topic 13- further energetics

Question 1

standard enthalpy change of lattice ΔlatticeH

Answer 1

The energy change when one mole of an ionic solid is formed from its gaseous ions
The value of the lattice energy depends on:
-Magnitude of charge of ions
-Ionic radii
-Lattice structure
-Extent of covalent character in the bond.

Question 2

Standard enthalpy change of atomisation: ΔatH

Answer 2

The standard enthalpy change when one mole of gaseous atoms are formed from an element in its standard state, at a stated temperature and 100kPa.
In equation form:
Mg(s) → Mg(g)
S8(s) → 8S(g)
½ N2 (g) → N(g)
½ Cl2(g) → Cl(g)

Question 3

First electron affinity: Eea(1)

Answer 3

The first electron affinity Eea(1) is the energy change when each atom in a mole of atoms in the gaseous state gains an electron and forms a 1- ion.
In equation form:
N (g) + e → N- (g)
S (g) + e → S- (g)
First electron affinity is negative (exothermic) except for noble gases, where the ‘new’ electron must go into an new shell

Question 4

Second electron affinity: Eea(2

Answer 4

The second electron affinity is when a mole of 1- ions gains a second electron and forms a 2- ion.
In equation form:
N- (g) + e → N2- (g)
S- (g) + e → S2- (g)
Second electron affinity is positive (endothermic) due to the already negatively charged ion repelling the incoming electron.

Question 5

theoretical lattice energy

Answer 5

Assume:
- ions are in contact
- ions are perfect spheres
- even distribution of charge throughout the volume

Question 6

poor agreement

Answer 6

ionic bonding model is not sufficient
due to the extent of covalent character
bigger discrepancy the bigger the extend of character

Question 7

entropy

Answer 7

measure of the degree of randomness or disorder of particles
higher entropy - highest disorder
moles of products > moles of reactants
- positive entropy for feasible reaction
-both entropy of surroundings and system is positive
because more way the particles can be arranged

Question 8

barium hydroxide and ammonium chloride

Answer 8

endothermic reaction
Ba(OH)2.8H2O (s) + 2NH4Cl (s) → BaCl2 (s) + 10 H2O (l) + 2 NH3 (g)
entrophy change is positive
thermodynamically spontaneous

Question 9

ammonium carbonate and ethanoic acid

Answer 9

endothermic
2 CH3COOH (l) + (NH4)2CO3 (s) → 2 CH3COONH4 (aq) + H2O (l) + CO2 (g)
entrophy change is positive
thermodynamically spontaneous

Question 10

magnesium and oxygen

Answer 10

exothermic reaction
2 Mg (s) + O2 (g) → 2 MgO (s)
entropy is positive as the surroundings overpowers
thermodynamically spontaneous

Question 11

total entropy

Answer 11

entropy of surroundings + entropy of system

Question 12

entropy of a system

Answer 12

products - reactants

Question 13

entropy of surroundings

Answer 13

-enthlapy change / temperature

Question 14

gibbs free energy

Answer 14

Calculate whether a reaction is feasable (spontaneous):
- If feasible gibbs is -ve
- If not feasible its +ve
- If equilibrium its 0

enthlapy change - temp(entropy change)

Question 15

calculate temp when feasable

Answer 15

Make ΔG=0
Rearrange equation

enthlapy change / entropy of system

Question 16

properties of ions that affect enthlapy change

Answer 16

• charge
• ionic radius

Question 17

close lattice energy values

Answer 17

mostly 100% ionic

Question 18

different lattice energy values

Answer 18

differing charges and ionic radius
so one polarises the other
so bonding has covalent character

Question 19

why expect entropy of system to be positive

Answer 19

increase number of moles
change of state - s - l - g

Question 20

enthalpy change of solution

Answer 20

the enthlapy change when one mol of an ionic solid dissolves in water to form an infinitely dilute solution

Question 21

enthlapy change of hydration

Answer 21

enthlapy change when one mole of an ion in its gaseous state is completely hydrated by water

Question 22

equation of equilib constant and gibbs

Answer 22

ΔG = -RTlnK

Question 23

speical cases of gibbs free energy

Answer 23

kintetic stability
-if thermodynamically feasable dosent necisary mean it will occur
activation energy too high
non standard conditions
-may not happen at standard conditions may need to be heated

Question 24

solubility of salts

Answer 24

if gibs of solution is negative the products are favoured at equilibrium and the salt is soluble
if the gibs of solution is positive than the solid salt is favoured at equilibrium so the salt is insoluble

Question 25

when gibbs is positive

Answer 25

not thermodynamicaly feasable
enthlapy>0, entropy of system<0
enthlapy>0, entropy of system>0 enthlapy>entropy
enthlapy<0, entropy of system<0 enthlapy<entropy

Question 26

gibbs negative

Answer 26

thermodynamicaly feasable - thermodynamicaly spontaneous
enthlapy<0, entropy of system>0
enthlapy<0, entropy of system<0 enthlapy>entropy
enthlapy>0, entropy of system>0 enthlapy<entropy

Question 27

standard enthalpy change of neutrilisation of a weak acid compared to a strong acid

Answer 27

weak acid is less negative as some energy is needed to break the bonds to completely disassociate

Question 28

calculating lattice energy

Answer 28

Standard enthalpy of atomisation
Electron affinity
Ionisation energy