2.2 Heat of Formation - Prairie Spirit Blogs

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Thermodynamics
1.2.2 Heat of Formation
2.2 Heat of Formation

Standard Heat of Formation ΔHof
 the amount of energy
gained or lost when
1 mole of the substance is
formed from its elements
under standard conditions
(25°C, 1 atm = 101.3 kPa)
Standard Heat of Formation
Ex. the formation reaction for liquid water is
described by the following equation:
 H2 (g) + ½O2 (g) → H2O(l) + 285.8 kJ
 The standard heat of formation is: 285.8
kJ. Since the reaction is exothermic: ΔHof
-285.8 kJ.

A heat of formation is a type of reaction
where one mole of the compound forms from
its elements
 The heat of formation for pure elements, such
as H2(g), O2(g), Al(s), etc. is 0 kJ/mole.You'll
find it useful to remember this.


Look over the Heat of formation table

See values in outline
Writing Heat of Formation Reactions

1.
2.
3.
Keep the following points in mind:
Balance the equation so that one mole of the
compound is produced.
Remember the diatomic (7) molecules and write
them correctly (H2, N2, O2, F2, Cl2, Br2, I2).
The reactants must be elements, not
polyatomic ions.
Examples of polyatomic ions are hydroxide, OH-,
carbonate, CO32-, and ammonium, NH4+.
Review


H2(g) + ½O2(g) → H2O(l) + 285 kJ.
If 285.8 kJ of energy are released during the
formation of one mole of H2O(l), how much energy
do you imagine would be released if two moles of
water were produced?
If you predicted 571.6 kJ of energy you're
right! Our formation reaction tells us that
285.8 kJ of energy are released for every one
mole of H2O. This can be written
mathematically as:
285.8 kJ/mol H2O×2 mol H2O =571.6 kJ
 Our new equation looks like this:
 2 H2(g) + O2(g) → 2 H2O(l) + 571.6 kJ

Practice problems

1. Write heats of formation reactions for each of the

following compounds. Be sure to include the energy term
with the equation, either as part of the equation or
separately as Δ H.You will need to refer to a Table of
Thermochemical Data.
CO2 (g),
CuCl2 (g),
CuCl (g),
N2H4 (l),
NH4Cl (s).




Answers - either format would be
acceptable as an answer





C (s) + O2 (g) → CO2 (g) + 393.5 kJ
or
C (s)
+ O2 (g) → CO2 (g)
Δ H = -393.5 kJ
Cu (s) + Cl2 (g) → CuCl2 (s) + 220.1 kJ
or
Cu (s) + Cl2 (g) → CuCl2 (s) Δ H = -220.1 kJ
Cu (s) + ½ Cl2 (g) → CuCl2 (s) + 137.2kJ
or
Cu (s) + ½ Cl2 (g) → CuCl2 (s) Δ H = -137.2 kJ
N2 (g) + 2H2 (g) + 50.6 kJ → N2H4 (l)
or
N2 (g) + 2H2 (g) → N2H4 (l) Δ H = +50.6 kJ
½N2 (g) + 2H2 (g) + ½Cl2 (g) → NH4Cl (s)+ 314.4 kJ or
½N2 (g) + 2H2 (g) + ½Cl2 (g) → NH4Cl (s)
Δ
H = -314.4 kJ
Example
The standard heat of formation, ΔHof, for sulfur
dioxide (SO2) is -297 kJ/mol. How many kJ of
energy are given off when 25.0 g of SO2 (g)
is produced from its elements?
answer

Step 1: Calculate moles SO2
a. Find the molar mass of SO2 = 32.1 + 2(16.0) = 64.1
g/mol
25.0 g
1 mol
b. moles SO2 =
1

 0.390 mol
64.1 g
Step 2: Determine kJ for 0.390 mol
We know from the question that 297 kJ of energy is
released for 1 mole of SO2 Determine how much energy will
be released for 0.390 mol of SO2:
0.39mol 297kJ
kJ 

 116 kJ
1
mol
Example
The heat of reaction for the combustion of 1
mol of ethyl alcohol is -9.50 × 102 kJ:
C2H5OH (l) + 3 O2 (g) →
2 CO2 (g) + 3 H2O (l) + 9.5 × 102 kJ
How much heat is produced when 11.5 g of
alcohol is burned?
Answer


First determine how many moles of ethyl alcohol are
being combusted.You need to begin by finding the
molar mass of C2H5OH, which is 46.0 g/mol.
moles C2H5OH = 11.5g  1mol  0.250mol
1

46.0g
From our balanced equation we see that 9.50 × 102
kJ of energy are released for every 1 mole of
C2H5OH . Now we determine how much energy will
be released for 0.250 mol:
0.250mol 9.50  10 kJ
2
kJ 

 2.38 10 kJ
1
mol
2
Example
ΔH for the complete combustion of 1 mol of
propane is -2.22 × 103 kJ:
C3H8 (g) + 5 O2 (g) → 3 CO2 (g) + 4 H2O (l)
Calculate the heat of reaction for the
combustion of 33.0 g of propane.
Answer
4. determine moles of propane actually used.
The molar mass of C3H8 is:
33.0 g
moles C3H8 =
 0.750mol
144.0 g / mol

From our balanced equation we see that 2.22 ×
103 kJ of energy are released for every 1 mole
of C3H8. Now determine how much energy
will be released for 0.750 mol:
0.750mol 2.22  103 kJ
kJ 

 1.67  103 kJ
1
mol

Assignment- practice problems
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