Gibbs Free Energy

Energy is the ability to do work.
Two states of energy
1. Potential energy −
Stored energy
2. Kinetic energy −
Active energy or the energy of movement.
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Energy cannot be created or destroyed only transformed from one form to another .
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nd
As energy conversions occur, some of the usable energy is converted into unusable energy in the form of heat energy (more entropy). Heat energy can do work but only if there is a heat gradient.
25% Efficient
75% Unusable Energy
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Represents an energy transformation
Free energy is energy that is available to do work.
The amount of free energy in a system is always lower than the total energy of the system.
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nd
Over time, entropy has increased with respect to the building in this picture.
• Over time, in a closed system, there is a decrease in the amount of free energy available to do work, and, as a result, the entropy of the system will continue to increase. This will continue until the system exhausts its supply of free energy.
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• As free energy decreases, there is an increase in the disorder of the system. It takes free energy to maintain a system’s order. Entropy is a measure of that disorder.
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Living Organisms Obey the 1 st and 2 nd Law of Thermodynamics
• All living organisms need a source of free energy to live, grow, and reproduce.
Most autotrophs rely on the sun or radiant energy. Heterotrophs rely on the food they consume.
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• During a chemical reaction, bonds are broken and bonds are remade. The chemical energy of the reactants will be different from the chemical energy of the products.
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• If you mix two moles of hydrogen gas H
2 with one mole of oxygen gas-nothing happens.
• If you add a spark to the container, the following reaction occurs. KABOOM
• 2H
2
+ O
2
→2 H
2
O
D G=−58 kcal/mole
In order for water to be produced H
2 must become 2H and the O
2 must become 2O as this frees up the electrons tied up in covalent bonds, to form chemical bonds forming water, H
2
O.
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The energy used to break the bonds in the reactants so they can be reformed in the products is called the energy of activation .
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Consider the combustion of methane gas. It does release heat energy.
CH
4
+ 2 O
2
→
CO
2
+ 2 H
2
O
Bonds Broke
(energy absorbed)
Bonds Made
(energy released)
4 C−H 4 × 98 = 392 kcal/mole
2 C=O 2 × 187 = 374 kcal/mole
2 O=O 2 × 116 = 232 kcal/mole
4 O−H 4 × 110 =440 kcal/mole
Total energy Reactants
624 kcal/mole Total energy products
814 kcal/mole
Net energy (enthalpy ΔH)= 624 −814 = −190 kcal/mole of heat energy released
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Exothermic and Endothermic Reactions
• A decrease in enthalpy ( D
H) is said to be an exothermic reaction and has a – DH .
Burning of wood is exothermic
• An increase in enthalpy ( D
H) is said to be an endothermic reaction and had a +
DH.
Photosynthesis overall is endothermic.
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Exergonic and Endergonic Reactions
Changes in energy can be calculated as
D
G = G final
– G initial
OR
D
G =
D
H – T D
S
Exergonic reaction is one that releases free energy to its surroundings.
Endergonic reaction is one that absorbs free energy from its surroundings.
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Photosynthesis is overall is endergonic.
6H
2
O + 6O
2
→ 6C
6
H
12
O
6
+ 6O
2
D
G=+686 kcal/mol
Energy is coming from the sun.
Cell Respiration is overall exergonic.
6C
6
H
12
O
6
+ 6O
2
→ 6H
2
O + 6O
2
D G=−686 kcal/mol
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Metabolism is the sum total of all the biochemical reactions occurring in a cell.
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Coupling Reactions
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Spontaneous reactions are reactions that are energetically favorable and occur without outside energy.
Systems in general move from a high energy state to low energy state. In general exothermic reactions are spontaneous -- but not always.
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• Consider a beaker of water. Over time it will evaporate. In order for the water molecules to have enough energy to escape and become a gas, energy must be absorbed from the surrounding environment. This is an endothermic reaction BUT it is also a spontaneous reaction.
If systems go from a high energy state to a lower energy state, there must be ANOTHER factor in determining Gibbs free energy. That second factor is entropy or randomization.
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How Reactions Can Be Spontaneous or Exergonic
• Changes in free energy can be quantified by the equation
D
G=
D H−T D
S
• G=free energy, H=enthalpy, T=absolute temperature (K),
S=entropy
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Exergonic and Endergonic Reactions
Changes in energy can be calculated as
D
G = G final
– G initial
OR
D
G =
D
H – T D
S
Exergonic reaction is one that releases free energy to its surroundings.
Endergonic reaction is one that absorbs free energy from its surroundings.
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D
G
• ( a)
D
G < 0 or -
D
G
( b)
D
G > 0 or +
D
G
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Effects of Changes in Entropy and Its Effect on the Free Energy of a System
Both reactions are spontaneous and free energy decreases.
CaCl
2(S)
+ H
2
O
(aq)
→ Ca 2+
(aq)
+ 2Cl
−
(aq)
Exergonic and exothermic
D
G =
D
H + T
D
S
Exergonic but endothermic!!!
NH
4
Cl
(s)
+ H
2
O
(aq)
→ NH
4
+
(aq)
+ Cl −
(aq)
D
G = +
D
H + T
D
S The change in entropy is greater than the change in enthalpy.
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Examples of Decreasing Amounts of
Free Energy
• I n general, energy transfers favor going from a higher energy state to a lower energy state.
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Changes in Entropy and Its Effect on
Membranes
• Changes in entropy affect living systems in other ways other beyond the molecular level.
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Entropy also Affects the Circulatory System
• The differences in the concentration of oxygen and carbon dioxide is the driving force moving these gases from the lungs to various body cells. Again these are spontaneous reactions.
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Entropy also Affects the Excretory System
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• The 2 nd law of thermodynamics is, in part, responsible for the declining amount of free energy as it is transferred from one trophic level to next trophic level.
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