BCM.17 - ENERGY AND ENTROPY

Universe

Everything there is

System

The part of the universe we are interested in

Surroundings

all of the universe that is not part of the system that is of interest to you

Boundary

a well defined point set by the experimenter across which energy and matter can move

Open system

Open system : system can exchange energy and matter with the surroundings ( eg living things )

closed system

only exchanges energy but no matter

isolated system

exchange neither matter nor energy with the surroundings

Universe as a whole is

isolated

Most systems in biology are

open ones approximated as closed ones

First law of thermodynamics

For any change in the energy of the system there is an equal and opposite change in the energy of the surroundings.

Conservation of energy

Energy cannot be created or destroyed

Energy

energy is the quantitative property that must be transferred to an object in order to perform work on, or to heat, the object.

Gravitational energy

Work done moving mass in the earths gravitational field

W = mgh

Mechanical energy

work done compressing a volume of gas

MW = p V

Electrical energy

work done moving charges in an electric fielf

EE = EP Z ( electrical potential x charge )

Thermal energy (Q)

energy associated with heat transfer

Q = T S

Work(W)

is an energy-transfer process where macroscopic numbers of particles move in a coordinated way.

Two energy transfer processes

1) Work

2) Heat

Heat

is an energy transfer process driven by differences in the average kinetic energy of particles between places

Enthalpy

total energy of a system

H =

U + pV

•Intrinsic energy of system (potential and kinetic energy of all its particles)

+

•Work done in shoving a volume (V) of the Earth’s atmosphere (at pressure p) out of the way

U =

W + Q

Internal energy of a system

mass x C^2

In an aqueous system at constant pressure, the enthalpy change of a reaction largely reflects the

change in bonding between molecules.

∆H ≈

-energy extracted from surroundings to break bonds

minus

-energy released to surroundings in forming new bonds

Bond breaking ....( endothermic )

Bond making .... ( exothermic )

requires energy

releases energy

Most reactions in aqueous systems (e.g. hydrolysis of ATP in a cell) have no effect on the volume of the system, so none of the intrinsic energy change is consumed in e.g. pushing the atmosphere out of the way during expansion.

Consequently...

the enthalpy change is more-or-less the same value as the intrinsic energy change,

second law of thermodynamics

Energy is dissipated by all spontaneous processes.

All processes increase the entropy of the universe.

Gases will mix in an isolated system ....

Experiment that shows this ?

without energy exchange with the surroundings (microscopic redistribution leading to sameness at the macroscopic level).

Iodine vapour/ oxygen experiment

The more particles there are the more likely they will meet a homogenous equilibrium eg have the same amount of each particles type in each bulb

Some systems will .... even as their temperature remains ... eg Ice melts to water at 0°C ( phase transition ) without change in temperature - ...

absorb heat

constant

the energy taken into the system to break bonds is used in making the molecules of water more dispersed. The water has a higher entropy than the ice, even though they are the same temperature.

Entropy, S, is a measure of

energy dissipation.

Q =

T S

∆Ssurroundings =

- energy extracted from system/ room temperature.

∆Suniverse =

Change in system entropy - change in surrounding entropy

When heat energy spontaneously flows from one place to another, the entropy of the universe ..

always increases (or – at best – does not change), because heat always flows from a hotter body to a colder one.

∆S universe ≥0

Boltzmann microstate equation

∆S=kB ln (Ω_after/Ω_before )

Ω is the number of microstates…

kB =

1.38 × 10−23 J K−1

The macrostate of a system is its ...

overall temperature, pressure, volume, etc ( bulk properties ).

The microstate of a system is the

precise configuration of every molecule in the macrostate

As long as the average molecular velocity remains the same, ...

As long as the total number of each kind of molecule remains the same....

As long as the average kinetic energy in each direction remains the same...

the temperature will be the same.

the overall composition will remain the same

the pressure will be unchanged.

There are many ... for each ...

microstates

macrostate

The most likely long-term macrostate of a system is the one with the ...

Eg..

largest number of compatible microstates.

Ice melts over time as it is more probable that it will end up in the water state as there are more ways of arranging molecules to make a water state than an ice state.

How to work out number of combinations of microstate

nCr = n!/r!(n-r)!

n is the number of grid squares and r is the number of molecules.

Protein folding components

Enthalpy : bonding within protein/ to water

Entropy : Protein configuration and water organisation

Hydrophobic effects tend to be ...

the mobility of the water associated with the protein is greatly increased when the hydrophobic regions of an unfolded protein .... This is because the ...

This increase in entropy... because of the sheer number of water molecules involved.

entropy-driven:

associate with one another.

water is no longer held in particular orientations around the hydrophobic regions.

greatly offsets the lower entropy of the folded protein overall compared to its unfolded state

The enthalpy change overall makes .... to protein folding because the protein itself has approximately the ....

a much smaller contribution

same number of hydrogen bonds whether the protein is folded or not

Zeroth Law of Thermodynamics

All systems at the same temperature are in thermal equilibrium with each other.

Why heat cannot be efficiently converted into other forms of energy

- Some heat energy always has to be dumped into the cold sink/surroundings as you extract mechanical work from the heat source.

Efficiency =

T source - T sink / T source

Why can we not create a 100% efficient engine

- Frictional energy losses so some energy is always lost to the surroundings and becomes unavailable to do useful work

- Impossible to create a 0K sink so some energy extracted from the source is always dumped into the sink, where it can no longer do useful work w.r.t. the heat source

Third law of thermodynamics

Absolute zero is unobtainable and the entropy of a perfect crystal at absolute zero (-273°C) is 0.

3 ways entropy is described

1) The amount of information needed to describe a system's possible microstates

(A liquid is more difficult to describe than a solid because particles in a solid vibrate around a mean position)

2) How close a system is to thermodynamic equilibrium

(A red-hot anvil insulated from a large block of ice have lower entropy than a cool anvil sitting in a tepid puddle)

3) How dispersed the energy in a system is

(The energy is much more evenly distributed once the ice melts and the anvil cools)