Thermodynamics Chemical reactions proceed according to the rules of thermodynamics The law of...
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Transcript of Thermodynamics Chemical reactions proceed according to the rules of thermodynamics The law of...
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!
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
Thermodynamics
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
Free Energy (G)
1. Change in free Energy (ΔG)
ΔG = Products – ReactantsΔG 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
Thermodynamics in a biological setting
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 EnergyEnthalpy
Enthalpy – average thermal energy of a collection of molecules i.e. bond energy
Change in enthalpy (H) = Hproducts – Hsubstrates
• Exothermic: H is negative i.e. C6H12O6 + 6O2 → 6CO2 + 6H2O + energy
• Endothermic: H is positive i.e. ADP + Pi → ATP
Chemical Reactions and Thermal EnergyEnthalpy and Entropy together
Entropy (S) – measure of randomness or disorder
Exothermic: H is negative, increase in S → reaction will occur spontaneously – negative G
Endothermic: H is positive, S is positive → reaction will occur spontaneously. It has to overcome the positive H
Free Energy: calculations
Free energy changes of reactions are additive (coupled reactions):Consider the phosphorylation of glucose to glucose 6-phosphate:
Go: glucose + Pi ↔ glucose-6-phosphate + H2O = 3.3 kcal/mol
Go: ATP + H2O ↔ ADP + Pi = -7.3 kcal/mol
Summing these reactions together:ATP + glucose ↔ ADP + glucose 6-phosphate
G° = +3.3 + (-7.3) = - 4kcal/mol (favourable)
Biological reactions
G = Go + RTln ([products]/[reactants])Where R = gas constant, T = temperature in Kelvin
Example:
glucose + ATP ↔ glucose-6-phosphte + ADP
Go: glucose + Pi ↔ glucose-6-phosphate + H2O = 3.3 kcal/mol
Go: ATP + H2O ↔ ADP + Pi = -7.3 kcal/mol
Glucose: [5mM]; ATP: [2mM]; ADP: [0.15mM]; glucose-6-phosphate: [0.05mM]
So, G = - 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
Go is zero
- Examples: glucose transport, ion transport across membranes
G = RTln ([inside]/[outside])Or for charged ions:
G = RTln ([inside]/[outside]) + zFEmwhere z = valence of the ion; F = Faraday constant and Em = membrane potential
G = RTln ([inside]/[outside]) + zFEmwhere 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
G = (1.987cal/K mol)(298K)(ln(10/120) + (-1)(23062 cal/V mol)(-0.08V) =
376 cal/mol
Transport across membranes