Transcript Document

17.4 Calculating Heats of Reaction > Standard Heats
of Formation
For a reaction that occurs at standard
conditions, you can calculate the heat
of reaction by using standard heats of
formation.
• The standard heat of reaction is the
difference between the standard
heats of formation of all the reactants
and products.
ΔH° = ΔHf°(products) – ΔHf°(reactants)
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17.4 Calculating Heats of Reaction >
Sample Problem 17.8
Calculating the Standard Heat
of Reaction
What is the standard heat of
reaction (ΔH°) for the reaction
of CO(g) with O2(g) to form
CO2(g)?
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17.4 Calculating Heats of Reaction >
Sample Problem 17.8
First write the balanced equation.
2CO(g) + O2(g) → 2CO2(g)
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17.4 Calculating Heats of Reaction >
Sample Problem 17.8
Second, find the enthalpy values from a table
2CO(g) + O2(g) → 2CO2(g)
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17.4 Calculating Heats of Reaction >
Interpret Data
Standard Heats of Formation (ΔHf°) at 25 °C and 101.3 kPa
Substance
ΔHf°
(kJ/mol)
Substance
Al2O3(s)
–1676.0
F2(g)
0.0
NO(g)
90.37
Fe(s)
0.0
NO2(g)
33.85
–822.1
NaCl(s)
Br2(g)
30.91
Br2(l)
0.0
Fe2O3(s)
C(s, diamond)
1.9
H2(g)
C(s, graphite)
0.0
–74.86
CH4(g)
H2O(g)
–241.8
O3(g)
142.0
H2O(l)
–285.8
P(s, white)
–187.8
P(s, red)
CO2(g)
–393.5
I2(g)
62.4
–1207.0
I2(s)
–635.1
N2(g)
0.0
–411.2
0.0
H2O2(l)
Cl2(g)
ΔHf°
(kJ/mol)
O2(g)
–110.5
CaO(s)
Substance
0.0
CO(g)
CaCO3(s)
5
ΔHf°
(kJ/mol)
NH3(g)
0.0
–18.4
S(s, rhombic)
0.0
0.0
S(s, monoclinic)
0.30
0.0
SO2(g)
–296.8
–46.19
SO3(g)
–395.7
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17.4 Calculating Heats of Reaction >
Sample Problem 17.8
The enthalpy values from the table
2CO(g) + O2(g) → 2CO2(g)
CO(g)
–110.5
O2(g)
0.0
CO2(g)
6
–393.5
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17.4 Calculating Heats of Reaction >
Sample Problem 17.8
Plug the values into the formula
2CO(g) + O2(g) → 2CO2(g)
ΔH° = ΔHf°(products) – ΔHf°(reactants)
CO(g)
–110.5
CO2(g)
–393.5
O2(g)
0.0
∆𝐻 𝑜 = 2 −395.5 − [2 −110.5 + 0]
∆𝐻 𝑜 = −566 𝑘𝐽
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17.4 Calculating Heats of Reaction >
Calculate the standard heat of reaction
for the following:
CH4(g) + Cl2(g) → C(s, diamond) + 4HCl(g)
ΔHf°(CH4(g)) = –74.86 kJ/mol
ΔHf°(C(s, diamond)) = 1.9 kJ/mol
ΔHf°(HCl(g)) = –92.3 kJ/mol
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17.4 Calculating Heats of Reaction >
Calculate the standard heat of reaction
for the following:
CH4(g) + Cl2(g) → C(s, diamond) + 4HCl(g)
ΔH°= ΔHf°(products) – ΔHf°(reactants)
ΔH°= [1.9 + 4(-92.3)] – [-74.86 + 0]
ΔH°= –367.3 – (–74.86 ) = –292.4 kJ
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