Spontaneous Change & Disorder

Question 1

What is a spontaneous change?

Answer 1

A change that occurs without an external source of energy
They move towards their equilibrium state without being driven by an external influence
They increase the total entropy of the universe

Question 2

What is matter?

Answer 2

Matter can be defined as atoms and molecules

Question 3

What is entropy?

Answer 3

Entropy is a measure of the disorder of matter and energy

Question 4

What is quanta?

Answer 4

Energy comes in discrete packages called quanta

This means it is possible to count the arrangements of quanta as you would count the arrangements of molecules

Question 5

How will a perfect gas act in a vacuum ?

Answer 5

A perfect gas will expand spontaneously

In spontaneous processes, matter and energy become more disordered

Question 6

What is a micro state?

Answer 6

The number of ways of arranging molecules are their energies

Question 7

What happens to entropy when the system becomes more disordered?

Answer 7

When the system becomes more disordered, entropy increases and the change in entropy will be positive

Question 8

What is the equation for entropy?

Answer 8

S= k lnW

k is the boltzmann constant 1.381x10 -23
W is the number of micro states

Question 9

What is the relation between micro states and entropy?

Answer 9

Microstates is the number arrangement of molecules and their energies
The larger the number of arrangements, the less ordered the system and the larger the entropy

Question 10

What is the probability of finding all gas molecules in one container?

Answer 10

There is very little probability of finding all gas molecules in one containing
Arrangements where the molecules are mixed up and distributed are much more likely

Question 11

What is a state function?

Answer 11

This describes a quantity that depends only on the current state of the system and not on the path by which the system arrived at its present state

Question 12

What does the 2nd law of thermodynamics state?

Answer 12

The entropy of the universe tends to increase through spontaneous process

Question 13

What is the equation for the change in entropy related to heat?

Answer 13

^S= qrev/ T
dS= drev/ T

qrev is the quantity of heat added reversibly

Question 14

What is qrev?

Answer 14

This means adding the heat reversibly
This means the heat is added very slowly so that at any stage, the temperature difference between the system and its surroundings is infinisimally small

Question 15

What happens to entropy change at higher qrev?

Answer 15

At higher qrev the entropy change will be larger because a large amount of heat has been added so the molecules have more energy and hence are more disordered

Question 16

What equation do you use for entropy change if the temperature is changing?

Answer 16

^S= qrev/ T
dS= drev/ T

You need to use an integral of the above equation with the initial and finial temperature as the limits

Question 17

What is fusion?

Answer 17

When a solid changes state to become a liquid

This is an endothermic change as bonds are broken and entropy increases

Question 18

What is the equation for the entropy change of fusion?

Answer 18

^fusS= ^fusH/ Tf

Where qrev= ^fusH

Question 19

What is vaporisation?

Answer 19

This is when a liquid changes state to become a gas

This is an endothermic change as bonds are broken and the entropy increases

Question 20

What is the equation for the entropy change of vaporisation?

Answer 20

^vapS= ^vapH/ Tb

Where qrev= ^vapH

Question 21

What is troutons rule?

Answer 21

This states that the entropy of vaporisation is almost the same value (85JK-1) for all liquids at their boiling points (^vapH/Tb)

Question 22

What is the equation for entropy change for volume change?

Answer 22

^S= nR ln(Vf/Vi)

n is the number of moles
R is the gas constant

You use this equation when you want to calculate the change in entropy for a changing volume

Question 23

What is the equation for entropy change for temperature changes?

Answer 23

^Sm= Cp,m ln(Tf/Ti)

Cp,m is the molar heat capacity
To find ^S, multiply by the number of moles (Sm is the molar entropy)
Use this equation when you want to calculate the change in entropy for a changing temperature

Question 24

How do you work out the molar heat capacity for a perfect gas at constant pressure?

Answer 24

Cp,m= Cv,m + R

Cv,m is the molar heat heat capacity of a perfect gas at constant volume?

Question 25

What is meant by monatomic?

Answer 25

Consisting of one atom

Question 26

What is meant by polyatomic?

Answer 26

Consisting of more than one atom

Question 27

If the gas is monatomic then what is Cvm and Cpm?

Answer 27

``` Cvm= 3/2R Cpm = 5/2 R ```

Question 28

If the gas is polyatomic then what is Cvm and Cpm?

Answer 28

Cvm>3/2 R Cpm> 5/2 R

Question 29

What is the 3rd law of thermodynamics?

Answer 29

This states that the entropies of all perfectly crystalline materials are the same at absolute zero (T=0K) The entropy of a perfect crystal at 0K is 0

Question 30

What is the standard molar entropy?

Answer 30

This is the entropy of 1 mole of a substance in its standard state JK-1mol-1

Question 31

What is residual entropy?

Answer 31

This is a small amount of entropy (normally still close to zero) which is present even after a substance has been cooled to absolute zero (0K)

Question 32

What will be the micro states value for a crystal at 0K?

Answer 32

A perfectly ordered system has an entropy 0 and at 0K all the molecules will be in their ground states This means there is only one way of arranging the molecules hence w=1

Question 33

Why is entropy sometimes not 0 and 0K?

Answer 33

(Residual entropy) This is because it is hard to achieve a perfect crystal as there normally are always the presence different orientations which leads to some disordered Also isotopes in a molecule can create disorder

Question 34

What is the Debye T3 law?

Answer 34

At temperatures very close to zero we use this law Cvm= aT3 Which gives Sm= 1/3Cvm

Question 35

What do you need to consider when determining entropy change?

Answer 35

You need to take into account any phase changes that occur Including melting and vaporisation

Question 36

Why do lattices have a lower standard entropy?

Answer 36

This is because lattice structures are more ordered that this with delocalised electrons and so have a lower standard entropy

Question 37

Why do solids with more complex structures have high standard entropies?

Answer 37

This is because larger molecules have more ways of distributing the energy within the vibrations of their many bonds This means the value of standard entropy increases as molecular complexity increases

Question 38

What does the second law of thermodynamics state?

Answer 38

The second law of thermodynamics states that spontaneous processes are those that increase the total entropy of the universe

Question 39

How do the system and surroundings interact?

Answer 39

They act to change each other's entropies

Question 40

What is the entropy change of reaction?

Answer 40

The standard entropy change of reaction is defined as the difference in fanfare entropy between the products and reactants of a reaction system

Question 41

How do you calculate the entropy change of reaction?

Answer 41

^rS*= EvSm*(products) - Evsm*(reactants)

Question 42

What is the total entropy change of the system made up of?

Answer 42

It is made up of two parts: the entropy change of the system plus the entropy change in the rest of the universe surrounding the system

Question 43

How do you calculate the entropy change of the surroundings?

Answer 43

^Ssurr= -^H/T At constant pressure q=^H so qsystem= ^H

Question 44

What must S always be?

Answer 44

S must always be positive as there is also some form of disorder

Question 45

How do you calculate the total entropy change?

Answer 45

^Stotal= ^S(system)+ ^S(surroundings)

Question 46

What do the values for the change in total entropy mean?

Answer 46

>0 then the process is spontaneous <0 then the process is non spontaneous =0 then the process is at equilibrium