2-Entropy

Question 1

What are the laws of thermodynamics?

Answer 1

1. Energy can never be created or destroyed
2. Entropy always increases in the transformation of energy
3. Entropy in a perfect crystal at 0K is 0

Question 2

What is entropy? What is it measured in?

Answer 2

A measure of the disorder within a system

J mol-1 K-1

Question 3

How does the number of arrangements of energy affect the entropy of a system?

Answer 3

As the number of arrangements increase, so does the entropy

Question 4

What is energy measured in in entropy?

Answer 4

Quanta

Question 5

How can molecules change their energy states?

Answer 5

Electrons moving between energy levels
Vibrate, translate and rotate
all involve energy

Question 6

Can you work out how many arrangements there are of quanta? What does it depend on?

Answer 6

Yes

Number of atoms and the number of quanta of energy

Question 7

What is meant by a perfect crystal?

Answer 7

One in which the internal lattice structure is the same at all times
(fixed and non-moving, without particles vibrating or rotating)

Question 8

What state has more entropy? Why?

Answer 8

Least- solid
liquid
Most- gas
There is greater movement in the particles so more ways to arrange the quanta

Question 9

What is a spontaneous reaction? How do you know its spontaneous?

Answer 9

One that will occur on their own, without any external influence. However, it may be fast or slow. Spontaneity gives no indication of the rate
Positive total entropies

Question 10

How does the complexity of the molecule affect the entropy?

Answer 10

The greater the complexity of the molecule, the more ways of sharing the quanta

Question 11

How does the number of moles affect the entropy?

Answer 11

The greater the number of moles of particles the more ways of sharing the quanta
H2 has less entropy than 2H

Question 12

What has the most to least effect on the entropy?

Answer 12

Most- State
Moles
Complexity
Temperature

Question 13

How do you work out the entropy using Boltzmann’s constant?

Answer 13

S=kInW
S is entropy
k is Boltzmann’s constant
InW is the natural log of the ways of arranging quanta

Question 14

Why does entropy increase when a solid dissolves?

Answer 14

The particles are free to move around the solution so there is greater disorder
Particularly with ionic solids, bonds between the particles and solute are made, reducing disorder

Question 15

Why is the entropy of a gas high?

Answer 15

The particles are free to move so there is greater disorder

Question 16

What will happen to the entropy of an exothermic reaction that produces a solid?

Answer 16

Entropy to decrease because solids have considerably more order than liquids or gases

Question 17

What will happen to the entropy of an endothermic reaction between two solids?

Answer 17

Depend on the physical state of the products

If a liquid or gas is produced the entropy of a system will decrease due to increased disorder

Question 18

How do you work out ΔSsystem?

Answer 18

ΔSsystem= Sproducts - Sreactants

Look up the standard entropy for the substances

Question 19

When looking up entropy values, what must you be careful of?

Answer 19

For molecules such as 02, values are quoted per mole of atoms so you may have to double the values

Question 20

How do you calculate ΔSsurroundings?

Answer 20

ΔSsurroundings = -ΔH/T

ΔH is normally in kJmol-1 so must be converted into Jmol-1 by x1000

Question 21

How do you calculate ΔStotal?

Answer 21

ΔStotal = ΔSsystem + ΔSsurroundings

Question 22

Why do some spontaneous reactions still occur even if they have a decrease in entropy?

Answer 22

Number of ways of arranging quanta in the universe is affected which affects ΔSsurroundings

Question 23

What is a spontaneous reaction?

Answer 23

One which tends to go without being driven by any outside agency

Question 24

Why can’t ΔH be used to predict whether a reaction is spontaneous?

Answer 24

• Doesn’t say anything about kinetic stability
• Only for standard conditions
• Doesn’t take into account other factors such as entropy

Question 25

What is the definition of lattice enthalpy?

Answer 25

The enthalpy of formation of one mole of an ionic compound from gaseous ions under standard conditions

Question 26

What are the trends in lattice enthalpy?

Answer 26

As the ionic radii of either ions increase, the lattice energy decreases
Solids consisting of divalent ions have much higher lattice energies than solids with monovalent ions

Question 27

What is the definition of the enthalpy of hydration?

Answer 27

The amount of energy released when 1 mole of the gaseous ions dissolve in water under standard conditions to produce a solution of concentration 1.0moldm-3

Question 28

How do you work out the enthalpy of hydration?

Answer 28

Hydration value of each ion added together

Question 29

What is the definition for the enthalpy of solution?

Answer 29

The enthalpy change when 1 mole of the compound dissolves in the stated amount of water under standard conditions
(difference between energy needed to separate ions from lattice and the energy given out when ions are hydrated)

Question 30

How do you work out the enthalpy of solution?

Answer 30

Hydration enthalpies - lattice enthalpy

Question 31

Why do some ionic compounds dissolve in water when the hydration energy is not larger than the lattice energy?

Answer 31

Dissolving ions in water leads to increase in entropy

Need to consider both enthalpy and entropy in ΔG which must be negative

Question 32

What is ΔG?

Answer 32

Free energy change