Energetics II

Question 1

what is lattice energy of formation

Answer 1

enthalpy change when 1 mole of an ionic compound is formed from its gaseous ions (under standard conditions)

Question 2

is lattice energy of formation exo/endo

Answer 2

exothermic, as when ions are combined to form an ionic solid lattice there is an extremely large release of energy

Question 3

what does large negative value of lattice energy suggest

Answer 3

ionic compound is much more stable than its gaseous ions
This is due to the strong electrostatic forces of attraction between the oppositely charged ions in the solid lattice
Since there are no electrostatic forces of attraction between the ions in the gas phase, the gaseous ions are less stable than the ions in the ionic lattice

Question 4

strength of bonds in regards to exothermic

Answer 4

more exothermic means stronger ionic bonds

Question 5

what is enthalpy change of atomisation

Answer 5

The standard enthalpy change of atomisation (ΔHatꝋ) is the enthalpy change when 1 mole of gaseous atoms is formed from its element under standard conditions

Question 6

is enthalpy change of atomisation endo or exo

Answer 6

always endothermic as energy is always required to break any bonds between the atoms in the element, to break the element into its gaseous atoms

Question 7

what is electron affinity

Answer 7

energy change when one mole of electrons is gained by one mole of gaseous atoms of an element to form one mole of gaseous ions under standard conditions

Question 8

difference between first and second affinity and why

Answer 8

The first electron affinity is always exothermic as energy is released when electrons are attracted to the atoms
However, the second electron affinity of an element can be endothermic

This is because a large force of repulsion must be overcome between the negatively charged ion and the second electron requiring a large input of energy

Question 9

what is theoretical lattice energy

Answer 9

assume a perfect ionic model where the ions are 100% spherical and the attractions are purely electrostatic

Question 10

why do theoretical lattice energy differ from measured

Answer 10

covalent character is introduced when there is polarisation of the anions
the cation attracts electrons from the anion therefore distorting electron density of the anion

Question 11

what type of cation has a larger polarising power

Answer 11

A cation with a large charge density

Question 12

do anions polarise or get polarised

Answer 12

get polarised ad this depends on ionic radius, The larger the ionic radius the more easily it will be distorted

Question 13

what is standard enthalpy change of solution

Answer 13

enthalpy change when 1 mole of an ionic substance dissolves in sufficient water to form an infinitely dilute solution

Question 14

is standard enthalpy change of solution (Hsol) exo or endo

Answer 14

can be exothermic (negative) or endothermic (positive)

Question 15

what is standard enthalpy change of hydration (ΔHhydꝋ)

Answer 15

enthalpy change when 1 mole of a specified gaseous ion dissolves in sufficient water to form an infinitely dilute solution

Question 16

difference between enthalpy of hydration and solution

Answer 16

solution is for ionic substance and hydration for gaseous ions

Question 17

are hydration enthalpy exo or endo

Answer 17

exothermic

Question 18

formula for enthalpy of hydration

Answer 18

ΔHhydꝋ = ΔHlattꝋ + ΔHsolꝋ

Question 19

what factors affect lattice enthalpy

Answer 19

charge and radius of the ions that make up the crystalline lattice

Question 20

how does lattice energy become less exothermic with ionic radius

Answer 20

as the ionic radius of the ions increases, this is because the charge on the ions is more spread out over the ion when the ions are larger, The ions are also further apart from each other in the lattice. Therefore, the electrostatic forces of attraction between the oppositely charged ions in the lattice are weaker

Question 21

how does lattice energy get more exothermic with ionic charge

Answer 21

The greater the ionic charge, the higher the charge density
results in stronger electrostatic attraction between the oppositely charged ions in the lattice

As a result, the lattice energy is more exothermic

Question 22

factors affecting enthalpy of hydration

Answer 22

The higher the charge density the greater the hydration enthalpy (e.g. smaller ions or ions with larger charges) as the ions attract the water molecules more strongly

Question 23

what is the entropy of a given system

Answer 23

how disordered or chaotic a system is

(number of possible arrangements of the particles and their energy in a given system)

Question 24

what happens if system is more disordered

Answer 24

When a system becomes more disordered, its entropy will increase
An increase in entropy means that the system becomes energetically more stable

Question 25

what is formula for total entropy change

Answer 25

ΔSΘ total = ΔS Θsys + ΔSΘsurr

Question 26

formula for standard entropy change of a system
(ΔS Θsys)

Answer 26

ΔSΘsystem = ΣΔSΘproducts - ΣΔSΘreactants

Question 27

formula for ΔSΘ surr

Answer 27

-(enthalpychange/Temperature)

Question 28

what is the feasibility of a reaction determines by

Answer 28

The enthalpy and entropy change which come together in a concept called Gibbs free energy

Question 29

gibbs equation

Answer 29

ΔGꝋ = ΔHreactionꝋ – TΔSsystemꝋ

Question 30

what happens when gibbs energy is negative

Answer 30

the reaction is spontaneous / feasible and likely to occur

Question 31

limitation of Gibbs

Answer 31

doesn’t take kinetics into account - there might be a large energy barrier (Ea) which the reacting species have to overcome before a reaction can occur

Some reactions are feasible since ∆G is negative, but kinetically not feasible since it just occurs too slowly
Such reactions are feasible but very slow