Calculation OF Enthalpy changes

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CALCULATION OF
ENTHALPY CHANGES
Molar Enthalpy


the enthalpy change for 1 mole of a substance
associated with a chemical, physical or nuclear
change
the units will J or kJ per mole
Example
A common refrigerant , Freon-12, has a molar mass of
120.91 g/mol. The molar enthalpy to vaporize
Freon is 34.99 kJ/mol. If 500 g of Freon is
vaporized, what is the enthalpy change?
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Specific Heat Capacity
the quantity of heat needed to change the
temperature of 1 g of substance by 1˚C
 the symbol is C
 the units are J/g˚C or Jg-1˚C-1
EX.
C water = 4.18 J/g˚C

Specific Heat Capacity

the quantity of heat needed to change the temperature of 1 g of substance
by 1˚C

the symbol is C

the units are J/g˚C or Jg-1˚C-1
EX.
C water = 4.18 J/g˚C
Specific heat capacity is a measure of how ‘easily’ a substance
changes temperature. A substance with a high heat capacity
can absorb more energy without changing temperature than a
substance with low specific heat.
Heat Transfer Problems
ΔH= mCΔT
Enthalpy Change
(J)
Mass of the substance
changing temperature
(g)
Change in
Temperature
Tf –Ti (°C)
Specific
Heat Capacity
(Jg-1°C-1 )
Heat Transfer Problems
ΔH= mCΔT
Enthalpy Change
(J)

Mass of the substance
changing temperature
(g)
Change in
Temperature
Tf –Ti (°C)
Specific
Heat Capacity
(Jg-1°C-1 )
When Tf is greater than Ti :
-energy is absorbed
- the process is ENDOthermic
- ΔH is positive
Heat Transfer Problems
ΔH= mCΔT
Enthalpy Change
(J)

Mass of the substance
changing temperature
(g)
Change in
Temperature
Tf –Ti (°C)
Specific
Heat Capacity
(Jg-1°C-1 )
When Tf is less than Ti :
-energy is released
- the process is EXOthermic
- ΔH is negative
Examples
1) A 150 g sample of Pb at 100.0 °C is plunged into
a beaker containing 50 g of water at 22.0 °C. The
final temperature of the mixture is 28.8 °C.
a) What is the energy change for the water?
b) How much energy is lost by the Pb?
c) Calculate the CPb
On Board
Examples
2) While running an adult expends 5.0 x 105 J/mile.
If this energy could be transferred as heat to a
beaker of water, how many grams of water could
be heated from 25.0 °C to 100.0 °C?
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CALORIMETRY
Calorimeter – an insulated device used to determine
temperature changes during a chemical or physical
process.
 the calorimeter is part of the surroundings
Two Kinds: 1) Simple
2) Bomb
Simple Calorimetry

the process occurs at constant pressure
ΔHobserved = mCΔT =

- ΔH rxn
RECALL that ΔH is + for an ENDOTHERMIC change
and – for an EXOTHERMIC
Assumptions



for a dilute aqueous solution, use Cwater = 4.18
J/g°C as the heat capacity for the solution
1 mL of solution = 1 g of solution
no heat is transferred to the outside of the
calorimeter( i.e., the calorimeter is perfectly
insulated)
Diagram
insulated lid
thermometer
insulated cup
reaction mixture
Example
1) 5.00 mL of 1.0 mol/L NaOH and 5.00 mL of 1.0
mol/L HCl are mixed in a simple calorimeter. The
initial temperature of each substance is 23.0 °C.
After mixing, the temperature rose to 27.5 °C.
a) What is the enthalpy change for this rxn?
b) Calculate the energy change per mole of HCl.
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Example
2) 1.50 g of NH4NO3 is added to 35.0 g of water in
a simple calorimeter. The initial temperature of the
water is 22.7°C and the final temperature of the
mixture is 19.4 °C. Calculate the enthalpy change
per mole of salt.
Bomb Calorimetry
Bomb Calorimetry
Not Really !
Bomb Calorimetry


the process occurs at constant volume
used to measure the heat released in a combustion
reaction
Diagram
Bomb Calorimetry


the process occurs at constant volume
used to measure the heat released in a combustion
reaction
q observed = C’ΔT = - q combustion
J °C -1

NOTE: q is used in place of ΔH for an energy
change at constant volume
Example
A food manufacturer wishes to know the number of calories
per serving of a new dessert. They hire a chemist at the
Dep’t of Consumer Affairs to help them out.
a) The chemist places a 1.00 g sample of the dessert in a
bomb calorimeter. The heat capacity of the calorimeter is
8.151 kJK-1. The temperature increases by 4.937 °C.
Calculate the number of food calories (kcal) in the sample
of dessert.
b) A single serving of the dessert weighs 30.00 g. How many
food calories are in the dessert?
On Board

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