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CHAPTER 6
Energy
General, Organic, & Biological
Chemistry
Janice Gorzynski Smith
CHAPTER 6: Energy
Learning Objectives:
 Definition of Energy, Kinetic Energy, Potential Energy
 Heat transfer in reactions
 Enthalpy
 Exothermic and endothermic
 Energy unit conversions and calculations
 Bond Strength
 Energy diagrams
 How to change the rate of a reaction
 Catalysts
 Equilibrium: definition and calculations
 Re-establishing equilibrium and Le Chatlier’s principle
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Smith. General Organic & Biolocial Chemistry 2nd Ed.
Energy
Definition of Energy
Total = Potential + Kinetic
Energy
Energy
Energy
Energy is the capacity to do work.
Potential energy is stored energy.
Kinetic energy is the energy of motion.
The law of conservation of energy states that the
total energy in a system does not change. Energy
cannot be created or destroyed.
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Energy
Kinetic Energy
Kinetic Energy (KE)
Energy of motion
KE = ½mv 2
Smith. General Organic & Biolocial Chemistry 2nd Ed.
http://www.petervaldivia.com/technology/energy/
http://scienceisntscary.wordpress.com/tag/kinetic-energy/
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Energy
Potential Energy
Reactants Products
Potential Energy
= Stored energy
Exists in natural attractions and repulsions
Chemical Energy
• PE possessed by chemicals
• Stored in chemical bonds
• Breaking bonds requires energy
• Forming bonds releases energy
Smith. General Organic & Biolocial Chemistry 2nd Ed.
P
PE
R
Higher PE
Unfavorable
Unstable
P
Lower PE
Favorable
Stable
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Energy
Units of Energy
•A calorie (cal) is the amount of energy needed to
raise the temperature of 1 g of water by 1 oC.
•A joule (J) is another unit of energy.
1 cal = 4.184 J
•Both joules and calories can be reported in the
larger units kilojoules (kJ) and kilocalories (kcal).
1,000 J = 1 kJ
1,000 cal = 1 kcal
1 kcal = 4.184 kJ
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Energy
Example: Energy in a Gummy Bear
A gummy bear is 9.000 Calories (nutritional calories).
How much energy is stored in a gummy bear in units of
Joules?
9.000 Cal = 9.000 kcal x 1000 cal = 9000. cal
1 kcal
9000. cal x 4.184 J = 37660. J = 37.66 kJ
1 cal
Gummy Bear Video:
https://www.youtube.com/watch?v=6YWGnfnEmgM&src_vid=Jzoi7dJAiSc&feature=iv&annotation_id=annotation_713078
Smith. General Organic & Biolocial Chemistry 2nd Ed.
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Energy
Breaking and Forming Bonds
Breaking bonds requires energy
Forming bonds releases energy
To cleave this bond, 58
kcal/mol must be added.
H = +58 kcal/mol
Endothermic
Cl
Cl
To form this bond, 58
kcal/mol is released.
H = −58 kcal/mol
Exothermic
H is the energy absorbed or released in a
reaction; it is called the heat of reaction or
the enthalpy change.
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Enthalpy Change &
Bond Dissociation Energy
Energy
The bond dissociation energy is the H for breaking
a covalent bond by equally dividing the e− between
the two atoms.
Bond dissociation energies are positive values,
because bond breaking is endothermic (H > 0).
H
H
H
+
H
H = +104 kcal/mol
Bond formation always has negative values,
because bond formation is exothermic (H < 0).
H
+
H
Smith. General Organic & Biological Chemistry 2nd Ed.
H
H
H = −104 kcal/mol
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Energy
Bond Strength
The stronger the bond, the higher its bond dissociation E.
H indicates the relative strength of the bonds broken
and formed in a reaction:
• H negative: Exothermic reaction: more energy required to
break products then reactant bonds: products have
stronger bonds.
• H positive: Endothermic reaction: less energy required to
break products then reactant bonds: products have weaker
bonds.
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Energy
Endothermic & Exothermic
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Reactions
Energy Diagrams
For a reaction to occur, two molecules must collide
with enough kinetic energy to break bonds.
