Reaction
Energetics
Announcements
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Still grading standards 3 and 4
updated “tentative” schedule on Blackboard (under Schedule tab)
Next standards will be 5-9
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5 and 6: Reaction Energetics
7-9: Cell membranes and membrane transport
Standards 5 and 6: Reaction Energetics
5. Given biologically relevant chemical reactions, determine which
molecules or atoms within a molecule are being oxidized and which
are being reduced.
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We will focus only on how carbon is reduced/oxidized
6. Given a biologically relevant chemical reaction, determine if there
is a positive or negative change in entropy, if there is a positive or
negative change in enthalpy and solve the Gibb’s Free energy
equation
• not the real equation, but counting bond types and molecule
#/size to determine if the reaction is spontaneous (exergonic,
catabolic) or nonspontaneous (endergonic, anabolic)
Energy
• All living organisms require energy
• Life requires the transfer and transformation of energy and matter
• ENERGY:
• The ability to do work
• Vital to all biological processes
• Plants and animals rely on exogenous energy sources
Kinetic energy: energy due to motion
Potential energy: stored energy
Nonpolar bonds (weak) have HIGH POTENTIAL ENERGY
Polar bonds (strong) have LOW POTENTIAL ENERGY
Equal sharing
(nonpolar)
Unequal sharing
(polar)
Longest, weakest
bonds
Shortest, strongest
bonds
Decreasing potential energy
© 2017 Pearson Education, Inc.
Thermodynamics
• Thermodynamics is the study of energy transformations
• There are two laws of thermodynamics that biological
systems cannot escape
Thermodynamics
• 1st Law of Thermodynamics:
• The total energy in a molecule is referred to as it
ENTHALPY (H)
• Enthalpy measures the potential energy of a molecule
• Changes in enthalpy are measured can be represented by
ΔH
Thermodynamics
• 1st Law of Thermodynamics:
• When a reaction releases heat:
• ΔH is negative
• The reactants have more potential energy than
the products
• Referred to as exothermic/exergonic reactions
• Releases heat/energy and are spontaneous
Thermodynamics
• 1st Law of Thermodynamics:
• When a reaction takes up heat (or energy):
• ΔH is positive
• The products of the reaction have more
potential energy than the reactants
• Referred to as endothermic/endergonic
reactions
• Require heat/energy input and are not spontaneous
Thermodynamics
• 2nd Law of Thermodynamics:
• There is an inevitable degradation of useful energy
into heat
• Heat energy is random molecular motion of atoms
• As we have already discussed
• Every energy transformation or transfer increases
the entropy of the universe
Thermodynamics
• 2nd Law of Thermodynamics:
• Entropy (S):
• A measure of thermal randomness
• If heat is gained, entropy is increased
• Also referred to as a measure of disorder
Thermodynamics
• 2nd Law of Thermodynamics:
• Entropy (S):
Measure of disorder
• Big molecules = low disorder, therefore low entropy
• Small molecules = high disorder, high entropy
Gibbs Free Energy
• If a reaction is spontaneous 
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Does not require energetic input
exothermic/exergonic – gives off heat (energy)
Energetically favorable
It should release energy
• Use as a fuel
• If a reaction is nonspontaneous 
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It is not energetically favorable
Requires the input of energy to occur
endothermic/endergonic – requires input
Typically used to store energy
• Increase potential energy
Figure 6.6
(a) Exergonic reaction: energy released, spontaneous
Free energy
Reactants
Amount of
energy
released
(ΔG < 0)
Energy
Products
Progress of the reaction
(b) Endergonic reaction: energy required,
nonspontaneous
Free energy
Products
Energy
Reactants
Progress of the reaction
© 2014 Pearson Education, Inc.
