Energetics II Flashcards
what is lattice energy of formation
enthalpy change when 1 mole of an ionic compound is formed from its gaseous ions (under standard conditions)
is lattice energy of formation exo/endo
exothermic, as when ions are combined to form an ionic solid lattice there is an extremely large release of energy
what does large negative value of lattice energy suggest
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
strength of bonds in regards to exothermic
more exothermic means stronger ionic bonds
what is enthalpy change of atomisation
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
is enthalpy change of atomisation endo or exo
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
what is electron affinity
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
difference between first and second affinity and why
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
what is theoretical lattice energy
assume a perfect ionic model where the ions are 100% spherical and the attractions are purely electrostatic
why do theoretical lattice energy differ from measured
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
what type of cation has a larger polarising power
A cation with a large charge density
do anions polarise or get polarised
get polarised ad this depends on ionic radius, The larger the ionic radius the more easily it will be distorted
what is standard enthalpy change of solution
enthalpy change when 1 mole of an ionic substance dissolves in sufficient water to form an infinitely dilute solution
is standard enthalpy change of solution (Hsol) exo or endo
can be exothermic (negative) or endothermic (positive)
what is standard enthalpy change of hydration (ΔHhydꝋ)
enthalpy change when 1 mole of a specified gaseous ion dissolves in sufficient water to form an infinitely dilute solution
difference between enthalpy of hydration and solution
solution is for ionic substance and hydration for gaseous ions
are hydration enthalpy exo or endo
exothermic
formula for enthalpy of hydration
ΔHhydꝋ = ΔHlattꝋ + ΔHsolꝋ
what factors affect lattice enthalpy
charge and radius of the ions that make up the crystalline lattice
how does lattice energy become less exothermic with ionic radius
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
how does lattice energy get more exothermic with ionic charge
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
factors affecting enthalpy of hydration
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
what is the entropy of a given system
how disordered or chaotic a system is
(number of possible arrangements of the particles and their energy in a given system)
what happens if system is more disordered
When a system becomes more disordered, its entropy will increase
An increase in entropy means that the system becomes energetically more stable
what is formula for total entropy change
ΔSΘ total = ΔS Θsys + ΔSΘsurr
formula for standard entropy change of a system
(ΔS Θsys)
ΔSΘsystem = ΣΔSΘproducts - ΣΔSΘreactants
formula for ΔSΘ surr
-(enthalpychange/Temperature)
what is the feasibility of a reaction determines by
The enthalpy and entropy change which come together in a concept called Gibbs free energy
gibbs equation
ΔGꝋ = ΔHreactionꝋ – TΔSsystemꝋ
what happens when gibbs energy is negative
the reaction is spontaneous / feasible and likely to occur
limitation of Gibbs
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
