3.6/3.7 - Enthalpy and Entropy

Question 1

What is thermodynamics?

Answer 1

Study of how heat and temperature relate to energy and work.

Question 2

What is the 1st law of thermodynamics?

Answer 2

Energy and matter passing into or out of a system must abide by the law of conservation of energy, energy cannot be created or destroyed, only transferred from one form into another.

Question 3

What is the second law of thermodynamics?

Answer 3

The sum of entropies of interacting thermodyamic systems naturally increase.

Question 4

What is heat and what other name does it have?

Answer 4

Heat is a form of energy which is transferred as a result of a temperature difference and provides an increase in disorder as to how particles behave.
The heat content of a system is called its enthalpy.

Question 5

What is bond dissociation energy?

Answer 5

Energy required to break one mole of gaseous bonds to give separated atoms. DeltaH
Always endothermic as you have to supply energy to break the bond.
Example
HCl(g) - H(g) + Cl(g) = +432kjmol-1

Question 6

What is lattice energy?

Answer 6

Enthalpy change when one mole of a solid ionic compound is formed from its gaseous ions
Always exothermic

Question 7

What simplified equation is linked to lattice enthalpy and what does this tell us in terms of proportionality?

Answer 7

The born-iande equation
Super simplified to E=the product of the charges/the sum of the radii
The lattice enthalpy is proportional to the product of the charges (as the charges increase, so does the E)
The lattice enthalpy is inversely proportional to the sum of the radii (as radii increases, the E decreases)

Question 8

What effect does charge have on lattice enthalpies?

Answer 8

• the greater the product of the charges, the greater the LE
• the LE depends on the electrostatic attraction between the ions
  -the greater the charge, the stronger the bond that is formed, and therefore more energy is released.

Question 9

What does the effect of radii have on lattice enthalpies?

Answer 9

• the smaller the ion, the greater the charge density and therefore the force of attraction is greater beween the ions, increasing LE

Question 10

What is the standard enthalpy of atomisation?

Answer 10

The enthalpy change when one mole of gaseous atoms are produced to form an element in its standard state (delta H a)
Endothermic as energy required to break an element into separate gaseous ions or atomise it from a solid
Example
Na(s) - Na(g) = +107kjmol-1

Question 11

What is first ionisation energy?

Answer 11

Delta H i
The energy required to remove one mole of electrons from one mole of gaseous atoms
Na(g) - Na+(g) + e-(g) = +496kjmol

Question 12

What is the first electron affinity?

Answer 12

Delta H e
The enthalpy change when one mole of gaseous atoms attracts one mole of electrons.
Opposite of first ionisation energy
Cl(g) +e-(g) - Cl-(g) = -349kjmol-1
Always exothermic value for first one.

Question 13

Why does second electron affinity require energy?

Answer 13

x-(g)+e- = x2- (g) (endothermic)
- electrostatics tells us that charges repel, the electron is negatively charged and so is the 1- ion
- that repulsion must be overcome before the 2- ion can be formed and energy is required for this change

Question 14

What is a born-haber cycle?

Answer 14

Hess’s law mixed in with all the enthalpy values. Process of formation of a product.
For example the process required for Na(s) +1/2Cl2(g) to turn into NaCl(s)
Draw the arrow cycle in the direction away from the enthalpy you want to find, if the arrows don’t match switch the sign and add all together.

Question 15

What is the standard enthalpy of hydration?

Answer 15

Delta H h
The enthalpy change when one mole of gaseous ions dissolves in water to create an infinitely dilute solution
Cl-(g) + aq - Cl-(aq) = -378kjmol-1

Question 16

Why is standard enthalpy of hydration always exothermic?

Answer 16

When a substance dissolves electrostatic attractions are formed between the polar water molecules and the ion in question.
- with positive ions, electrostatic attraction or even dative covalent bonds can form between the oxygens lone pairs and the positve ion
- with negative ions are stabilised by the positive hydrogen of the water molecules
So either way bonds are formed and energy is released.

Question 17

What are the factors affecting the size of the hydration enthalpy?

Answer 17

• the size of the hydration enthalpy depends on the strength of the attraction between the ions and the water molecules
• smaller ions have stronger attractions so greater E due to a higher charge density relative to other ions with the same charge
• hydration enthalpies falls you go down the the group because the ions get bigger
• ions with a greater charge but with similiar size have stronger attractions

Question 18

What is the standard enthalpy change of solution?

Answer 18

Delta H sol
The enthalpy change when one mole of an ionic substance dissolves in water to create an infinitely dilute solution
example - KOH(s) +(aq)= K+(aq) +OH- (aq) = -58kjmol-1
Delta H sol can be positive or negative, so when ionic substancees dissolve it can be endothermic or exothermic
(example coffee needs hot water to form a coffee solution)

Question 19

What links lattice, solution and hydration enthalpy/

Answer 19

You can draw a hess diagram linking all values to find a missing value.
Usually lattice on top, hydration on left and sol on right hand side.
So in this case it would be lat + sol = hydration

Question 20

What is delta H sol in endothermic dissolving?

Answer 20

Delta H sol is more than 0

Question 21

What is delta H sol in exothermic dissolving?

Answer 21

Delta H sol is less than 0

Question 22

What is entropy?

Answer 22

The degree of disorder in a system is measured by its entropy (s)
A system becomes more stable when is energy is spread out in a more disorded state
‘The number of possible arrangements of the particles and their energy in a given system’.
Particles adapt a more disordered state with high entropy.

Question 23

What is entropy like in solids and gasses?

Answer 23

Solid state = low entropy
Gaseous state = high entropy
Therefore the entropy of a system increases as a solid changes to a liquid and a liquid changes to a gas.

Question 24

What 2 factors affect entropy?

Answer 24

• increasing the number of particles present in the gaseous state increases the entropy of a system
• increasing the temperature will increase the entropy as more energy is available to be shared between the particles

Question 25

What is the standard entropy?

Answer 25

S (standard^)
Of a substance is its entropy under standard conditions (units Jk-1mol-1)

Question 26

How can the entropy change of a system be calculated?

Answer 26

Delta H Ssystem = the sum of delta S (products) - the sum of delta S (reactants)
Can expect a decrease (negative value) if number of moles of gas decreases in the equation.

Question 27

What is the equation for delta S surroundings?

Answer 27

Delta S surroundings = -deltaH/T

Question 28

What is the equation for delta S total?
When is a reaction feasible?

Answer 28

Delta S total = delta S system + delta S surroundings
It is feasible when deltaS total is greater than 0

Question 29

What is the equation for gibbs free energy change?

Answer 29

Delta G = delta H reaction - T delta Ssystem

Question 30

When is delta gibbs feasible?

Answer 30

If delta gibbs is less than zero.
If H- and S+, the reaction is feasible at any temperature
If H+ and S-, the reaction is not feasible at any temperature
If H+ and S+, the reaction is only feasible at a high temperature
If H- and S-, the reaction is only feasible at a low temperature
- exothermic reactions occur at low temperatures
- all reaction with s+ are feasible at high temperatures

Question 31

Even if delta gibbs is feasible, what other factors limit the process?

Answer 31

• a high Ae may stop the reaction from proceeding
• a kinetically stable system has no tendancy to react
• a kinetically inert system has the potential to react but the rate is very slow due to the high activation energy