determination of enthalpy change using Hess' Law

Question 1

what is the definition of Hess’ Law?

Answer 1

the enthalpy change for a chemical reaction is the same regardless of whether the reaction takes place in 1 step or several steps, provided that the initial states of the reactants & the final states of the products are the same

Question 2

how do we apply Hess’ Law?

Answer 2

by Hess’ Law,
total enthalpy change for path 1 (direct) = total enthalpy change for path 2 (indirect)

Question 3

what is the formula for calculating ΔHrθ from enthalpy change of formation, ΔHfθ?

Answer 3

ΔHrθ = ∑ΔHfθ (products) - ∑ΔHfθ(reactants)

Question 4

what is the Born-Haber cycle used for?

Answer 4

to determine the enthalpy change of formation of ionic compounds

Question 5

what is the definition of lattice energy?

Answer 5

lattice energy, ΔHlattθ is defined as the heat evolved when 1 mole of solid ionic compound is formed rom its constitutional gaseous ions at 298K & 1 bar

Question 6

is lattice energy always endothermic or exothermic?

Answer 6

ALWAYS EXOTHERMIC!!

Question 7

what is the definition of ionisation energy?

Answer 7

the 1st ionisation energy of an element is the minimum energy required to completely remove 1 mole of valence electrons from 1 mole of ground state atoms in the gaseous state

Question 8

is the 1st ionisation energy always exothermic or endothermic?

Answer 8

ALWAYS ENDOTHERMIC!!

Question 9

why is the 1st ionisation energy always endothermic?

Answer 9

because 1 mole of ground state atoms in the gaseous state need to take in heat to remove 1 mole of valence electrons!!

Question 10

what is the definition of electron affinitiy?

Answer 10

the 1st electron affinity is the enthalpy change when 1 mole of gaseous atoms gains 1 mole of electrons to form 1 mole of gaseous singly-charged anions

Question 11

is the 1st electron affinity usually exothermic or endothermic?

Answer 11

it is usually exothermic!!

Question 12

why is the enthalpy change for the 2nd electron affinity always positive?

Answer 12

• energy needs to be supplied to overcome the repulsion between 2 negatively charged species, as 1 mole of valence electrons are now being added to a negatively charged ion

Question 13

what are the steps to construct a Born-Haber cycle?

Answer 13

1. F
2. A
3. I
4. L

Question 14

what does F in the 1st step in constructing a Born-Haber cycle represent?

Answer 14

formation of the solid ionic compound from its constituent elements in their standard states

Question 15

what does A in the 2nd step in constructing a Born-Haber cycle represent?

Answer 15

Atomisation of the constituent elements in their standard states to the gaseous state

Question 16

what does I in the 3rd step in constructing a Born-Haber cycle represent?

Answer 16

-ionisation (in the case of the metal atoms) of the gaseous atoms into gaseous ions

• electron affinity (in the case of the non-metal atoms) of the gaseous atoms into gaseous ions

Question 17

what does L in the 4th step in constructing a Born-Haber cycle represent?

Answer 17

lattice energy of the solid ionic compound formed from its constituent gaseous ions

Question 18

what does lattice energy, ΔHlattθ ,measure?

Answer 18

it measures the strength of ionic bonds within an ionic compound

Question 19

what are the factors affecting the magnitude of lattice energy?

Answer 19

1. size of ion
2. charge of ion

Question 20

how does the size of the ion affect the magnitude of lattice energy?

Answer 20

as the size of the ion (ionic radius) increases, the |ΔHlattθ| decreases, and the strength of the ionic bond decreases

Question 21

how does the charge of the ion affect the lattice energy?

Answer 21

as the charge of the ion increases, |ΔHlattθ| increases, and the strength of the ionic bond increases

Question 22

what is theoretical lattice energy?

Answer 22

it is lattice energy of an ionic compound based on the simple ionic model where the compound is considered to be fully ionic in nature

Question 23

what is experimental lattice energy?

Answer 23

it is the lattice energy of a compound derived from the Born-Haber cycle

Question 24

what does it mean when the theoretical & experimental lattice energies of an ionic compound are approximately the same?

Answer 24

it means that the ionic model is satisfactory for the compound, and the ionic compound is considered to be fully ionic

Question 25

what does it mean when the theoretical & experimental lattice energies of an ionic compound differ significantly?

Answer 25

it means that the ionic model is not satisfactory for that compound, and there is some degree of covalent bonding in that compound