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17.2 Measuring and Expressing
>
Enthalpy Changes
17.2 Measuring and Expressing
>
Enthalpy Changes
Thermochemistry
Remember: The
concept of specific
heat allows you to
measure heat flow in
chemical and physical
processes.
17.1 The Flow of Energy
17.2 Measuring and Expressing
Enthalpy Changes
17.3 Heat in Changes of State
17.4 Calculating Heats of Reaction
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17.2 Measuring and Expressing
> Calorimetry
Enthalpy Changes
& YOU
How can you measure the amount of
heat released when a match burns?
Chapter 17
1
CHEMISTRY
2
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17.2 Measuring and Expressing
> Calorimetry
Enthalpy Changes
Calorimetry:measurement of heat flow into or
out of a system for chemical & physical
processes.
•  Calorimeter: insulated device used to
measure the absorption/release of heat in
chemical or physical processes.
•  heat absorbed by system (+q)
• Foam cups
= heat released by surroundings (-q).
• Open to environment (constant pressure)
•  heat released by system (-q)
= heat absorbed by its surroundings (+q).
3
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17.2 Measuring and Expressing
> Calorimetry
Enthalpy Changes
17.2 Measuring and Expressing
> Calorimetry
Enthalpy Changes
Constant-Pressure
Calorimeters
Constant-Pressure Calorimeters
To calculate the heat absorbed or
released by the surroundings
(qsurr) :
The value of ΔH of a reaction can be
determined by measuring the heat flow
of the reaction at constant pressure.
qsurr = m x s x ΔT
q = ΔH
ΔT = Tf -Ti of water (°C)
m = mass (g)
s = specific heat (J/g°C)
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17.2 Measuring and Expressing
> Calorimetry
Enthalpy Changes
6
17.2 Measuring and Expressing
> Calorimetry
Enthalpy Changes
Constant-Pressure Calorimeters
Constant-Pressure Calorimeters
The enthalpy change for the reaction
(ΔH) can be written as follows:
The heat absorbed by the
surroundings is equal to, but
has the opposite sign of,
the heat released by the
system.
qsys = ΔH = - qsurr
q = m x s x ΔT
•  ΔH is positive for an endothermic reaction
qsurr = - qsys
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•  ΔH is negative for an exothermic reaction.
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17.2 Measuring and Expressing
> Calorimetry
Enthalpy Changes
17.2 Measuring and Expressing
>
Enthalpy Changes
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17.2 Measuring and Expressing
>
Enthalpy Changes
Sample Problem 17.3
10
Sample Problem 17.3
1 Analyze List the knowns and the unknown.
•  Use dimensional analysis to determine the
mass of the water.
When 25.0 mL of water containing 0.025 mol HCl
at 25.0°C is added to 25.0 mL of water containing
0.025 mol NaOH at 25.0°C in a foam-cup
calorimeter, a reaction occurs. Calculate the
enthalpy change (in kJ) during this reaction if the
highest temperature observed is 32.0°C. Assume
that the densities of the solutions are 1.00 g/mL
and the volume of the final solution is equal to the
sum of the volumes of the reacting solutions.
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17.2 Measuring and Expressing
>
Enthalpy Changes
Enthalpy Change in a Calorimetry
Experiment
11
& YOU
What type of calorimeter would you use to
measure the heat released when a match
burns? Describe the experiment and how you
would calculate the heat released.
Energy Content of Food Lab
9
CHEMISTRY
•  Calculate ΔT
•  Use ΔH = qsurr = m x s x ΔT
12
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17.2 Measuring and Expressing
>
Enthalpy Changes
Sample Problem 17.3
17.2 Measuring and Expressing
>
Enthalpy Changes
1 Analyze List the knowns and the unknown.
KNOWNS
swater = 4.18 J/(g·°C)
Sample Problem 17.3
2 Calculate Solve for the unknown.
UNKNOWN
First calculate the total mass of the
water.
