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Wize University Chemistry Textbook > Thermodynamics: Part 1: (0th law, 1st law, Calorimetry, Enthalpy)

Heats of Formation Method

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Heats of Formation

Heat of formation is the amount of heat that is required to form 1 mole of a compound from its constituent elements in their natural/standard state (the state they are in under standard conditions: P=
1
atm, T=
25
oC,
1
M concentration, pH
7
.

Wize Concept
The ΔHof=0 for an element in standard state.

Wize Tip
**The following elements in their standard state should be memorized and the phase they are in is important too!

C(s) as graphite is in standard state.
I2(s)
Br2(l)
Hg(l)

Diatomic molecules (BrINClHOF)
Examples:
Cl2(g), H2(g), O2(g)


Watch Out!
If you are shown O(g) on your exam, this is NOT oxygen in its standard state. Remember oxygen in its standard state has to be gaseous AND diatomic! O2(g) would be in standard state.

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Example: The formation reaction for C4H10O(l) would be:

4C(s)+5H2(g)+12O2(g)→C4H10O(l)4C_{(s)}+5H_{2(g)}+\frac{1}{2}O_{2(g)}\to C_4H_{10}O_{(l)}

The enthalpy of formation for this compound is -327kJ/mol.
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Calculating the Enthalpy of Reaction Using Heats of Formations

If you know the enthalpies of formation for each reactant and product in any chemical equation, you can find the enthalpy change for that reaction:

ΔHorxn=[(∑nΔHofproducts)−(∑nΔHofreactan⁡ts)]\boxed{\Delta H^orxn=\left[\left(\sum_{ }^{ }n\Delta H^of_{products}\right)-\left(\sum_{ }^{ }n\Delta H^of_{reac\tan ts}\right)\right]}

In other words, Δ= Final - Initial

Example: Calculating the Enthalpy of a Reaction

Calculate ΔHorxn for the following reaction:
2NO(g) + O2(g) ⇋ 2NO2(g)2NO(g)\ +\ O_2(g)\ \leftrightharpoons \ 2NO_2(g)
ΔHof(NO(g))=90.75kJ/mol
ΔHof(NO2(g))=33.18kJ/mol
ΔHof(O2(g))=
0


ΔHorxn=[(∑nΔHofproducts)−(∑nΔHofreactan⁡ts)]\boxed{\Delta H^orxn=\left[\left(\sum_{ }^{ }n\Delta H^of_{products}\right)-\left(\sum_{ }^{ }n\Delta H^of_{reac\tan ts}\right)\right]}
ΔHorxn=[(2molx33.18kJ/mol)-(2molx90.75kJ/mol + 0)]
ΔHorxn=[(66.36kJ-181.5kJ)]
=-115.14 kJ
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Practice: Formation Reaction

Write a reaction for the heat of formation for dimethylaminopyridine, (CH3)2NC5H4N.

Practice: Calculate the Enthalpy of the Reaction

Find ΔHorxn for the following reaction at 25°C:
C6H10O3(l)+7O2(g)→6CO2(g)+5H2O(l)C_6H_{10}O_{3\left(l\right)}+7O_{2\left(g\right)}\to6CO_{2\left(g\right)}+5H_2O_{\left(l\right)}
Given the following data:

Extra Practice
Find the standard molar enthalpy of combustion of C2H5OH using the standard enthalpy of formations below.
ΔHf(C2H5OH(l))=−277.7 kJ/mol\Delta H_f(C_2H_5OH_{(l)}) = -277.7\ kJ/mol
ΔHf(CO2(g))=−393.5 kJ/mol\Delta H_f(CO_{2(g)}) = -393.5\ kJ/mol
The standard enthalpy of formation for the amino acid glycine C2H5NO2(s) is –528 kJ/mol at 25°C. Which reaction below has an enthalpy change equal to this value?
Which of the following represents a proper formation reaction for the product indicated? (select all that apply)
Liquification of fuels is an important process that allows fuel to be transported more safely and more easily around the world. Oxidizing methane transforms it from a gas to a liquid, methanol, which is much easier to transport. Using the data provided calculate ΔH\Delta Hfor the oxidation of methane (CH4) shown below
CH4(g) + 1/2 O2(g)→CH3OH(l)ΔHfo(CH4(g))=−74.9kJ mol−1ΔHfo(CH3OH(l)=−238.6 kJ mol−1\begin{array}{c} CH_{4(g)}\ +\ 1/2\ O_{2(g)} \rightarrow CH_3OH_{(l)} \\ \\ \Delta H^o_f(CH_{4(g)})=-74.9 kJ\ mol^{-1} \\ \Delta H_f^o(CH_3OH_{(l)}=-238.6\ kJ\ mol^{-1} \end{array}
Heats of Formation

Heats of Formation
How much heat is required for the following reaction to go given that we start with 37g of CO2(g)?
CO2(g) + 2H2O(l) --> CH4(g) + 2O2(g) ∆H= +890.4kJ/mol
Heats of Formation

Enthalpy of Formation
Calculate ΔHf\Delta H_f of SO2SO_2
S+1.5O2→SO3ΔH=−395.8kJ2SO2+O2→2SO3ΔH=−198.2kJ\begin{array}{}S+1.5 O_2\rightarrow SO_3\quad \Delta H=-395.8 kJ\\2SO_2+O_2\rightarrow 2SO_3\quad \Delta H=-198.2kJ\end{array}
Find ΔHo for the following reaction if 12.0 g of N2H4 is consumed in excess N204.
2N2H4(l)+N2O4(g)→3N2(g)+4H2O(l)2N_2H_{4(l)}+N_2O_{4(g)}\to 3N_{2(g)}+4H_2O_{(l)}
The following enthalpies of formation are provided:
Find the ΔH for the atomization of gaseous propane (CH3CH2CH3). The bond enthalpy for C-H bonds is 414 kJ/mol and the bond enthalpy for C-C bonds is 347 kJ/mol.
Please report your answer to four significant figures in kJ/mol. Do NOT include units in the answer field
Enthalpy of Reactions
Practice: Formation Reaction
Write a reaction for the heat of formation for dimethylaminopyridine, (CH3)2NC5H4N.