Download Hess`s Law (Spring 2013)

Hess’s Law of Additivity of
Reaction Enthalpies
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I will be able to use proper scientific
terminology to describe Hess’s Law
I will be able to use Hess’s law to calculate
enthalpies in multiple step reactions
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describe Hess’s law
use Hess’s law to calculate enthalpies in
multiple step reactions
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Calorimetry is an excellent technique for
determining enthalpy changes, BUT is no
longer useful when reactions are too slow and
temperature changes would be too small to
measure
Chemists utilize methods to deal with this
problem based on the principle that net (or
overall) changes in some properties of a
system are independent of the way the
system changes from initial state (reactants)
to the final state (products)
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The value of the ∆H for any reaction that can
be written in steps equals the sum of the
values of ∆H for each of the individual steps.
OR
If two or more equations with known enthalpy
changes can be added together to form a new
“target” equation, then their enthalpy changes
may be similarly added together to yield the
enthalpy change of the target equation
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Written as an equation
∆Htarget = ∆H1 + ∆H2 + ∆H3 + …
∆Htarget = ∑ ∆Hknown
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Example 1
Carbon can react with oxygen to form carbon
monoxide. The carbon monoxide then reacts
with oxygen to produce carbon dioxide. The
two equations below represent this pathway.
C(s) + ½ O2(g)  CO(g)
CO(g) + ½ O2(g)  CO2(g)
∆Hº = -110.5 kJ
∆Hº = -283.0 kJ
Carbon can also react with oxygen to produce
carbon dioxide directly.
C(s) + O2(g)  CO2(g)
∆Hº = -393.5 kJ
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Hess’s Law allows chemists to determine the
enthalpy change of a reaction without direct
calorimetry, using two familiar rules for
chemical equations and enthalpy changes
1. If a chemical equation is reversed, then
the sign of the ∆H changes.
2. If the coefficients of a chemical equation
are altered by multiplying or dividing by a
constant factor, then the ∆H is altered in
the same way.
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Practice 1
A typical automobile engine uses a lead-acid battery.
During discharge, the following chemical reaction takes
place.
2PbO(s) + 2H2SO4(l)  2PbSO4(aq) + 2H2O(l)
Determine the enthalpy change of this reaction, given the
following equations.
(1)
(2)
2PbO (s) + 2SO3(g)  2PbSO4(s) ∆Hº = -775 kJ
SO3(g) + H20(l)  H2SO4(l)
∆Hº = -133 kJ
Answer: -509 kJ
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Practice 2
Mixing household cleansers can result in the
production of hydrogen chloride gas, HC1(g). Not
only is this gas dangerous in its own right, but it
also reacts with oxygen to form chlorine gas and
water vapour.
4HCl(g) + O2(g)  2Cl2(g) + 2H2O(g)
Determine the enthalpy change of this reaction,
given the following equations.
(1) H2(g) + Cl2(g)  2HC1(g)
(2) H2(g) + ½ O2(g)  H2O(l)
(3)
H2O(g)  H2O(l)
∆Hº = -185 kJ
∆Hº = -285.8 kJ
∆Hº = -40.7 kJ
Answer: -120.2 kJ
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Practice
pp. 326-327 UC # 1, 2, 3
Multi-Step Enthalpy Practice
p. 329 UC # 4, 5
Additional Practice
p. 330 UC # 1, 2, 3
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