1.8 Thermodynamics

Question 1

What does Hess’s law state

Answer 1

The enthalpy change for a reaction is independent of the route taken

Question 2

Define standard enthalpy of formation

Answer 2

The enthalpy change when one mole of a compound is formed from its elements under standard conditions in its standard states

Question 3

Whats the standard enthalpy of an element

Answer 3

0

Question 4

Define standard enthalpy of combustion

Answer 4

The enthalpy change when one mole of a substance is burnt completely in excess oxygen

Question 5

Define standard enthalpy of atomisation

Answer 5

Enthalpy change when one mole of gaseous atoms is formed from a compound in its standard state in standard conditions

Question 6

Define first ionisation energy

Answer 6

Enthalpy change when one mole of electrons is removed from one mole of gaseous atoms to form one mole of gaseous 1+ ions

Question 7

Define second ionisation energy

Answer 7

Enthalpy change when one mole of electrons is removed from one mole of gaseous 1+ ions to form one mole of gaseous 2+ ions

Question 8

Define first electron affinity

Answer 8

Enthalpy change when one mole of gaseous atoms gains one mole of electrons to form one mole of gaseous 1- ions

Question 9

Define second electron affinity

Answer 9

Enthalpy change when one mole of gaseous 1- ions gains one mole of electrons to form one mole of gaseous 2- ions

Question 10

Define lattice enthalpy of formation

Answer 10

Enthalpy change when one mole of solid ionic lattice is formed from its gaseous ions

Question 11

Define lattice enthalpy of dissociation

Answer 11

Enthalpy change when one mole of solid ionic lattice is dissociated into its gaseous ions

Question 12

Define enthalpy of hydration

Answer 12

Enthalpy change when one mole of gaseous ions become hydrated in water to infinite dilution

Question 13

Define enthalpy of solution

Answer 13

Enthalpy change when one mole of solute dissolves completely in a solvent to infinite dilution

Question 14

Define mean bond dissociation enthalpy

Answer 14

Enthalpy change when one mole of covalent bonds is broke, with all species in the gaseous state

Question 15

What factors effect the lattice enthalpy of an ionic compound

Answer 15

Size of ions
Charge on the ions

Question 16

How can you increase lattice enthalpy and why

Answer 16

Smaller ions - charge centres will be closer together
Increased charge - greater electrostatic FOA between oppositely charted ions

Question 17

How can born haber cycles be used to see if compounds could theoretically exist

Answer 17

Use known data to predict certain values and see if they would be thermodynamically stable

Question 18

What happens when a solid is dissolved in terms of interactions of ions with water

Answer 18

Break lattice –> gaseous ions, dissolve in water. Aqueous ions are surrounded by water molecules which have a permanent dipole due to polar O-H bond

Question 19

What is the perfect ionic model

Answer 19

Assumes that ions are perfectly spherical and that there is an even charge distribution

Question 20

Define the terms spontaneous and feasible

Answer 20

If a reaction is spontaneous and feasible, it will take place of its own accord

Question 21

Is a reaction with a positive or negative enthalpy change more likely to be spontaneous

Answer 21

negative - exothermic

Question 22

Define entropy

Answer 22

Randomness/disorder of a system
Higher value = more disordered

Question 23

What units is entropy measured in

Answer 23

JK-1mol-1

Question 24

What is the second law of thermodynamics

Answer 24

Entropy always increases, as it is overwhelmingly more likely for molecules to be disordered than ordered

Question 25

Is a reaction with positive or negative entropy more likely to be spontaneous

Answer 25

positive - reactions always try and increase the amount of disorder

Question 26

Compare the general entropy values for solids, liquids and gases

Answer 26

Solids < liquids < gases

Question 27

How would you calculate the entropy change for a reaction

Answer 27

Entropy change = sum of products entropy - sum of reactants entropy

delta S = delta S (products) - delta S (reactants)

Question 28

Define gibbs free energy using an equation

Answer 28

Delta G = delta H - T Delta S

Gibbs free energy = enthalpy change - temperature x entropy change

Question 29

What does the value for gibbs free energy for a reaction show

Answer 29

If G<0 reaction is feasible
If G = 0 reaction is JUST feasible
If G > 0 reaction isnt feasible

Question 30

What is the significance of the temperature at which G = 0

Answer 30

This is the temperature (in kelvin) at which the reaction becomes feasible

Question 31

Limitations of using G as an indicator whether a reaction will occur

Answer 31

Only indicates if a reaction is feasible
Doesnt take into account rate of the reaction

Question 32

Why is entropy 0 at 0K

Answer 32

No disorder
Molecules/atoms are not moving or vibrating and cannot be arranged in any other way
Maximum possible state of order