The orientation of the two molecules must be correct
as well.
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Reactions
Energy Diagrams
Ea, the energy of activation, is the difference in energy
between the reactants and the transition state. It can be
thought of as the energy barrier that must be overcome for
the reaction to occur.
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Reactions
Energy Diagrams
When the Ea is high, few molecules have enough energy to cross the
energy barrier, and the reaction is slow.
When the Ea is low, many molecules have enough energy to cross the
energy barrier, and the reaction is fast.
H is negative,
the reaction is exothermic:
Smith. General Organic & Biological Chemistry 2nd Ed.
H is positive,
the reaction is endothermic:
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Summary
Energy & Reactions
ENDOTHERMIC
E
PE
increases
as bonds
break
Reactants
PE
decreases
as bonds
form
Heat + Reactants  Products
Products have weaker bonds and a higher
energy then Reactants.
Heat is absorbed by the system.
ΔE +
ΔH +
ENDOTHERMIC
Heat absorbed
Heat released
Products
EXOTHERMIC
EXOTHERMIC
Reactants  Products + heat
Products have stronger bonds and a lower
energy then Reactants.
Heat is released by the system.
ΔE -
ΔH -
Reactions
Ex: Splitting Water
Requirments: Very Endothermic
o Need a minimum of 1.23 V to split water
o Kinetically infrared light could do this,
but the reaction is very slow
o The potential really needs to be at
least 3.0 V to utilize the full spectrum
of light
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Reactions
Rates of Reactions
Increasing the concentration of the reactants:
•Increases the number of collisions
•Increases the reaction rate
Increasing the temperature of the reaction:
•Increases the kinetic energy of the molecules
•Increases the reaction rate
A catalyst is a substance that speeds up the rate
of a reaction and can be recovered unchanged.
•Catalysts lower Activation Energy, Ea.
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Reactions
Catalysts
•The uncatalyzed reaction (higher Ea) is slower.
•The catalyzed reaction (lower Ea) is faster.
H is the same for both reactions.
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Reactions
Catalysts: Photosystem II
PQ + H2O --> PQH2 + O2 (g)
The overall reaction of Photosystem II
is the oxidation of water and the
reduction of plastoquinone.
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Reactions
Equilibrium
A reversible reaction can occur in either direction.
The forward reaction
proceeds to the right.
CO(g) + H2O(g)
CO2(g) + H2(g)
The reverse reaction
proceeds to the left.
•The system is at equilibrium when the rate of the
forward reaction equals the rate of the reverse reaction.
•The net concentrations of reactants and products
do not change at equilibrium.
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Reactions
Equilibrium
The relationship between the concentration of the
products and the concentration of the reactants is
the equilibrium constant, K.
aA + bB
equilibrium
constant = K =
cC + dD
[products]
[reactants] =
[C]c [D]d
[A]a [B]b
*Brackets, [ ], are used to symbolize concentration
in moles per liter (mol/L).
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Reactions
Equilibrium
N2(g) + O2(g)
equilibrium
constant
2 NO(g)
= K
=
[NO]2
[N2] [O2]
*The coefficient becomes the exponent.
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Equilibrium
Reactions
HOW TO Calculate the Equilibrium
Constant for a Reaction
Step [1]
A2
+ B2
Step [2]
K =
Write the expression for the equilibrium
constant from the balanced equation.
2 AB
K =
[AB]2
[A2][B2]
Substitute the given concentrations in
the equilibrium expression and calculate K.
[AB]2
[A2][B2]
=
Smith. General Organic & Biological Chemistry 2nd Ed.
[0.50]2
[0.25][0.25]
=
0.25
0.0625
=
4.0
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Reactions
Le Châtelier’s Principle
If a chemical system at equilibrium is disturbed or
stressed, the system will react in a direction that
counteracts the disturbance or relieves the stress.
Some of the possible disturbances:
1) Concentration changes
2) Temperature changes
3) Pressure changes
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Reactions
Le Châtelier’s Principle
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