bio.lesson document 12

8/6/2019 bio.lesson document 12

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BIO 202 Biochemistry II

bySeyhun YURDUGL

Lecture IBasics of Thermodynamics


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C

ontent Outline Definition

The laws of thermodynamics The free energy concept


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W

hat is thermodynamics? the field of physics that studies the

properties of systems

that have a temperature

and involve the flow of energy from oneplace to another.


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Laws of thermodynamics

Zeroth Law First Law

Second Law


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Another demonstration for Zeroth

Law


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Heat

defined as energy in transit from a hightemperature object

to a lower temperature object.

An object does not possess "heat";

the appropriate term for the microscopicenergy in an object: internal energy.


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Heat The internal energy :

may be increased by transferring energy tothe object from a higher temperature(hotter) object

this is properly called heating.


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Heat


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Specific Heat

amount of heat per unit mass required to raise the

temperature by one degreeC

elsius. The relationship between heat and temperaturechange:

usually expressed in the form shown below wherec is the specific heat.

does not apply if a phase change is encountered, because the heat added or removed during a phase

change does not change the temperature.


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Specific Heat The specific heat of water:

1 calorie/gram C = 4.186 joule/gram C

higher than any other common substance.

As a result, water plays a very importantrole in temperature regulation.

The specific heat per gram for water :

much higher than that for a metal.


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Specific Heat


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Heat Transfer

normally from a high temperature object toa lower temperature object.

changes the internal energy of both systemsinvolved according to the First Law ofThermodynamics.


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Heat Conduction

heat transfer by means of molecular agitationwithin a material without any motion of the

material as a whole. If one end of a metal rod : at a higher temperature,then energy : transferred down the rod toward thecolder end,

because the higher speed particles: collide with the slower ones with a net transfer ofenergy to the slower ones


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Heat Convection

heat transfer by mass motion of a fluid such as air or water;

when the heated fluid is caused to move away from thesource of heat, carrying energy with it.

Convection above a hot surface occurs

because hot air expands, becomes less dense, and rises (aslike in Ideal Gas Law).

Hot water : likewise less dense than cold water and rises,causing convection currents which transport energy.


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Heat Convection


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Heat Transfer by Vaporization

If part of a liquid evaporates,

it cools the liquid remaining behind

because it must extract the necessary heat ofvaporization from that liquid;

in order to make the phase change to thegaseous state.


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Heat Transfer by Vaporization therefore an important means of heat

transfer in certain circumstances:

such as the cooling of the human body

when subjected to ambient temperaturesabove the normal body temperature


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Explanation of the figure for Zeroth

Law IfA and C: in thermal equilibrium with B,

then A : in thermal equilibrium with B.

Practically this means that all three are atthe same temperature,

and it forms the basis for comparison of

temperatures. so named because it logically precedes the

First and Second Laws of Thermodynamics


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More on first law of

thermodynamics In the context of physics, the common

scenario:

one of adding heat to a volume of gas

and using the expansion of that gas to dowork,

as in the pushing down of a piston in aninternal combustion engine.


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thermodynamics The first law of thermodynamics:

the application of the conservation ofenergy principle to heat and thermodynamic

processes:


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thermodynamics It is just that

W : defined as the work done on the systeminstead of work done by the system.


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Isothermal Process


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Adiabatic Process

An adiabatic process : one in which no heat gained or lost by the system.

The first law of thermodynamics with Q=0 showsthat: all the change in internal energy is in the form of

work done. This puts a constraint on the heat engine process

leading to the adiabatic condition can be used to derive the expression for the work

done during an adiabatic process.


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Isothermal Process

For a constant temperature processinvolving an ideal gas,

pressure can be expressed in terms of thevolume:


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Pressure-Volume (PV) Diagrams

Pressure-Volume (PV) diagrams: a primary visualization tool for the study of heat

engines. Since the engines usually involve a gas as a

working substance, the ideal gas law relates the PV diagram to the

temperature

so that the three essential state variables for thegas:

can be tracked through the engine cycle.


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Pressure-Volume (PV) Diagrams Since work is done only

when the volume of the gas changes,

the diagram gives a visual interpretation ofwork done.


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Pressure-Volume(PV) Diagrams

Since the internal energy of an ideal gas dependsupon its temperature,

the PV diagram along with the temperaturescalculated from the ideal gas law

determine the changes in the internal energy of thegas

so that the amount of heat added can be evaluatedfrom the first law of thermodynamics.


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In summary, the PV diagram:

provides the framework for the analysis ofany heat engine

which uses a gas as a working substance.


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For a cyclic heat engine process, the PVdiagram will be closed loop.

The area inside the loop:

a representation of the amount of work doneduring a cycle.


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Some idea of the relative efficiency of anengine cycle:

can be obtained by comparing its PVdiagram with that of a Carnot cycle,

the most efficient kind of heat engine cycle


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Pressure-Volume (PV) Diagrams


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Adiabatic Process


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C

onstant Volume Process


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System Work

When work:

done by a thermodynamic system, it is usually a gas that is doing the work.

