected as a temperature change in water.
Calorimeters
3 Types:
Coffee cup calorimeter:
– Constant volume calorimeter
(open to the atmosphere)
-reasonably accurate measurements of heat transfer
– Measure initial and final temp of the solution.
Calorimeters
Laboratory calorimeter:
– Can provide precise data for changes of state
or chemical reactions that do not involve
gases.
Bomb Calorimeter:
–Container inside which a fuel is burned
–Much more accurate measurements of heat
transfer.
–Pressure changes depending on the reaction
–Often described as ‘isolated’
Assumptions made in Calorimetry
– > There is no/negligible heat transfer between
the environment and the calorimeter.
– > The calorimeter doesn’t absorb energy (or
we are saying it is negligible
– > All dilute, aqueous solutions are assumed to
have the same density and specific heat capacity
equal to water.
pwater= 1.00g/mL cwater = 4.18 J/g°C
Calorimetry Calculations
• q=heat transferred in a reaction to the surroundings
and...
if q is + then it is endothermic
if q is – then it is exothermic
• m=mass (in grams)
• c=specific heat capacity (in J/(g °C) see chart in
textbook)
• ΔT=the change in temperature. (measured in Celsius of Kelvin)
ΔT=T2 ‐T1
Calorimetry cont…
• Therefore:
HEAT LOST by the PROCESS = HEAT GAINED by the
WATER
OR
HEAT GAINED by the PROCESS = HEAT LOST by the WATER
Sample Problem #1
• A student places 125mL of water at 20°C into a calorimeter
and adds a sample of silver at 125°C. The final temperature
of the water is 22.5°C. Calculate the quantity of thermal
energy absorbed by the water.
Sample Problem 2
Many water heaters use the combustion of natural gas (assume
methane) to heat the water in the tank. When 150.0 L of water at
10.0°C is heated to 65.0°C, how much heat flows into the
water?
m = 150L x 1kg/L
= 150 kg
= 150 000 g
c = 4.18 J/g°C
T = (65-10) = 55°C
q = mcΔT
= 150 000g x 4.18 x 55.0
= 34485000
q = 3.45 x 107 J
Enthalpy
• What is enthalpy (H)?
– Total amount of thermal energy in a substance
Molar enthalpy change (ΔHr) [r is reaction type]
– Enthalpy change that occurs when 1 mol of a
substance undergoes a physical, chemical or nuclear
change (kJ/mol)
– ****This value is given – in charts in the back of your textbook
& pg307
ΔH = nΔHr
Sample Problem #3
• What amount in g of ethylene glycol would
vaporize while absorbing 200.0 kJ of heat?
• (ΔHvap = 65.6 kJ/mol)
ry, Enthalpy and Bond Energies.notebook
Enthalpy Change
q … meet ΔH…
Decem
• Enthalpy change(ΔH): energy change that
Enthalpy
( H): energy
change
occurs in achange
system during
a reaction
(kJ orthat
J)
occurs in a system during a reaction (kJ or J)
• ΔH
– endothermic
is +ve;-ve
exothermic
is -ve
H +ve
is endothermic,
is exothermic
ΔH and q
Dec 4-2:42 PM
• Under constant pressure, the enthalpy change
of a system is equal to the flow of thermal
energy in and out of the system
system
system
Dec 4-2:42 PM
6
Heat Transfer and Enthalpy Change
It is not possible to measure all of these energies for a system
Instead, we study the energy absorbed or released to the
surroundings during a change in the system – the change in
enthalpy, ΔH
Changes in Kinetic and Potential Energy
high potential energy
high kinetic energy
This example of
exothermic
shows that the
change in
energy of the system (ΔH) equals the
kinetic energy of the surroundings (q)
change
potential
change in
ΔH
Energy
low kinetic energy
q
low potential energy
Reaction Progress
ΔHsystem = ±lq
l
Sample Problem #4
• In a calorimetry experiment, 3.50 g of potassium chloride is
dissolved in 200.0mL of water at an initial internal
temperature of 23.2°C. The final temp is 19.5°C. What is
the molar enthalpy of potassium chloride?
Bond Energies
Apr 3-12:33 PM
• Bond dissociation energy: the energy
required to break a chemical bond (kJ/mol)
• You can break covalent bonds, if you supply
enough energy...that energy is called bond
dissociation energy
• Table 1 on Page 307 shows several bond
energies for single bonds (multiple bonds are
on Pg. 308)
• They are measured in kJ/mol, for example it
takes 413 kJ to break 1 mol of C‐H bonds