Amount of
energy
required
(ΔG > 0)
2 * C8H18 (octane) + 25 * O2

16 * CO2 + 18 * H2O
2 * C8H18 + 25 * O2  16 * CO2 + 18 * H2O
To determine enthalpy (H), count the number and type of bonds
on both sides of the reaction
Reactants
O=O
Electronegativity
C: 2.5
O: 3.5
H: 2.1
2 * C8H18 + 25 * O2  16 * CO2 + 18 * H2O
To determine enthalpy (H), count the number and type of bonds
on both sides of the reaction
Products
O=C=O
O
H
H
Electronegativity
C: 2.5
O: 3.5
H: 2.1
2 * C8H18 + 25 * O2  16 * CO2 + 18 * H2O
Reactants
• C8H18 – All Carbon-hydrogen or carbon-carbon bonds, nonpolar – 18 C-H bonds + 7
C-C bonds = 25 nonpolar bonds * 2 molecules = 50 NONPOLAR bonds
• O2 - All nonpolar bonds (O=O) – 2 nonpolar bonds * 25 molecules = 50 nonpolar
bonds
Products
• CO2 – All polar bonds (C=O) – 4 polar bonds * 16 molecules = 64 polar bonds
• H2O – All polar bonds (O-H) – 2 polar bonds * 18 molecules = 36 polar bonds
2 * C8H18 + 25 * O2  16 * CO2 + 18 * H2O
• Reactants = 100 NONPOLAR bonds total
• Products = 100 POLAR bonds total
• Potential energy has decreased
• ΔH is negative (enthalpy has decreased)
Enthalpy (H)
Reactants
Products
-∆H
Enthalpy (H)
Reactants
Products
2 * C8H18 + 25 * O2  16 * CO2 + 18 * H2O
To determine entropy (S) count the number and relative size of
molecules on each side of the reaction
You may be given a chemical formula like this, or the actual
molecules structures present in the reaction.
2 * C8H18 + 25 * O2  16 * CO2 + 18 * H2O
• The reactants have 27 molecules, AND octane (C8H18)
is by far the largest molecules in the reaction
• The products have 34 molecules, and they are all
small
• Entropy has increased (ΔS is positive)
Entropy (S)
Reactants
Products
Entropy (S)
+∆S
Reactants
Products
REDOX reactions
• Chemical reactions that involve the gain or loss of one or more
electrons are called reduction-oxidation reactions (redox
reactions)
• When an atom or molecule loses an electron is it oxidized
• Electrons have moved farther from an element or atom
• Oxidation is exergonic
• When an atom or molecule gains an electron it is reduced
• Electrons have moved closer to an element or atom
• Reduction is endergonic
REDOX reactions
• Just remember:
• O – oxidation
• I – is
• L – loss of electrons
• R – reduction
• I – is
• G – gain of electrons
• OIL RIG
REDOX reactions
In ORGANIC reactions:
• If carbon is oxidized  reaction will release energy (spontaneous)
• If carbon is reduced  reaction will need energy input (stores
energy) (nonspontaneous)
2 * C8H18 + 25 * O2  16 * CO2 + 18 * H2O
O=C=O
Is carbon being reduced or oxidized?
Electronegativity
C: 2.5
O: 3.5
H: 2.1
2 * C8H18 + 25 * O2  16 * CO2 + 18 * H2O
O=C=O
All bonds in octane are nonpolar (equally sharing electrons)
All bonds in CO2 are polar (carbon loses electrons to O)
Thus carbon has been oxidized
Spontaneous reactions
• Enthalpy decreases (-ΔH)
• Entropy increases (+ΔS)
• Carbon is oxidized (loses
electrons)
Nonspontaneous reactions
• Enthalpy increases (+ΔH)
• Entropy decreases (-ΔS)
• Carbon is reduced (gains
electrons)
Sometimes S or H may be 0; in that case, rely on other metrics (like the redox
reaction) to determine is spontaneous or not!
6 * CO2 + 6 * H2O

C6H12O6 + 6*O2
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Is ΔH positive or negative?
Is ΔS positive or negative?
Is carbon being reduced or oxidized?
Is this reaction spontaneous or not?
Electronegativity
C: 2.5
O: 3.5
H: 2.1
If it helps, this is C6H12O6
6 * CO2 + 6 * H2O  C6H12O6 + 6*O2
O=C=O
Reactant with carbon
Is carbon being reduced or oxidized?
Product with carbon
C3H8 (propane) + 5 O2
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3 CO2 + 4 H2O
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Is ΔH positive or negative?
Is ΔS positive or negative?
Is carbon being reduced or oxidized?
Is this reaction spontaneous or not?