Standard Molar Enthalpies of Formation
Standard Molar Enthalpies
of Formation
Δ
H
f
o
Focus Questions
1) What are formation reactions?
2) What is standard molar enthalpy of
formation?
3) How do we write a formation equation?
4) How do we calculate the ∆H using the
standard molar enthalpy of formations?
5) How does this method relate to Hess’s
Law?
Formation Reactions
In a
formation reaction
, a substance is
formed from elements in their
standard states
standard states
.
From what elements is water formed?
H
2(g)
+ ½O
2 (g)
H
2
O
(l)
Δ
H
f
o
= -285.8 kJ
The enthalpy change of a formation reaction is
called the
standard molar enthalpy of
formation
,
∆H˚
f
.
Definition of ∆H˚
f
The standard molar enthalpy of
formation is the quantity of energy that
is absorbed or released when one mole
of a compound is formed directly from
its elements in their standard states.
Formation Equations and ∆H˚
f
Note
The standard enthalpies of formation of
most compounds are negative.
By definition, the enthalpy of formation
of an element in its standard state is
zero
Writing a formation equation
Always write the elements in their
standard state (l, g, or s).
A formation equation should also show
the formation of exactly one mole of
the compound of interest.
Calculating Enthalpy Changes
You can calculate the enthalpy change of a
chemical reaction by
adding the heats of
formation of the products
and
subtracting the heats of formation of
the reactants
.
∆H˚ = Σ(n∆H˚f products) -
Σ(n∆H˚f reactants)
Note
As usual, you need to begin with a
balanced chemical equation.
If a given reactant or product has a
molar coefficient that is not 1, you need
to multiply its ∆H˚f by the same molar
coefficient.
Try Solving using Standard enthalpy of formation equation…
How does this method of adding
heats of formation relate to Hess’s
law?
Solve previous question using Hess’s Law…
Note
It is important to realize that, in most
reactions, the reactants do not actually break
down into their elements and then react to
form products.
Since there is extensive data about enthalpies
of formation, however, it is useful to calculate
the overall enthalpy change this way.
Challenging Question:
When octane burns in a car engine, heat
is released to the air and to the metal of
the engine, but a significant portion is
absorb by the liquid in the cooling
system-an aqueous solution of ethylene
glycol. What mass of octane is completely
burned to cause the heating of 20kg of
ethylene glycol from 10
o
C to 70
o
C?
Assume water is produced as a gas and
that all the heat flows into the coolant. (
ethylene glycol 3.5J/g
o
C)
Formation reactions involve substances being created from elements in their standard states, with the enthalpy change known as the standard molar enthalpy of formation (Hf). This enthalpy represents the energy released or absorbed when one mole of a compound is formed from its elements in their standard states. Formation equations are written to show the formation of exactly one mole of the compound of interest, and enthalpy changes can be calculated by summing the heats of formation of products and subtracting the heats of formation of reactants, in alignment with Hess's Law.
Download Presentation
Please find below an Image/Link to download the presentation.
The content on the website is provided AS IS for your information and personal use only. It may not be sold, licensed, or shared on other websites without obtaining consent from the author. Download presentation by click this link. If you encounter any issues during the download, it is possible that the publisher has removed the file from their server.
E N D
Presentation Transcript
Standard Molar Enthalpies of Formation Hfo
Focus Questions 1) What are formation reactions? 2) What is standard molar enthalpy of formation? 3) How do we write a formation equation? 4) How do we calculate the H using the standard molar enthalpy of formations? 5) How does this method relate to Hess s Law?
Formation Reactions In a formation reaction, a substance is formed from elements in their standard states. From what elements is water formed? H2(g)+ O2 (g) H2O (l) Hfo= -285.8 kJ The enthalpy change of a formation reaction is called the standard molar enthalpy of formation, H f.
Definition of Hf The standard molar enthalpy of formation is the quantity of energy that is absorbed or released when one mole of a compound is formed directly from its elements in their standard states.
Note The standard enthalpies of formation of most compounds are negative. By definition, the enthalpy of formation of an element in its standard state is zero
Writing a formation equation Always write the elements in their standard state (l, g, or s). A formation equation should also show the formation of exactly one mole of the compound of interest.
Calculating Enthalpy Changes You can calculate the enthalpy change of a chemical reaction by adding the heats of formation of the products and subtracting the heats of formation of the reactants. H = (n H f products) - (n H f reactants)
Note As usual, you need to begin with a balanced chemical equation. If a given reactant or product has a molar coefficient that is not 1, you need to multiply its H f by the same molar coefficient.
Try Solving using Standard enthalpy of formation equation CH4(g) + 2O2(g) C02(g) + 2H2O(g) H = [(n H f of C02(g) ) + 2(n H f of H2O(g) )] [(n H f of CH4(g)) + 2(n H f of O2(g) )] H = [(-393.5 kJ/mol) + 2(-241.8 kJ/mol )] [(-74.4 kJ/mol) + 2(0 kJ/mol)] = -802.7 kJ/mol of CH4
How does this method of adding heats of formation relate to Hess s law? Solve previous question using Hess s Law (1) H2(g) + O2(g) H2O2(g) H f= -241.8 kJ (2) C(s) + O2(g) CO2(g) H f = -393.5 kJ (3) C(s) + 2H2(g) CH4(s) H f =-74.4 kJ 2 x (1) 2H2(g) + O2(g) 2H2O2(g) H f= 2(-241.8) kJ (2) C(s) + O2(g) CO2(g) H f = -393.5 kJ -1 x (3) CH4(s) C(s) + 2H2(g) H f =-1(-74.4) kJ CH4(g) + 2O2(g) 2H2O(g) + CO2(g) H f = -802.7 kJ
Note It is important to realize that, in most reactions, the reactants do not actually break down into their elements and then react to form products. Since there is extensive data about enthalpies of formation, however, it is useful to calculate the overall enthalpy change this way.
Challenging Question: When octane burns in a car engine, heat is released to the air and to the metal of the engine, but a significant portion is absorb by the liquid in the cooling system-an aqueous solution of ethylene glycol. What mass of octane is completely burned to cause the heating of 20kg of ethylene glycol from 10oC to 70oC? Assume water is produced as a gas and that all the heat flows into the coolant. ( ethylene glycol 3.5J/goC)
