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1
CALORIMETRY
p. 661-667 (simple), 673-675 (bomb)
Calorimeter:
• A device in which the heat associated with a specific process is measured.
•
There are two basic types of calorimeters:
1. Constant-pressure calorimeter: also called simple calorimeters
2. Constant-volume calorimeter: also known as bomb calorimeters
Constant-Pressure Calorimeter
•
Used to measure the heat absorbed or released by a process or reaction
taking place at constant pressure.
•
Examples: The heat of acid-base neutralizations or the heat associated
with dissolving a salt in water can be measured.
•
A constant-pressure calorimeter can be constructed using styrofoam cups.
• This is called a coffee-cup calorimeter.
•
A coffee-cup calorimeter is considered to be an isolated system.
•
If an exothermic process occurs, the heat released will be absorbed by
the water in which the reaction took place and by the calorimeter.
•
If an endothermic process takes place, heat will be absorbed from the
water and from the calorimeter.
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•
Recall from the First Law of Thermodynamics that:
qsystem = -qsurroundings
•
For a process in a simple calorimeter, the system is the chemical reaction,
and the surroundings are the water and the calorimeter, so:
qreaction = - (qcalorimeter+ qH2O)
•
For simple calorimeters, qcalorimeter is often so small that it can be omitted
leaving us with:
qreaction = - qH2O
•
Heat absorbed or released from the H2O is calculated from q = mc∆T.
Examples:
1.
10.0 g of metal X at 280.0 oC is dropped into 200.0mL of H2O at 20.0 oC
in a coffee cup calorimeter. Metal X and H2O reach thermal equilibrium
at 25.0 oC. Calculate the specific heat capacity of X.
It may help to visualize the equilibrium temperature as follows:
o
Temperature ( C)
Temperature vs. Time
300
250
200
150
100
50
0
(mcΔT)X
(mcΔT)H2O
0
5
10
15
Time (arbitrary)
20
25
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2.
4.00 g of CaCl2 (s) is dissolved in 100 g of water in a simple
calorimeter. Before CaCl2was added, the water’s temperature was
22.0 oC. When all the CaCl2 dissolved, the water was at a final
temperature of 24.2 oC. What is the heat of solution for this process?
Note:
a)
For a process where the reactants dissolve, the mass of the
surroundings is the total mass of the solution. That is:
b)
We will assume that the solution is dilute enough so that we can
use the specific heat capacity of H2O.
Note: Since qCaCl2 is negative, we know that dissolving this salt in
water (the heat of solution) is an exothermic process!
3.
In the previous example, dissolving 4.00 g of CaCl2(s) released 0.96 kJ
of energy. What is the molar enthalpy (∆H) of solution for CaCl2(s)?
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4.
Consider the following neutralization reaction.
HCl(aq) + NaOH(aq) Æ NaCl(aq) + H2O(l)
When 50.0 mL of 1.00 mol/L HCl(aq) and 50.0 mL of 1.00 mol/L NaOH(aq)
are mixed in a coffee cup calorimeter the temperature of the resulting
solution increased from 21.0 oC to 27.5 oC. What is the heat of this
reaction, measured in kJ per mole of HCl(aq)?
Note:
a) Assume that density, dsolution = dH2O (1g = 1mL)
b) Assume that specific heat capacity, csolution = cH2O
c) Total mass, msurroundings = mNaOH + mHCl
d) Assume that all heat released is absorbed by H2O.
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Constant-Volume Calorimeter
•
Constant-volume calorimeters are also called bomb calorimeters.
•
Used to measure the heat released by a combustion reaction taking place
at constant volume.
•
A bomb calorimeter consists of several basic components:
•
A substance is placed in a steel container (the bomb!)
•
The bomb is then filled with high pressure oxygen.
•
The bomb is surrounded by water, and the entire system is isolated from
the external environment.
•
The substance in the bomb is ignited electrically.
•
The resulting change in temperature of the water and the bomb is
measured at the end of the reaction.
•
A bomb calorimeter is considered to be an isolated system.
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•
Applying the First Law of Thermodynamics gives us:
qsystem = -qsurroundings
•
For a process in a bomb calorimeter, the system is the chemical reaction,
and the surroundings are the water and the calorimeter, so:
qreaction = - (qcalorimeter+ qH2O)
•
Bomb calorimeters are normally calibrated for a constant mass of water,
with the heat capacity supplied by the manufacturer. This leaves us with
the calculation:
qreaction = -(qcalorimeter)
•
The heat absorbed by the calorimeter is calculated from q= C∆T.
Example
A 1.00 g sample of sucrose (C12H22O11) is burned in a bomb calorimeter
which has a heat capacity of 7.00kJ/oC. The temperature increases from
25.0oC to 27.3oC. How much heat is released from this combustion reaction?
What is the molar enthalpy of combustion (∆Hcomb)?
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THERMOCHEMISTRY WORKSHEET #6
Calorimetry
1.
A student mixed 100.0 mL of 1.0 mol/L sulfuric acid with 100.0 mL of 1.0 mol/L potassium
hydroxide in a simple calorimeter. Both solutions were initially at 19.67oC and the highest
temperature reached by the reaction mixture was 34.06oC.
a)
Find the heat change for this reaction. (Ans: -12.44 kJ)
b) Find the molar heat of neutralization for the acid. (Ans: -1.2 × 102 kJ/mol)
2.
A 5.00 g sample of acetylene undergoes complete combustion in an oxygen bomb calorimeter.
The calorimeter has a heat capacity of 6.49 kJ/oC. The maximum temperature increase is recorded
as 9.50oC. Find the molar heat of combustion for acetylene, C2H2. (Ans: -3.21 × 102 kJ/mol)
3.
A very cold piece of silver with a mass of 75.84 g is added to a simple calorimeter that contains
150.0 mL of water. The temperature of the calorimeter water changes from 19.47oC to 16.24oC.
Note: Do not add the mass of silver to total mass since it does not dissolve in the water.
a)
Find the heat change for the piece of silver. (Ans: 2.03 kJ)
b)
How cold was the piece of silver, originally (find Ti)? (cAg = 0.237 J/goC) (Ans: -97 oC)
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4.
5.
A 2.24 g sample of ammonium nitrate is added to 78.44 mL of water in a simple calorimeter. The
sample completely dissolves and causes the water temperature to change from 22.13oC to 20.15oC.
a)
Calculate the heat change associated with dissolving the ammonium nitrate.
(Ans: 0.669 kJ)
b)
Is the process exothermic or endothermic? Explain.
c)
What assumptions are being made in your solution to the question?
d)
Find the molar heat of solution for ammonium nitrate. (Ans: 23.9 kJ/mol)
A 5.00 g sample of solid benzene (C6H6) is added to a simple calorimeter that contains 0.125 kg of
ethanol. Given that the specific heat capacity of ethanol is 2.4 kJ/kg. oC, and that the temperature
changed from 21.05oC to 19.39oC, what is the molar heat of fusion for the benzene?
Note: Ethanol is the surroundings, not water and benzene dissolves in the ethanol.
(Ans: 8.1 kJ/mol)