determination of enthalpy change using Hess' Law Flashcards
what is the definition of Hess’ Law?
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
how do we apply Hess’ Law?
by Hess’ Law,
total enthalpy change for path 1 (direct) = total enthalpy change for path 2 (indirect)
what is the formula for calculating ΔHrθ from enthalpy change of formation, ΔHfθ?
ΔHrθ = ∑ΔHfθ (products) - ∑ΔHfθ(reactants)
what is the Born-Haber cycle used for?
to determine the enthalpy change of formation of ionic compounds
what is the definition of lattice energy?
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
is lattice energy always endothermic or exothermic?
ALWAYS EXOTHERMIC!!
what is the definition of ionisation energy?
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
is the 1st ionisation energy always exothermic or endothermic?
ALWAYS ENDOTHERMIC!!
why is the 1st ionisation energy always endothermic?
because 1 mole of ground state atoms in the gaseous state need to take in heat to remove 1 mole of valence electrons!!
what is the definition of electron affinitiy?
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
is the 1st electron affinity usually exothermic or endothermic?
it is usually exothermic!!
why is the enthalpy change for the 2nd electron affinity always positive?
- 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
what are the steps to construct a Born-Haber cycle?
- F
- A
- I
- L
what does F in the 1st step in constructing a Born-Haber cycle represent?
formation of the solid ionic compound from its constituent elements in their standard states
what does A in the 2nd step in constructing a Born-Haber cycle represent?
Atomisation of the constituent elements in their standard states to the gaseous state
what does I in the 3rd step in constructing a Born-Haber cycle represent?
-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
what does L in the 4th step in constructing a Born-Haber cycle represent?
lattice energy of the solid ionic compound formed from its constituent gaseous ions
what does lattice energy, ΔHlattθ ,measure?
it measures the strength of ionic bonds within an ionic compound
what are the factors affecting the magnitude of lattice energy?
- size of ion
- charge of ion
how does the size of the ion affect the magnitude of lattice energy?
as the size of the ion (ionic radius) increases, the |ΔHlattθ| decreases, and the strength of the ionic bond decreases
how does the charge of the ion affect the lattice energy?
as the charge of the ion increases, |ΔHlattθ| increases, and the strength of the ionic bond increases
what is theoretical lattice energy?
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
what is experimental lattice energy?
it is the lattice energy of a compound derived from the Born-Haber cycle
what does it mean when the theoretical & experimental lattice energies of an ionic compound are approximately the same?
it means that the ionic model is satisfactory for the compound, and the ionic compound is considered to be fully ionic
what does it mean when the theoretical & experimental lattice energies of an ionic compound differ significantly?
it means that the ionic model is not satisfactory for that compound, and there is some degree of covalent bonding in that compound
