Energy Changes in
Chemical Reactions #1
Atoms, ions and molecules can form chemical bonds when
they collide, PROVIDED that the particles contain sufficient
KINETIC energy to overcome the repulsive forces of the
electron clouds.
Activation Energy – The minimum energy colliding particles
must have in order to react.
Heat of Reaction
Energy Changes in Chemical Reactions #2
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Energy Changes in Chemical Reactions #3
Exothermic Reactions C(s) + O2(g) ---------> CO2(g) + heat
The energy stored in the bonds
Endothermic Reactions NH4NO3(s) + H2O(l) + heat --------> NH4NO3(aq)
Sample Problem #1
When 1 mol of methane is combusted, 890.0 kJ of energy
are released. Calculate the ΔH of a reaction in calories
when
of methane are burned.
Step #1 - Write the balanced equation
Step #2 - Convert grams to moles
Step #3 - Calculate kilojoules for this number of moles
Step #4 - Convert to calories
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Sample Problem #2
The reaction that occurs in the heat packs to treat sports injuries
is the synthesis reaction with Iron and Oxygen to produce the
Iron(III) product. When the reaction is balanced 1652 kJ of heat
energy is released. How much heat is released when
of
iron is reacted with excess oxygen?
Step #1 - Write the balanced equation
Step #2 - Convert grams to moles
Step #3 - Calculate kilojoules for this number of moles
Heat of Solution
During the process of solvation heat may be
NaOH (s) + H2O (l) -----> Na+ (aq) + OH– (aq)
CaCl2 (s) + H2O (l) -----> Ca2+ (aq) + 2Cl– (aq)
NH4NO3 (s) + H2O (l)
-----> NH4+ (aq) + NO3– (aq)
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Enthalpy (Heat)
ΔH =
If ΔH is negative
CH4 + 2O2 ----> CO2 + 2H2O + 802.24 kJ
Standard Conditions for ΔH: 1 atm and 25°C
Enthalpy changes occur when
All free elements (those not bonded to a DIFFERENT element)
Standard Heat of Formation
Standard Heat of Formation
The ΔH for a reaction in which
For a reaction the ΔH of the reaction is:
All free elements (those not bonded to a DIFFERENT
element)
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Sample Problem #1
Show that for:
CH4 + 2O2 ----> CO2 + 2H2O
the correct ΔH for the reaction is – 802.24 kJ
if ΔHf0 CH4(g) =
ΔHf0 CO2(g) =
ΔHf0 H2O(g) =
Sample Problem #2
What is the enthalpy change when Sulfur dioxide
combines with Oxygen to form Sulfur trioxide?
ΔHf0 Sulfur dioxide =
ΔHf0 Sulfur trioxide =
First Write the
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Hess’s Law
Hess’s Law - The enthalpy change of individual reactions that
occur in series can be
Given: C(s) + O2 (g) -> CO2 (g)
ΔH =
CO2 (g) -> CO (g) + 1/2 O2 (g)
ΔH =
What is the ΔH for the reaction:
C(s) + O2 (g) ->
CO (g) + 1/2 O2 (g)
Sample Problem
Calculate the ΔH for the following reaction:
2C(s) + H2(g) ------> C2H2(g)
Given the following:
C2H2(g) + 5/2 O2(g) ----> 2CO2(g) + H2O(g)
C(s) + O2(g) ------> CO2(g)
H2(g) + 1/2 O2(g) ----> H2O(g)
ΔH =
ΔH =
ΔH =
Solution - Step #1
Step #2
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Sample Problem (con’t)
Step #2
Add Result of
Final Step
Reversing
Sample Problem (con’t #2)
Final Step
Add the equations from step #2 and the final step together
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Entropy (S) - Disorder
Law of Disorder - The state of a system will move naturally in
the direction of
The entropy of the phases of matter are:
Entropy (S) - Disorder
When a substance is divided into more parts,
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Entropy (S) - Disorder
Entropy
Entropy (S) - Disorder
Entropy
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Entropy Calculations
Entropy - (S)
Entropy Units - J/K
Standard Entropy – S0
Entropy change is designated by ΔS.
Standard Entropy change is designated by ΔS0.
Sample Problem
Calculate the standard entropy change when 1 mol of water
changes from a liquid to a gas?
H2O (l) -------> H2O (g)
S0 H2O(l) = 69.94 (J/K x mol)
S0 H2O(g) = 188.7 (J/K x mol)
Sample Problem
Calculate the standard entropy change when hydrogen
peroxide decomposes?
S0 H2O2(l) =
S0 H2O(l) =
S0 O2(g) =
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Free Energy
Free Energy – The energy available to
Not all energy
Spontaneous Reactions – Naturally favor the
Non-Spontaneous Reactions – Do NOT favor the
Do not confuse spontaneous reactions with
Spontaneous Reactions
A reaction may not be
Together the size and direction of the
Change in Heat
(Enthalpy-ΔH)
Change in Disorder
(Entropy-ΔS)
Spontaneous
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Free Energy Calculations
Gibbs Free Energy Change – The amount of energy that can be
coupled to another process
Sample Problem
Is the following reaction spontaneous at 25°C.
C(s) + O2(g) -------> CO2(g)
ΔHf0 Carbon dioxide =
S0 Carbon dioxide =
S0 Carbon =
S0 Oxygen =
Sample Problem Solution
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