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
EX. 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?
SPECIFIC HEAT CAPACITY
- the symbol is C
- the units are J/g˚C or Jg-1˚C-1
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
- When Tf is greater than Ti :
- When Tf is less than Ti:
EX. 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 (q) for the water?
b) How much energy is lost by the Pb?
c) Calculate the CPb.
EX. 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?
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)
2)
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( ie the calorimeter is perfectly
insulated)
See Diagram
EX. 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.
EX. 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
- the process occurs at constant volume
- used to measure the heat released in a combustion reaction
See Diagram
q observed = C’ΔT =
- q combustion
NOTE:
EX. 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?