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 Kinetic

– trapped energy – energy of movement

Energy Categories: more definitions

• • • • •

Radiant energy

object to another – energy released from one

Mechanical energy

from place to place – energy to move objects

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 –

ΔG o : 298 K (25 o C), 1 atm pressure, pH 7.0 and 1M [initial] for all reactants and products

Thermal Energy

 Thermal energy molecules   movement of

Most chemical reactions involve changes in thermal energy

• •

Exo

thermic reactions – release heat

Endo

thermic 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 ( D H) =

H

products –

H

substrates • Exothermic : D H is negative i.e. C 6 H 12 O 6 → 6CO 2 + 6H 2 O + energy + 6O 2 • Endothermic : D H is positive i.e. ADP + Pi → ATP

Chemical Reactions and Thermal Energy

Enthalpy and Entropy together

Entropy

(S) – measure of randomness or disorder Exothermic : D H is negative, increase in D S → reaction will occur spontaneously – negative D G Endothermic : D H is positive, D S is positive → reaction will occur spontaneously. It has to overcome the positive D H

Free Energy: calculations

Free energy changes of reactions are additive (coupled reactions): Consider the phosphorylation of glucose to glucose 6-phosphate:

 D G o : glucose + Pi ↔ glucose-6-phosphate + H 2 O = 3.3 kcal/mol  D G o : ATP + H 2 O ↔ ADP + Pi = -7.3 kcal/mol Summing these reactions together: ATP + glucose ↔ ADP + glucose 6-phosphate D

G ° = +3.3 + (-7.3) = - 4kcal/mol (favourable)

Biological reactions

D G = D G o + RTln ([products]/[reactants])

Where R = gas constant, T = temperature in Kelvin Example: glucose + ATP ↔ glucose-6-phosphte + ADP

D G o : glucose + Pi ↔ glucose-6-phosphate + H 2 O = 3.3 kcal/mol D G o : ATP + H 2 O ↔ ADP + Pi = -7.3 kcal/mol Glucose: [5mM]; ATP: [2mM]; ADP: [0.15mM]; glucose-6 phosphate: [0.05mM] So, D

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

D

G o is zero - Examples: glucose transport, ion transport across membranes

D G = RTln ([inside]/[outside])

Or for charged ions:

D

G = RTln ([inside]/[outside]) + zFEm

where z = valence of the ion; F = Faraday constant and Em = membrane potential

Transport across membranes

D

G = 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 D

G = (1.987cal/K mol)(298K)(ln(10/120) + (-1)(23062 cal/V mol)(-0.08V) = 376 cal/mol