Thermodynamics
Chemical reactions proceed according to the
rules of thermodynamics
• The law of conservation of energy – energy can
be converted from one form to another but the
total amount of energy is constant
• Entropy – the universe is becoming more
chaotic
ACK!
Thermodynamics
Some constants
Gas constant: R = 8.315 Joules/K* mol or
1.9872 cal/K.mol
Faradays constant: F = 96485 Joules/Volt.mol or
23062 cal/Volt* mol
Energy: definitions
Energy – ability to do work
Energetics – energy transfer
Types of energy
• Potential – trapped energy
• Kinetic – energy of movement
Energy Categories: more definitions
• Radiant energy – energy released from one
object to another
• Mechanical energy – energy to move objects
from place to place
• Electrical energy – energy that results from the
movement of charged particles down a charge
gradient
• Thermal energy – reflected in the movement of
particles and serves to increase temperature
• Chemical energy – energy that is held within
chemical bonds
Energy Categories, Cont.
Animals rely on all five types of energy,
which are interconvertible
Food Webs are Transfers of Energy
Figure 2.3
Thermodynamics in a biological setting
Free Energy (G)
1. Change in free Energy (ΔG)
ΔG = Products – Reactants
ΔG negative – reaction will proceed forward →
ΔG positive – reaction will proceed backward ←
ΔG zero – reaction at equilibrium ↔
2. Standard free Energy – ΔGo: 298 K (25oC), 1
atm pressure, pH 7.0 and 1M [initial] for all
reactants and products
Thermal Energy
 Thermal energy   movement of
molecules
Most chemical reactions involve changes
in thermal energy
• Exothermic reactions – release heat
• Endothermic reactions – absorb heat
Chemical Reactions and Thermal Energy
Enthalpy
Enthalpy – average thermal energy of a collection
of molecules i.e. bond energy
Change in enthalpy (DH) = Hproducts – Hsubstrates
• Exothermic: DH is negative i.e. C6H12O6 + 6O2
→ 6CO2 + 6H2O + energy
• Endothermic: DH is positive i.e. ADP + Pi → ATP
Chemical Reactions and Thermal Energy
Enthalpy and Entropy together
Entropy (S) – measure of randomness or disorder
Exothermic: DH is negative, increase in DS →
reaction will occur spontaneously – negative DG
Endothermic: DH is positive, DS is positive →
reaction will occur spontaneously. It has to
overcome the positive DH
Free Energy: calculations
Free energy changes of reactions are
additive (coupled reactions):
Consider the phosphorylation of glucose to
glucose 6-phosphate:
DGo: glucose + Pi ↔ glucose-6-phosphate + H2O = 3.3
kcal/mol
DGo: ATP + H2O ↔ ADP + Pi = -7.3 kcal/mol
Summing these reactions together:
ATP + glucose ↔ ADP + glucose 6-phosphate
DG° = +3.3 + (-7.3) = - 4kcal/mol (favourable)
Biological reactions
DG = DGo + RTln ([products]/[reactants])
Where R = gas constant, T = temperature in Kelvin
Example:
glucose + ATP ↔ glucose-6-phosphte + ADP
DGo: glucose + Pi ↔ glucose-6-phosphate + H2O = 3.3 kcal/mol
DGo: ATP + H2O ↔ ADP + Pi = -7.3 kcal/mol
Glucose: [5mM]; ATP: [2mM]; ADP: [0.15mM]; glucose-6phosphate: [0.05mM]
So, DG = - 4.0 kcal/mol + 1.9872cal/K mol)(298K)ln((0.05*0.15)/(5*2))
= -8.26kcal/mol
ΔG for reactions that don’t make or break bonds
DGo is zero
- Examples: glucose transport, ion transport
across membranes
DG = RTln ([inside]/[outside])
Or for charged ions:
DG = RTln ([inside]/[outside]) + zFEm
where z = valence of the ion; F = Faraday constant and Em =
membrane potential
Transport across membranes
DG = RTln ([inside]/[outside]) + zFEm
where z = valence of the ion; F = Faraday constant and Em
= membrane potential
Example: Diffusion of Cl- from out to in
Cl- outside cell: 120mM; Cl- inside cell:
10mM; Em = -80mV
DG = (1.987cal/K mol)(298K)(ln(10/120) +
(-1)(23062 cal/V mol)(-0.08V) =
376 cal/mol