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

Energy Categories, Cont.

Animals rely on all five types of energy, which are interconvertible

Food Webs are Transfers of Energy

Figure 2.3

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