Topic 12

Question 1

Entropy

Answer 1

disorderliness of the system
more molecules / phase of the substance can increase the entropy

Question 2

ΔS(system)

Answer 2

change of state / change of number of molecules between solid and gases
ΔS(system) = ∑S(products) - ∑S(reactants)

Question 3

ΔS(surrounding)

Answer 3

ΔS(surrounding) = -ΔH/T
exothermic –> energy given out into surroundings, more ways arranging energy –> increase entropy
endothermic –> energy transferred from surroundings, number of ways to arrange energy decreased –> decrease entropy

Question 4

ΔS(total)

Answer 4

ΔS(total)=ΔS(surrounding)+ΔS(system)
positive ΔS(total) –> reaction is THERMODYNAMICALLY stable and is spontaneous

Question 5

Enthalpy change of formation

Answer 5

The energy required to form a mole of the compound from its elements under standard conditions. (at elements normal state)

Question 6

Enthalpy of atomisation

Answer 6

Enthalpy change when a mole of gaseous atom is formed from the element in its standard state.
Na(s) –> Na(g)
1/2 O2(g) –> O(g)

Question 7

First ionisation enthalpy

Answer 7

Enthalpy change required to remove a mole of electrons from a mole of gaseous atoms to form a mole of gaseous ion with a +1 charge

Question 8

Second ionisation enthalpy

Answer 8

Enthalpy change required to remove a mole of electrons from a mole of gaseous ion with +1 charge to form a mole of gaseous ion with a +2 charge

Question 9

First electron affinity

Answer 9

Enthalpy change that is required when a mole of gaseous atoms gain a mole of electrons to form a mole of gaseous ions with -1 charge

Question 10

Secondary electron affinity

Answer 10

Enthalpy change that is required when a mole of gaseous -1 ion gains a mole of electron to form a gaseous ion with -2 charge
(endothermic as it requires energy to overcome the repulsive force between -1 ion and the electron)

Question 11

Enthalpy of lattice formation

Answer 11

Enthalpy change when a mole of an ionic crystal lattice is formed from its constituent ion in gaseous form.

Question 12

How does size of ions and charges of ion affect the enthalpy of lattice formation

Answer 12

Larger ions, less negative the enthalpies of lattice formation as the ions are larger the charges become further apart and so have a weaker attractive force between them
Bigger the charge of ion, greater the attraction between the ions so stronger the lattice enthalpy (more negative value)

Question 13

Born Haber Cycle

Answer 13

remember to add electron for ionisation energy
if need 2 like MgCl2 , when doing atomisation and electron affinity need to times 2
if one like NaCl, but Cl2 is standard state, need to write 1/2 Cl2

Question 14

Perfect ionic model

Answer 14

ions are 100% ionic and spherical and attractions are purely electrostatic

Question 15

Theoretical value vs Experimental lattice enthalpies

Answer 15

Because have covalent character due to polarisation. Negative ion becomes distorted and more covalent –> polarised by the positive ion.

Question 16

Polarising power of cation

Answer 16

Increases when positive ion is small and has larger charge = larger charge density –> polarise anion better

Question 17

Enthalpy of hydration

Answer 17

Enthalpy change when a mole of gaseous ions becomes hydrated such that further dilute causes no further heat change

Question 18

Enthalpy of solution

Answer 18

Enthalpy change when a mole of an ionic solid dissolves in a large enough amount of water

Question 19

Calculation to determine enthalpy change of solution

Answer 19

Δsol. H = -ΔLE H + ∑Δhyd H
Δsol. H: MgCl2(s) + aq –> Mg2+(aq) + 2Cl- (aq)
ΔLE H: Mg2+(g) + 2Cl- (g) –> MgCl2 (s)
Δhyd H: Mg2+ (g) + aq –> Mg2+ (aq)

Question 20

Why some ionic solids are soluble and some are not

Answer 20

Δsol.H is exothermic to be soluble
Δsol.H is endothermic to be insoluble