Thermodynamics ( A-level )

Question 1

What does Hess’s law state?

Answer 1

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

Question 2

What is the standard enthalpy of an element?

Answer 2

Zero

Question 3

Define standard enthalpy of atomisation

Answer 3

the enthalpy change when of gaseous compound is formed from a compound in its standard state

Question 4

Define first electron affinity

Answer 4

Enthalpy change when one mole of gaseous atom gains one mole of electron to form one mole of a gaseous 1- ions

Question 5

Define second affinity

Answer 5

Enthalpy change when one mole of gaseous 1- ion gains one mole of electron to form one mole of gaseous 2- ion

Question 6

Define lattice enthalpy of formation

Answer 6

Enthalpy change when one mole of a solid ionic compound is formed from its consistuents ions in their gaseous state

Question 7

Define lattice enthalpy of dissociation

Answer 7

Enthalpy change when mole of a solid ionic compound dissociates into its consistuent ions in their gaseous state

Question 8

Define enthalpy of hydration

Answer 8

Enthalpy change when one mole of gaseous ions dissolves completely in water to form a infinitvely dilute solution

Question 9

Define enthalpy of solution

Answer 9

Enthalpy change when one mole of a solid ionic compound dissolves completely in water to form a infinitvely dilute solution

Question 10

Define mean bond dissociation enthalpy

Answer 10

Enthalpy change when one mole of covalent is broken, with all species in their gaseous state

Question 11

What is the Born-Haber cycle?

Answer 11

a cycle that shows all the enthalpy changes involved in the formation of an ionic compound. starting with all elements in their standard state

Question 12

what factors affect the lattice enthalpy of an ionic compound?

Answer 12

Size of the ions, charge on the ions

Question 13

what happens when a solid is dissolved in terms of interactions of the ions with water?

Answer 13

• The lattice is broken forming gaseous ions
• gaseous ions are dissolved to form aqueous ions
• The aqueous ions are surrounded by the water molecules due to the polarity of the O-H bond

Question 14

What is a perfect Ionic model?

Answer 14

• model that suggests that ions are perfectly spherical and there is even charge distribution
• acts as point charges

Question 15

why is the perfect ionic model not accurate?

Answer 15

• ions are not perfectly spherical
• polarisation occurs between the interaction of small positive ions or large negative ions leading to covalent character

Question 16

what kind of bonds will be the most ionic?

Answer 16

bonds between large positive ions and small negative ions

Question 17

define entropy

Answer 17

-measure of randomness/disorder of a system
- High value for entropy = more disorder

Question 18

what unit is entropy measured in?

Answer 18

JK-1mol-1

Question 19

what is the second law of thermodynamics?

Answer 19

entropy always increases, as its overwhelmingly more likely for molecules to be disordered than ordered

Question 20

compare the general entropy values for solids,liquids and gases

Answer 20

solids- low entropy
liquid- medium entropy
gases- high entropy

Question 21

define Gibbs free energy using an equation

Answer 21

ΔG = ΔH - TΔS

Question 22

what does the value for Gibbs free energy in a reaction show?

Answer 22

• if G < 0, reaction is feasible
• if G = 0, reaction is just feasible
• if G > 0, reaction is not feasible

Question 23

how would you calculate the temperature at which a reaction becomes feasible?

Answer 23

T = ΔH / ΔS

Question 24

if the reaction is exothermic and the entropy increases, what is the value of G and what does it mean?

Answer 24

G is always negative so the reaction is always feasible

Question 25

if the reaction is endothermic and entropy decreases, what is the value of G and what does it mean?

Answer 25

G is always positive so the reaction is never feasible

Question 26

why is entropy 0 at 0K?

Answer 26

molecules/atoms are not moving or vibrating and cannot be arranged in any other way

Question 27

what are two key things to look out for to decide if entropy increases/decreases/stays relatively constant?

Answer 27

• Number of moles, if more moles are made there is an increase in entropy
• Going from solid to liquid/gas or liquid to gas