Energetics II

Question 1

what is lattice energy

Answer 1

the energy given out when gaseous ions combine to make a solid lattice

Question 2

what is the definition for standard lattice energy

Answer 2

energy change when 1 mole of an ionic solid is formed from its gaseous ions under standard conditions 298K / 100kPa

Question 3

what two things affect lattice energy

Answer 3

ionic charge: higher the charge the more energy released when ionic lattice formed due to stronger electrostatic forces between ions. More energy released means lattice energy will be more negative.

size: smaller the ionic radii the more exothermic the lattice energy as higher charge density and sit closer together in lattice so attraction of ions are stronger
This is because the charge on the ions is more spread out over the ion when the ions are larger

Question 4

what is enthalpy of atomisation

Answer 4

the enthalpy change when 1 mole of gaseous atoms is formed from the element in its standard state

Question 5

what is electron affinity

Answer 5

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 6

what does theoretical lattice energies assume

Answer 6

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

Question 7

why do theoretical lattice energy differ from measured lattice energy

Answer 7

covalent character introduced when polarisation of anions. Cations attracts electrons from anion distorting electron density of the anion

Question 8

what does the polarisability of the anion depend on

Answer 8

ionic radius, the larger the more easily distorted

Question 9

what is standard enthalpy change of solution (ΔHsolꝋ)

Answer 9

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

Question 10

what is enthalpy of hydration

Answer 10

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

Question 11

are hydration enthalpies exo or endo

Answer 11

exothermic

Question 12

what happens when a ionic solid dissolves in water

Answer 12

water is polar with δ- oxygen (O) atom and δ+ hydrogen (H) atoms form ion-dipole attractions with ion present in solution

Question 13

what is hydration enthalpies

Answer 13

measure of the energy that is released when there is an attraction formed between the ions and water molecules

Question 14

what is route 1 and route 2 for constructing this energy cycle

Answer 14

gaseous ions → ionic solid → ions in aqueous solution (this is the indirect route)

going from gaseous ions → ions in aqueous solution (this is the direct route)

ΔHhydꝋ = ΔHlattꝋ + ΔHsolꝋ

Question 15

what makes the ions attract the water molecules more strongly

Answer 15

The higher the charge density the greater the hydration enthalpy so ions attract water molecules more strongly

Question 16

why is hydration enthalpies exothermic

Answer 16

as energy is given out as water molecules bond to the metal ions

Question 17

what is ionisation energy

Answer 17

the amount of energy required to remove an electron from an atom or ion in the gas phase

Question 18

What is Kc

Answer 18

Equilibrium expression linking the equilibrium concentration of reactants and products at equilibrium

Question 19

When does Kc chance

Answer 19

if the temperature of the reaction changes

Question 20

What is Kp

Answer 20

Equilibrium expression linking the partial pressures of reactants and products at equilibrium

Question 21

What is ignored in heterogeneous reactions

Answer 21

Solids and liquid are ignored

Question 22

For an endothermic reaction what does an increase in temperature cause

Answer 22

the products to increase and reactants to decrease therefore equilibrium constant will increase vice versa

Question 23

when does lattice energy become less exothermic

Answer 23

The lattice energy becomes less exothermic as the ionic radius of the ions increases
or as the ionic charge of the ions increases

Question 24

why does lattice energy become less exothermic with ionic radius

Answer 24

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 25

why does lattice energy become less exothermic with ionic charge

Answer 25

The greater the ionic charge, the higher the charge density
This results in stronger electrostatic attraction between the oppositely charged ions in the lattice
As a result, the lattice energy is more exothermic

Question 26

what factors affect enthalpy of hydration

Answer 26

Hydration enthalpies are exothermic as energy is given out as water molecules bond to the metal ions

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 27

Polarisability of cations and anions

Answer 27

A cation with a large charge density will have a larger polarising power So a cation that is small and highly charged will have the greatest polarising power

Anions are polarised
The polarisability of the anion depends of its ionic radius
The larger the ionic radius the more easily it will be distorted

Question 28

What is the entropy of a given system

Answer 28

number of possible arrangements of the particles and their energy in a given system - it is a measure of how disordered or chaotic a system is.

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

Question 29

ΔSΘ total =

Answer 29

ΔS Θsys + ΔSΘsurr

Question 30

unit for entropy

Answer 30

J K-1 mol-1

Question 31

ΔSΘsystem =

Answer 31

ΣΔSΘproducts - ΣΔSΘreactants

Question 32

ΔSΘ surr =

Answer 32

• enthalpy change / T

Question 33

what is the feasibility of a reaction determined by

Answer 33

The enthalpy and entropy change

For a reaction to be feasible, ΔGꝋ must be equal or less than zero

Question 34

gibs equation

Answer 34

ΔGꝋ = ΔHreactionꝋ – TΔSsystemꝋ

Question 35

equations for kc directly related to gibbs free energy

Answer 35

ΔGꝊ = -RT lnK
determines if forward or backward favoured

Question 36

what does a negative ΔG value for a reaction indicates

Answer 36

The reaction is feasible / spontaneous
The equilibrium concentration of the products is greater than the equilibrium concentration of the reactants
The value of the equilibrium constant is greater than 1