Energetics II Flashcards
what is lattice energy
the energy given out when gaseous ions combine to make a solid lattice
what is the definition for standard lattice energy
energy change when 1 mole of an ionic solid is formed from its gaseous ions under standard conditions 298K / 100kPa
what two things affect lattice energy
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
what is enthalpy of atomisation
the enthalpy change when 1 mole of gaseous atoms is formed from the element in its standard state
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
what does theoretical lattice energies 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 lattice energy
covalent character introduced when polarisation of anions. Cations attracts electrons from anion distorting electron density of the anion
what does the polarisability of the anion depend on
ionic radius, the larger the more easily distorted
what is standard enthalpy change of solution (ΔHsolꝋ)
enthalpy change when 1 mole of ionic substance dissolves in sufficient water to form an infinity dilute solution
what is enthalpy of hydration
enthalpy change when 1 mole of gaseous ion dissolves in sufficient water to form an infinitely dilute solution
are hydration enthalpies exo or endo
exothermic
what happens when a ionic solid dissolves in water
water is polar with δ- oxygen (O) atom and δ+ hydrogen (H) atoms form ion-dipole attractions with ion present in solution
what is hydration enthalpies
measure of the energy that is released when there is an attraction formed between the ions and water molecules
what is route 1 and route 2 for constructing this energy cycle
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
what makes the ions attract the water molecules more strongly
The higher the charge density the greater the hydration enthalpy so ions attract water molecules more strongly
why is hydration enthalpies exothermic
as energy is given out as water molecules bond to the metal ions
what is ionisation energy
the amount of energy required to remove an electron from an atom or ion in the gas phase
What is Kc
Equilibrium expression linking the equilibrium concentration of reactants and products at equilibrium
When does Kc chance
if the temperature of the reaction changes
What is Kp
Equilibrium expression linking the partial pressures of reactants and products at equilibrium
What is ignored in heterogeneous reactions
Solids and liquid are ignored
For an endothermic reaction what does an increase in temperature cause
the products to increase and reactants to decrease therefore equilibrium constant will increase vice versa
when does lattice energy become less exothermic
The lattice energy becomes less exothermic as the ionic radius of the ions increases
or as the ionic charge of the ions increases
why does lattice energy become less exothermic with ionic radius
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
why does lattice energy become less exothermic with ionic charge
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
what factors affect enthalpy of hydration
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
Polarisability of cations and anions
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
What is the entropy of a given system
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
ΔSΘ total =
ΔS Θsys + ΔSΘsurr
unit for entropy
J K-1 mol-1
ΔSΘsystem =
ΣΔSΘproducts - ΣΔSΘreactants
ΔSΘ surr =
- enthalpy change / T
what is the feasibility of a reaction determined by
The enthalpy and entropy change
For a reaction to be feasible, ΔGꝋ must be equal or less than zero
gibs equation
ΔGꝋ = ΔHreactionꝋ – TΔSsystemꝋ
equations for kc directly related to gibbs free energy
ΔGꝊ = -RT lnK
determines if forward or backward favoured
what does a negative ΔG value for a reaction indicates
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