The work done by a gas at constant pressure

is:


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Work at constant pressure:


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E

nthalpy It is a useful quantity for tracking chemical

reactions.

If as a result of an exothermic reaction someenergy is released to a system,

it has to show up in some measurable form

in terms of the state variables.


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E

nthalpy An increase in the enthalpy H = U + PV:

might be associated with an increase ininternal energy,

which could be measured by calorimetry,

or with work done by the system, or a

combination of the two.


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InternalE

nergy The internal energy U : thought of as the energy required to create a

system in the absence of changes in temperature orvolume. But if the system: created in an environment of

temperature T,

then some of the energy can be obtained byspontaneous heat transfer; from the environment to the system.


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Helmholtz free energy But as discussed in defining enthalpy,

an additional amount of work PV must be done

if the system is created from a very small volumein order to "create room" for the system.

an environment at constant temperature T willcontribute an amount TS to the system:

reducing the overall investment necessary forcreating the system.


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Helmholtz free energy

This net energy contribution for a systemcreated in environment temperature T from

a negligible initial volume : Gibbs free energy.


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Free energy (G)

Defined as Gibbs free energy

The energy which has an ability to carry outwork

Standard version (Gr) : performed inconditions as 1M concentrates, 25r C

temperature and 1 atm(760 mm Hg)pressure


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Another definition by formula for

Gibbs free energy


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Gibbs free energy

The change in Gibbs free energy, G, in a reaction is a very useful parameter.

It can be thought of as the maximum amount ofwork obtainable from a reaction. For example, in the oxidation of glucose, the

change in Gibbs free energy:

G = 686 kcal = 2870 kJ. This reaction : the main energy reaction in living

cells.


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Entropy

The amount of this spontaneous energytransfer: TS where S is the final entropy of

the system. In that case, you don't have to put in as

much energy.

Note that if a more disordered (higherentropy) final state is created,

less work is required to create the system.


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Entropy

The Helmholtz free energy then:

a measure of the amount of energy you haveto put in to create a system;

once the spontaneous energy transfer to thesystem from the environment is accounted

for.


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Why is the heat of vaporization

more at body temperature?

An interesting feature of the process of cooling thehuman body by evaporation that:

the heat extracted by the evaporation of a gram ofperspiration from the human skin at bodytemperature (37C)

quoted in physiology books as 580 calories/gmrather than the nominal 540 calories/gm at the

normal boiling point. The question is, why is it larger at bodytemperature?


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more at body temperature? The main part of the answer

the binding energy of the water molecules:

greater at that lower temperature,

and it therefore takes more energy to breakthem apart into the gaseous state.


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more at body temperature? The change in the heat of vaporization:

roughly calculated using what we know

from the specific heat of water, 1 calorie/gm C.

It takes 37 calories to heat a gram of water

from 0C

to 37C

, but the change in the kinetic energy is muchless than that:


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more at body temperature? the kinetic energy of the water molecules only increases

by:

61.7 - 45 = 16.7 calories/gm

when the water is heated from zero to 100C but we know it takes 100 calories to do that heating.

Therefore the contribution to weakening the water bonds is83.3 calories/gm.

Using the result for water at 37C : 52.4 calories of additional energy must be supplied at 37C

to vaporize the water


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more at body temperature? one additional element in modeling the heat of

vaporization at body temperature:

- the PdV work required to expand the water intoits gaseous form: slightly less at 37C.

By analogy with the work calculation above,

that work is found to be 34.2 calories/gm,

6.8 calories/gm less than at 100C.


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more at body temperature? This model then suggests a heat of

vaporization at 37C:

Body temperature heat of vaporization =539 cal/gm + 52.4 cal/gm - 6.8 cal/gm =585 cal/gm.

So this simple model agrees fairly well withthe quoted 580 cal/gm.


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Perspiration Cooling of Body

When the ambient temperature is abovebody temperature, then radiation,

conduction and convection: all transfer heat into the body rather than

out.


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Since there must be a net heat transfer,

the only mechanisms left under those

conditions: the evaporation of perspiration from the

skin

and the evaporative cooling from exhaledmoisture.


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Even when one :

unaware of perspiration,

an amount of about 600 grams per day of"insensate loss" of moisture from the skin isobserved.


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The cooling effect of perspirationevaporation:

makes use of the very large heat ofvaporization of water.

This heat of vaporization:

540 calories/gm at the boiling point, but is even larger, 580 cal/gm, at the normalskin temperature.


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LITERATURECITED

Devlin,T.M. Textbook of Biochemistry with ClinicalCorrelations,Fifth Edition,Wiley-Liss Publications,NewYork, USA, 2002.

Lehninger, A. Principles of Biochemistry, Secondedition, Worth Publishers Co., New York, USA, 1993.

Matthews, C.K. and van Holde, K.E., Biochemistry,Second edition, Benjamin / Cummings PublishingCompany Inc., San Francisco, 1996.

Segel, I.H. Biochemical Calculations, Second Edition,John Wiley and Sons, New York, 1976.

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bio.lesson document 12