ThermodynamicsThermodynamics

BCH 341 lecture 3

Thermodynamics is the study of the patterns Thermodynamics is the study of the patterns of energy change.of energy change.

ThermoThermo means means energyenergy DynamicsDynamics means means Patterns of changePatterns of change

BCH 341 lecture 3

Thermodynamics deals with:Thermodynamics deals with:

a)a) energyenergy

b) The stability of molecules and direction of b) The stability of molecules and direction of change.change.

BCH 341 lecture 3

ExamplesExamples::

Imagine a brick resting on window ledge 3 meters high. Imagine a brick resting on window ledge 3 meters high. As the brick rests on the ledge it has As the brick rests on the ledge it has potential energy.potential energy.

If you knock the brick out of the ledge, the potential If you knock the brick out of the ledge, the potential energy is converted to energy is converted to kinetic energykinetic energy( the brick ( the brick

accelerated towards the ground).accelerated towards the ground).

When the brick hits the ground the kinetic energy When the brick hits the ground the kinetic energy converted to converted to light energylight energy (sparks), (sparks), Sound energySound energy and and

chemical energychemical energy (the brick breaks). (the brick breaks).

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To study thermodynamics we will introduce To study thermodynamics we will introduce two new terms which are:two new terms which are:

1- 1- The systemThe system

2- 2- The surroundingsThe surroundings

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Two types of exchange can occur between the Two types of exchange can occur between the system and surroundings.system and surroundings.

1- Energy exchange1- Energy exchange (heat, radiation….etc). (heat, radiation….etc).

22--Matter exchangeMatter exchange (movement of molecules across (movement of molecules across the boundary between the system and the the boundary between the system and the surroundings)surroundings)..

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Based on the types of exchange which take Based on the types of exchange which take place, three types of systems are identified.place, three types of systems are identified.

1- Isolated systems1- Isolated systems: no exchange of matter or : no exchange of matter or energy.energy.

2-Open systems2-Open systems: Exchange of both matter and : Exchange of both matter and energy.energy.

3- Closed system3- Closed system: no exchange of matter but : no exchange of matter but exchange of energy. exchange of energy.

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The total energy of a system before the The total energy of a system before the process has taken place, called the process has taken place, called the initial state.initial state.

While the total energy of a system after the While the total energy of a system after the process has taken place, calledprocess has taken place, called final state. final state.

The The equilibrium stateequilibrium state is that condition in is that condition in which no further change is occurring within which no further change is occurring within the system or between the system and its the system or between the system and its surroundings.surroundings.

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Thermodynamic process are either:Thermodynamic process are either:

--ReversibleReversible: happens slow enough to be reversed: happens slow enough to be reversed

--IrreversibleIrreversible: can not be reserved: can not be reserved

BCH 341 lecture 3

The first law of thermodynamicsThe first law of thermodynamics

Energy can be exchanged between the system and its Energy can be exchanged between the system and its surroundings.surroundings.

But the total energy of the system But the total energy of the system ++ the surrounding is the surrounding is constant. constant. i.ei.e The energy of the universe, remains constant. The energy of the universe, remains constant.

So, the 1So, the 1stst law of thermodynamics stated that law of thermodynamics stated that

“ “ energy is conserved”energy is conserved”

or “ energy can neither created nor destroyed”or “ energy can neither created nor destroyed”

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Heat and work changesHeat and work changes

The energy of a system will change if heat is transferred The energy of a system will change if heat is transferred to or from the system or work is done by the system.to or from the system or work is done by the system.

11stst law stated that, if some amount of heat (Q) is added law stated that, if some amount of heat (Q) is added into the system, it must either do work or increase the into the system, it must either do work or increase the total energy of the system.total energy of the system.

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In biological systemIn biological system

If the capacity to do work is represented by the If the capacity to do work is represented by the symbol symbol “W”“W” and and “H”“H” stands for enthalpy (or stands for enthalpy (or heat content), then the first law can be heat content), then the first law can be expressed as:expressed as:

ΔΔH = Q - WH = Q - W

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Isothermal nature of cellular processIsothermal nature of cellular process

Although heat is the simplest and more Although heat is the simplest and more familiar medium by which energy may be familiar medium by which energy may be

transferred or used in man-made machines, it transferred or used in man-made machines, it is not useful form of energy for performing is not useful form of energy for performing

biological work.biological work. Why?Why?

“ “ because heat can do work if there is a because heat can do work if there is a temperature differential through which it can temperature differential through which it can

act”act”

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Heat must either pass from one body to another one Heat must either pass from one body to another one having lower temperature, having lower temperature,

or the temperature of a given hot body is lowered.or the temperature of a given hot body is lowered.

Since this is not possible because there are very small Since this is not possible because there are very small temperature differentials in cells which is the basis of temperature differentials in cells which is the basis of doing cellular work……Sodoing cellular work……So

“ “ living organisms are isothermal”living organisms are isothermal”

Here we must define another form of energy calledHere we must define another form of energy called

“ “free energy”free energy”

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