Thermodynamics

Question 1

enthalpy of lattice formation

Answer 1

• enthalpy change when one mole of a solid ionic compound
• formed from it’s gaseous ions

Question 2

A calculation of the enthalpy of lattice formation of silver iodide based on a perfect ionic model gives a smaller numerical value than the value calculated
Explain this difference

Answer 2

• AgI contains covalent character
• Bonds (holding the lattice together) are stronger

Question 3

The enthalpy of lattice formation for caesium iodide in Table 1 is a value obtained by experiment. –1 The value obtained by calculation using the perfect ionic model is –582 kJ mol
Deduce what these values indicate about the bonding in caesium iodide.

Answer 3

(Almost/Mostly) purely/ perfectly ionic

Question 4

State the meaning of the term periodicity.

Answer 4

Repeating pattern/trends (of physical or chemical properties/reactions)

Question 5

State why there is a difference between theoretical and experimental values

Answer 5

covalent character

Question 6

Explain why the hydration becomes less exothermic from Li+ to K+

Answer 6

• size of ion increases from Li+ to K+
• attraction between lone pair on O and + ion decreases

Question 7

state meaning of enthalpy change

Answer 7

• heat energy change at constant pressure

Question 8

The enthalpy of hydration of Ca2+(g) is –1650 kJ mol–1 Suggest why this value is less exothermic than that of Mg2+(g)

Answer 8

• Ca2+ (ion) bigger/lower charge to size ratio (than Mg2+)
• weaker attraction/bond to (Oδ- in) water

Question 9

explain why standard entropy value for CO2 is greater than carbon

Answer 9

CO2 / gas is more disordered (than solid)

Question 10

State the temperature at which the standard entropy of aluminium is 0 J K–1 mol–1

Answer 10

0 K

Question 11

standard enthalpy of formation

Answer 11

• enthalpy change when one mole of a compound is formed from its elements
• under standard conditions
• reactants and products in standard states

Question 12

standard enthalpy of combustion

Answer 12

• enthalpy change when one mole of a compound
• completely burned in oxygen
• under standard conditions
• reactants and products in standard states

Question 13

standard enthalpy of atomisation

Answer 13

• enthalpy change when one mole gaseous atoms are formed
• from an element in its standard state

Question 14

mean bond enthalpy

Answer 14

• standard enthalpy change
• when one mole of gaseous molecules each break a covalent bond
• to form 2 free radicals
• averaged over a range of compounds

Question 15

first ionisation energy

Answer 15

• standard enthalpy change when one mole of electrons
• is removed from one mole of gaseous atoms
• to form one mole of gaseous ions each with single positive charge

Question 16

second ionisation energy

Answer 16

• standard enthalpy change when one mole of electrons
• is removed from one mole of gaseous 1+ ions
• to form one mole of gaseous ions each with 2+ charge

Question 17

first electron affinity

Answer 17

• standard enthalpy change when one mole of gaseous atoms
• converted into one mole of gaseous ions
• each with single negative charge
• under standard conditions

Question 18

second electron affinity

Answer 18

• standard enthalpy change when one mole electrons
• added to one mole of gaseous ions
• with single negative charge
• to form one mole of ions with 2- charge

Question 19

lattice formation

Answer 19

• enthalpy change when one mole of solid ionic compound
• is formed from its gaseous ions

Question 20

lattice dissociation

Answer 20

• enthalpy change when one mole of solid ionic compound
• dissociates into its gaseous ions

Question 21

enthalpy of hydration

Answer 21

• standard enthalpy change when one mole of gaseous ions
• is converted into one mole of aqueous ions

Question 22

enthalpy of solution

Answer 22

• standard enthalpy change when one mole of solute
• dissolves in enough solvent to form solution with ions are enough apart not to interact with each other

Question 23

what does the perfect ionic model predict

Answer 23

• ions act as point charges
• ions are perfect spheres which cannot be distorted
• ions show purely ionic bonding with no covalent character

Question 24

if a ionic compound has more exothermic lattice formation what does this suggest

Answer 24

covalent character

Question 25

positive entropy change means

Answer 25

more disorder

Question 26

negative entropy change means

Answer 26

more ordered

Question 27

when is the reaction feasible

Answer 27

when Gibbs is less than or equal to 0

Question 28

calculating temperature that reaction becomes feasible

Answer 28

T = enthalpy change/entropy change
T = H/S

Question 29

equation that links Gibbs to y=mx + c

Answer 29

Gibbs = -ST + H
y = mx + c

Gibbs = y-axis
temp = x-axis
gradient = -S
y-intercept = H

Question 30

In terms of electrostatic forces, suggest why the electron affinity of fluorine has a negative value.

Answer 30

• attraction between nucleus and extra electron
• energy released when electron is gained

Question 31

write equation for first electron affinity for chlorine

Answer 31

Cl (g) + e- —> Cl- (g)

Question 32

Explain why the bond enthalpy of a Cl–Cl bond is greater than that of a Br–Br bond

Answer 32

• bonding pair closer to nucleus/Cl smaller atom
• so attraction is stronger

Question 33

Suggest why the electron affinity of chlorine is an exothermic change.

Answer 33

• attraction between chlorine nucleus and extra electron

Question 34

explain why there is a difference between the hydration enthalpies between magnesium and sodium ions

Answer 34

• Mg2+ smaller ion AND more highly charged
• So more strongly attracted to water

Question 35

state meaning of mean bond enthalpy of O-H bonds

Answer 35

• standard enthalpy change when 1 mole of O-H molecules break to form covalent bond
• averaged over a range of compounds

Question 36

explain why value of Kw increases as temperature increases

Answer 36

• [H2O] is constant
• only partially dissociates

Question 37

Explain why kw value increases as temperature increases

Answer 37

• breaking bonds is endothermic
• equilibrium moves to right hand side

Question 38

state why there is a difference between theoretical and experimental values

Answer 38

covalent character

Question 39

explain why enthalpy of hydration becomes less exothermic from Li+ and K+

Answer 39

• ion size increases
• weaker attraction between metal ion and O- on water