Calorimetry
AP Chemistry
Calorimetry

Calorimetry is the measurement of
heat flow.

It allows us to calculate the amount
of energy required to heat up a
substance or to make a substance
change states.
Calorimetry Terms
 Molar
Heat of Fusion — The heat
absorbed by one mole of a
substance when changing from a
solid to a liquid.
 For water, it = 6.0 kiloJoules/mole
 Heat
of solidification is opposite of
heat of fusion (heat is released).



Molar Heat of Vaporization — The heat
absorbed by one mole of a substance when
changing from a liquid to a gas.
For water, it = 40.7 kiloJoules/mole.
Heat of condensation is the opposite of heat
of vaporization (heat is released)
Heat Required For a Phase Change – Or Latent Heat
Process
Heat Absorbed or Released = q
q = (moles) x (Molar Heat
Fusion/Vaporization)
Calculating Heat Required To
Change State



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Example #1: How much heat is needed
to melt 56.0 grams of ice into liquid (the
molar heat of fusion for ice is 6.0
kJ/mol)?
56.0 g
1 mole H2O
6.0 kJ =
18.0 g
1 mole
= 18.7 kJ will be absorbed
q = + 18.7kJ
Example #2

How much heat energy will be released
when 200grams steam condenses back to
a liquid water?

Molar Heat
condensation
= 40.7kJ/mol
q = (moles) x (Molar Heat condensation)
200gram
1 mole
40.7 kJ
18gram
1 mole
So, q = - 452kJ
= 452 kJ released
Heating a Substance with
No Phase Change




Also known as a sensible heat process.
Depends on Specific Heat Capacity
Specific Heat Capacity--The amount of
energy required to raise one gram of a
substance one degree Celcius.
Every substance and every phase of that
substance has its own unique specific
heat capacity.
Water’s Specific Heat (as a liquid)
= 4.184 Joules/gram oC
or
unit cans be J/gK
Specific heat (J/goC) = heat capacity
(J/oC)
 You will see both terms used.
 You can also use Molar Heat Capacity
 This is the heat energy required to raise 1
mole of a substance 1oC or 1K.
 Molar heat capacity can be determined
from the specific heat by multiplying it by
the molar mass of the substance.

Energy to Change Temperature
Heat
Measured in Joules
Change in Temperature
Tfinal – Tinitial
In OCelcius or kelvin
q = (mass) ( C ) ( T )
Mass
In grams
Specific Heat
Capacity
Exothermic vs. Endothermic

Exothermic Change – Heat Energy is released
to the surroundings. q and ∆H are (-)
– Molecules slow down, extra energy is
transferred to surrounding.
– Cooling phase changes are exothermic
•Endothermic Change – Heat energy is absorbed
by the system. q and ∆H are (+)
-Molecules move faster as they absorb energy
-Phase changes that require energy (or heat) are
endothermic
Changes In State
Solid
Freezing
Sublimation
Deposition
Melting
Vaporization
Liquid
Condensation
Gas
Example #3

How much energy is needed to heat
80grams of water from 10oC to 55oC at
constant pressure?
q=mC
T
= m C (Tfinal – Tinitial )
q = (80grams) ( 4.184 J/goC) (55oC – 10oC)
q = + 15062 joules
divide by 1000 to get kilojoules
15062 J
1 kJ
1000J
=
15.06 kJ absorbed

Example #4 -How much energy is needed to
change 150grams of ice from 0oC to 50oC?
This problem requires two steps. Since water is solid ice at
0oC, we need to melt the ice and then heat it up to 50oC.
Step 1 – Calculate heat required to melt 150grams ice
150g
1 mole
18grams
6.0 kJ
1 mole
=
50 kJ
Step 2 - Calculate heat required to heat liquid water from 0oC to 50oC
q = mC
T
= (150g)(4.184 J/goC)(50oC)
= 31380 J
 convert to kJ = 31.38kJ
*Add both heat values together for your final answer
50 kJ + 31.38kJ = 81.38 kJ heat absorbed.
Multiple Step Calorimetry Problems
•Add each individual energies together for total
Use
e
q = Moles x Molar Heat vap/fus
Vaporization
c
b
Solid
Heats
Gas
Heats
d
Liquid
Heats
melting
a
Use q = mC
qtotal = a + b + c + d + e
T
Calorimetry

Experimental way of measuring heat
generation/consumption by essentially catching
all the heat energy in a water bath or water bath
+ metal apparatus.

Coffee Cup Calorimetry
Styrofoam cup insulates the contents so any
heat generated or consumed in the water can be
measured by the temperature change
 q = -(Hrxn) = mCH2O∆T
 Reactions must take place in water, then you
measure the change in temperature.

Bomb-Calorimeter
Used to measure heats of reaction.
 Usually do combustion reactions in it.
 Metal innards absorb heat. You have to
keep track of heat absorbed by metal and
water.
 q = mCH2O∆T + Cp ∆T

qwater
+ qapparatus


Cp is the heat capacity of the apparatus –
determined experimentally.
Conservation of Energy
Under conditions of constant pressure
Heat Lost = Heat gained
qp-lost = qp-gained
For a Reaction heat gained by
water = heat released by
reaction.
Example- When 100mls of 0.1M HCl is
mixed with 100mls of 0.1M NaOH in a
coffee cup calorimeter, the temperature
increases from 25oC to 29.8oC. Assume
that the coffee cup is a perfect insulator,
pressure is constant and the density of the
solution is 1g/ml and specific heat capacity
for the mixture is that of water, 4.18 J/gK.
 What is the enthalpy change for the
reaction?

∆H = q = mC∆T
Since density is 1g/mL , all volumes are gram
values.
 Total volume must be used = 200mLs =
200grams.
 ∆T = Tfinal – Tinitial = 29.8oC – 25oC = 4.8oC

q = mC ∆T = (200g)(4.18J/gK)(4.8K)
 q = 4013 J = 4.013 kJ
 ∆H = - 4.013 kilojoules
 Since temperature of water increased, it is
exothermic, which means a - ∆H
