2-Entropy Flashcards
What are the laws of thermodynamics?
- Energy can never be created or destroyed
- Entropy always increases in the transformation of energy
- Entropy in a perfect crystal at 0K is 0
What is entropy? What is it measured in?
A measure of the disorder within a system
J mol-1 K-1
How does the number of arrangements of energy affect the entropy of a system?
As the number of arrangements increase, so does the entropy
What is energy measured in in entropy?
Quanta
How can molecules change their energy states?
Electrons moving between energy levels
Vibrate, translate and rotate
all involve energy
Can you work out how many arrangements there are of quanta? What does it depend on?
Yes
Number of atoms and the number of quanta of energy
What is meant by a perfect crystal?
One in which the internal lattice structure is the same at all times
(fixed and non-moving, without particles vibrating or rotating)
What state has more entropy? Why?
Least- solid
liquid
Most- gas
There is greater movement in the particles so more ways to arrange the quanta
What is a spontaneous reaction? How do you know its spontaneous?
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
How does the complexity of the molecule affect the entropy?
The greater the complexity of the molecule, the more ways of sharing the quanta
How does the number of moles affect the entropy?
The greater the number of moles of particles the more ways of sharing the quanta
H2 has less entropy than 2H
What has the most to least effect on the entropy?
Most- State
Moles
Complexity
Temperature
How do you work out the entropy using Boltzmann’s constant?
S=kInW
S is entropy
k is Boltzmann’s constant
InW is the natural log of the ways of arranging quanta
Why does entropy increase when a solid dissolves?
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
Why is the entropy of a gas high?
The particles are free to move so there is greater disorder
What will happen to the entropy of an exothermic reaction that produces a solid?
Entropy to decrease because solids have considerably more order than liquids or gases
What will happen to the entropy of an endothermic reaction between two solids?
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
How do you work out ΔSsystem?
ΔSsystem= Sproducts - Sreactants
Look up the standard entropy for the substances
When looking up entropy values, what must you be careful of?
For molecules such as 02, values are quoted per mole of atoms so you may have to double the values
How do you calculate ΔSsurroundings?
ΔSsurroundings = -ΔH/T
ΔH is normally in kJmol-1 so must be converted into Jmol-1 by x1000
How do you calculate ΔStotal?
ΔStotal = ΔSsystem + ΔSsurroundings
Why do some spontaneous reactions still occur even if they have a decrease in entropy?
Number of ways of arranging quanta in the universe is affected which affects ΔSsurroundings
What is a spontaneous reaction?
One which tends to go without being driven by any outside agency
Why can’t ΔH be used to predict whether a reaction is spontaneous?
- Doesn’t say anything about kinetic stability
- Only for standard conditions
- Doesn’t take into account other factors such as entropy
What is the definition of lattice enthalpy?
The enthalpy of formation of one mole of an ionic compound from gaseous ions under standard conditions
What are the trends in lattice enthalpy?
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
What is the definition of the enthalpy of hydration?
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
How do you work out the enthalpy of hydration?
Hydration value of each ion added together
What is the definition for the enthalpy of solution?
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)
How do you work out the enthalpy of solution?
Hydration enthalpies - lattice enthalpy
Why do some ionic compounds dissolve in water when the hydration energy is not larger than the lattice energy?
Dissolving ions in water leads to increase in entropy
Need to consider both enthalpy and entropy in ΔG which must be negative
What is ΔG?
Free energy change