ΔH = ? kJ
Vfinal = VHCl + VNaOH
mwater = 50.0 mL x
= 25.0 mL + 25.0 mL = 50.0 mL
1.00 g
= 50.0 g
1 mL
Ti = 25.0°C
Tf = 32.0°C
Assume that the densities of the
solutions are 1.00 g/mL to find the
total mass of the water.
densitysolution = 1.00 g/mL
13
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17.2 Measuring and Expressing
>
Enthalpy Changes
Sample Problem 17.3
14
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17.2 Measuring and Expressing
>
Enthalpy Changes
2 Calculate Solve for the unknown.
Sample Problem 17.3
2 Calculate Solve for the unknown.
Now calculate ΔT.
Use the values for mwater, swater,
and ΔT to calculate ΔH.
ΔT = Tf – Ti = 32.0°C – 25.0°C = 7.0°C
ΔHw = qsurr = mwater s water ΔT
= (50.0 g)(4.18 J/(g·oC))(7.0°C)
= 1500 J = 1.5 kJ
ΔHreaction = -1.5 kJ
15
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17.2 Measuring and Expressing
>
Enthalpy Changes
The initial temperature of the water in a
constant-pressure calorimeter is 24°C. A
reaction takes place in the calorimeter, and the
temperature rises to 87°C. The calorimeter
contains 367 g of water, which has a specific
heat of 4.18 J/(g·°C). Calculate the enthalpy
change during this reaction.
17.2 Measuring and Expressing
>
Enthalpy Changes
The initial temperature of the water in a constantpressure calorimeter is 24°C. A reaction takes
place in the calorimeter, and the temperature rises
to 87°C. The calorimeter contains 367 g of water,
which has a specific heat of 4.18 J/(g·°C). Calculate
the enthalpy change during for this reaction.
ΔHw = m x s x ΔT
= 367 g x 4.18 J/(g·°C) x (87°C – 24°C)
= 97000 J = 97 kJ
ΔHreaction = -97 kJ
17
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17.2 Measuring and Expressing
> Calorimetry
Enthalpy Changes
18
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Thermochemical
17.2 Measuring and Expressing
> Equations
Enthalpy Changes
Constant-Volume Calorimeters
•  Measuring the
temperature increase
of the water, to
calculate the quantity of
heat released during
the combustion
reaction.
19
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Thermochemical Equations
In a chemical equation, the enthalpy
change for the reaction can be written
as either a reactant or a product.
20
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Thermochemical
17.2 Measuring and Expressing
> Equations
Enthalpy Changes
Thermochemical
17.2 Measuring and Expressing
> Equations
Enthalpy Changes
Calcium oxide and water create an exothermic reaction.
Heats of Reaction
The heat of reaction is the enthalpy change
for the chemical equation as it is written.
The enthalpy change can be considered a
product.
CaO(s) + H2O(l) → Ca(OH)2(s) + 65.2 kJ
•  Heats of reaction à ΔH.
•  The physical state of the reactants and
products must be given.
•  Assume reaction is carried out at 1 atm and
25°C (room temperature).
21
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Thermochemical
17.2 Measuring and Expressing
> Equations
Enthalpy Changes
22
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Thermochemical
17.2 Measuring and Expressing
> Equations
Enthalpy Changes
A chemical equation that includes the enthalpy
change is called a thermochemical equation.
Heats of Reaction
1 mole of calcium oxide and 1 mol of
water react to form calcium hydroxide
and 65.2 kJ of heat.
CaO(s) + H2O(l) → Ca(OH)2(s) + 65.2 kJ
CaO(s) + H2O(l) → Ca(OH)2(s)
ΔH = –65.2 kJ
•  In exothermic processes, the chemical
potential energy of the reactants is higher than
the chemical potential energy of the products.
23
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Thermochemical
17.2 Measuring and Expressing
> Equations
Enthalpy Changes
Heats of Reaction
Thermochemical
17.2 Measuring and Expressing
> Equations
Enthalpy Changes
Heats of Reaction
Baking soda (sodium bicarbonate) decomposes
heated. This process is endothermic.
2NaHCO3(s) + 85 kJ → Na2CO3(s) + H2O(l) + CO2(g)
2NaHCO3(s) + 85 kJ → Na2CO3(s) + H2O(l) + CO2(g)
2NaHCO3(s) → Na2CO3(s) + H2O(l) + CO2(g)
ΔH = 85 kJ
ΔH is positive for endothermic reactions.
