Hess’s Law of Heat Summation
Hess’s law
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Chemists can determine the enthalpy change of any
reaction using an important law, known as Hess’s
law of heat summation.
The enthalpy change of a physical or chemical
process depends only on the beginning conditions
(reactants) and the end conditions (products).
The enthalpy change is independent of the pathway
of the process and the number of intermediate steps
in the process. It is the sum of the enthalpy changes
of all the individual steps that make up the process.
Hess’s Law
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For example, carbon and oxygen can form carbon
dioxide via two pathways.
1. Carbon can react with oxygen to form carbon
monoxide. The carbon monoxide then reacts with
oxygen to produce carbon dioxide.
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The two equations below represent this pathway.
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C(s) + 12O2(g) → CO(g) ΔH° = −110.5 kJ/mol
CO(g) + 12O2(g) → CO2(g) ΔH° = −283.0 kJ/mol
2. Carbon can also react with oxygen to produce
carbon dioxide directly.
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C(s) + O2(g) → CO2(g) ΔH° = −393.5 kJ/mol
In both cases, the net result is that one mole of
carbon reacts with one mole of oxygen to produce
one mole of carbon dioxide.
Hess’s Law
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Representing pathways using an
enthalpy diagram.
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Hess’s law allows you to determine the
energy of a chemical reaction without
directly measuring it.
Use Hess’s law to calculate the enthalpy
change of a chemical reaction:
1. by combining chemical equations
algebraically
2. by using the enthalpy of formation
Hess’s Law
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According to Hess’s law, the pathway
that is taken in a chemical reaction has
no effect on the enthalpy change of the
reaction.
One way is to add equations for
reactions with known enthalpy changes,
so that their net result is the reaction
you are interested in.
Hess’s Law
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For example, you can combine
thermochemical equations (1) and (2)
below to find the enthalpy change for
the decomposition of hydrogen
peroxide, equation (3).
(1) H2O2(l) → H2(g) + O2(g)
(2) H2(g) + 12O2(g) → H2O(l)
(3) H2O2(l) → H2O(l) + 12O2(g)
ΔH° = +188 kJ/mol
ΔH° = −286 kJ/mol
ΔH° = ?
Hess’s Law
Hess’s Law
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1.
2.
In many cases, however, you will need to
manipulate the equations before adding them. There
are two key ways in which you can manipulate an
equation:
Reverse an equation so that the products become
reactants and the reactants become products. When
you reverse an equation, you need to change the
sign of ΔH° (multiply by −1).
Multiply each coefficient in an equation by the same
integer or fraction. When you multiply an equation,
you need to multiply ΔH° by the same number.
Hess’s Law
Practice
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P 681 #11-14
Using Standard Molar
Enthalpies of Formation
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the following equation shows the
formation of liquid water from its
elements under standard conditions.
H2(g) + 12O2(g) → H2O(l) ΔH°f = −285.8 kJ/mol
Practice
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P 685 #15-18
Calculating Enthalpy Changes
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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.
ΔH°rxn = Σ(nΔH°f products) − Σ(nΔH°f reactants)
In this equation, n represents the molar
coefficient of each compound in the balanced
chemical equation and Σ means “the sum of.”
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CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)
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ΔH°rxn = [(ΔH°f of CO2(g)) + 2(ΔH°f of H2O(g))]
− [(ΔH°f of CH4(g)) + 2(ΔH°f of O2(g))]
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ΔH°rxn = [(−393.5 kJ/mol) + 2(−241.8 kJ/mol)]
− [(−74.8 kJ/mol) + 2(0 kJ/mol)]
= −802.3 kJ/mol of CH4
Use heats of formation to find
heat of reaction
solution
Practice
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P 687 #19-22
Using Bond Energies
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Breaking bonds is an exothermic
process, while making bonds is an
endothermic process.
A specific amount of energy is needed
to break each type of bond-- bond
energy
Bond energy is usually measured in
kJ/mol.
Bond Energies
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Every chemical reaction involves bond
breaking and bond formation.
Since there are different types of bonds in the
reactants and products, the total energy
required for bond breaking and the total
energy released when new bonds form are
different.
The difference represents the energy change
for the reaction.
Bond Energies
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To use bond energies to estimate the
enthalpy change for a reaction:
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add together the total bond energies of the
reactants. Bond breakage requires energy,
so the sign will be positive.
From the total, subtract the total bond
energies of the products.
Bond energies
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Since bond energies are only a way of
estimating the energy of a reaction,
your answer will not agree exactly with
the recognized enthalpy of reaction.
Bond energies are not exact; rather,
they are averages.
Practice
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P 690 #23-26
Section Review: At this point you should
be able to answer p 691 # 1,3-7