•  The decomposition of 2 mol of sodium
bicarbonate requires 85 kJ of heat.
•  The decomposition of 4 mol would require twice
as much heat, or 170 kJ.
• 170 kJ = (85 kJ x 2)
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17.2 Measuring and Expressing
>
Enthalpy Changes
Sample Problem 17.4
26
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17.2 Measuring and Expressing
>
Enthalpy Changes
Sample Problem 17.4
1 Analyze List the knowns and the unknown.
Use the thermochemical equation to write a
conversion factor relating kJ of heat and moles of
NaHCO3. Then use the conversion factor to
determine ΔH for 2.24 mol NaHCO3.
Using the Heat of Reaction to Calculate
Enthalpy Change
2NaHCO3(s) + 85 kJ → Na2CO3(s) + H2O(l) CO2(g)
Calculate the amount of heat (in kJ)
required to decompose 2.24 mol
NaHCO3(s).
KNOWNS
amount of NaHCO3(s) that decomposes = 2.24 mol
ΔH = 85 kJ for 2 mol NaHCO3
UNKNOWN
ΔH = ? kJ for 2.24 mol NaHCO3
27
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17.2 Measuring and Expressing
>
Enthalpy Changes
Sample Problem 17.4
17.2 Measuring and Expressing
>
Enthalpy Changes
2 Calculate Solve for the unknown.
Sample Problem 17.4
2 Calculate Solve for the unknown.
Write the conversion factor relating kJ of
heat and moles of NaHCO3.
Using dimensional analysis, solve for ΔH.
85 kJ
ΔH = 2.24 mol NaHCO3(s) x 2 mol NaHCO (s)
3
= 95.2 kJ
85 kJ
2 mol NaHCO3(s)
The thermochemical equation
indicates that 85 kJ are needed
to decompose 2 mol NaHCO3(s).
29
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17.2 Measuring and Expressing
>
Enthalpy Changes
30
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Thermochemical
17.2 Measuring and Expressing
> Equations
Enthalpy Changes
Heats of Reaction
Why are the physical state of the reactants and
products important?
Total
energy
released
31
H2O(l) → H2(g) +
Total energy
absorbed
H2O(g) → H2(g) +
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32
1
2
1
2
O2(g)
O2(g)
ΔH = 285.8 kJ
ΔH = 241.8 kJ
difference = 44.0 kJ
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Thermochemical
17.2 Measuring and Expressing
> Equations
Enthalpy Changes
Thermochemical
17.2 Measuring and Expressing
> Equations
Enthalpy Changes
Heats of Combustion
Small amounts of natural gas within crude
oil are burned off at oil refineries.
Heats of Combustion
The heat of combustion is the heat of
reaction for the complete burning of one mole
of a substance.
CH4(g) + 2O2(g) → CO2(g) + 2H2O(l) + 890 kJ
•  Exothermic
•  Burning 1 mol of CH4 releases 890 kJ of
heat.
•  ΔH = - 890 kJ / mol CH4
33
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17.2 Measuring and Expressing
>
Enthalpy Changes
Heats of Combustion at 25°C
35
Substance
Formula
ΔH (kJ/mol)
Hydrogen
H2(g)
–286
Carbon
C(s,
graphite)
–394
Methane
CH4(g)
Acetylene
C2H2(g)
–1300
Ethanol
C2H6O(l)
–1368
Propane
C3H8(g)
–2220
Glucose
C6H12O6(s)
–2808
Octane
C8H18(l)
–5471
Sucrose
C12H22O11(s)
–5645
–890
Interpret Data
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17.2 Measuring and Expressing
>
Enthalpy Changes
Which of the following thermochemical
equations represents an endothermic
reaction?
Remember, heats of
reaction/combustion
are reported as the
ΔH when the
reactions are carried
out at STP.
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A. Cgraphite(s) + 2 kJ
B. 2H2(g) + O2(g)
36
Cdiamond(s)
2H2O + 483.6 kJ
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17.2 Measuring and Expressing
>
Enthalpy Changes
Which of the following thermochemical
equations represents an endothermic
reaction?
A. Cgraphite(s) + 2 kJ
B. 2H2(g) + O2(g)
37
Cdiamond(s)
2H2O + 483.6 kJ
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