a2 chemistry- chemical energetics

ionic bond strength is determined by

lattice energy

covalent bond strength is determined by

bond energy

lattice formation enthalpy

is always exothermic
"energy released when 1 mole of an ionic solid is formed from its gaseous cation and anion under standard condition"

lattice dissociation enthalpy

endothermic.
"energy absorbed when 1 mole of an ionic solid is converted into gaseous cation and anion under standard condition"

lattice energy depends on

ionic charge and ionic radius- charge density

lattice energy is directly proportional to

charge of ions

lattice energy is inversely proportional to

ionic radius

when is lattice energy highly exothermic

if the charge of the ion is higher, size of the ion is smaller, charge density increases, electrostatic attraction (b/w cation and anion) will become stronger so more energy is released

which one has higher value of lattic energy

anion same--->explanation on basis of cation
charge same--->ionic radius

priority given to the

charge of the ion

born haber cycle is an application of

hess's law

born haber cycle is only applicable for

pure ionic crystals (only exhibit ionic bonding, consists of only 2 elements)

impure ionic crystals

consists of more than three elements
consists of both ionic and covalent bonding

standard enthalpy change of formation

enthalpy change that occurs when 1 mole of a compound is formed from its constituent elements in their standard states under standard condition- can be exo/endo

standard enthalpy change of atomisation (for metals)

1. enthalpy change occurs when 1 mole of gaseous atom is formed from its constituent elements in their standard states under standard condition
2. is always endothermic

energy values need to be

multiplied with number of moles

standard enthalpy change of atomisation (for non-metals)

1. decomposition
2. is always endothermic
3. if it's not given, then half the bond energy value
4. do not multiply the enthalpy change of vap/sub with no. of moles

ionisation energy of metals

1. don't use ionisation energy of non metals
2. amount of energy required to remove one mole of an electron from one mole of neutral gaseous atom to give a monopositive ion
3. is always endothermic
4. charge of the ion decide the ionisation energy, not the moles of electron

electron affinity of non-metals

1. only in non metals
2. energy released when one mole of an electron is added to one mole of neutral gaseous atom to give an anion with a charge of 1-
3. it is a measure of the force of attraction between incoming electron and nucleus
4. stronger the force of attraction, more energy released and vice versa
5.exo/endo

why the first electron affinity is exo while the second and third are endo?

both anion and electron are negatively charged, experience repulsion, to overcome this, absorption of energy takes place.

electron affinity depends on

1. atomic size
2. shielding effect
3. nuclear charge

electron affinity (down the group)

atomic size increases, number of shells increase and so does the shielding effect, nucleus to electron attraction becomes weak so electron affinity becomes less exo

elctron affinity (across the period)

nuclear charge increases, nucleus to electron attraction increases, more energy is released so electron affinity becomes more exo

dissolving ionic solids in water

1. standard enthalpy change of solution
2. lattice dissociation enthalpy
3. hydration enthalpy

standard enthalpy change of solution

enthalpy change occus when 1 mole of compound is dissolved in large amount of water to give an infinite dilute solution under standard condition
can be exo/endo
exo if soluble in water
endo if partially soluble/insoluble

lattice dissociation enthalpy-2

amount of energy required to break 1 mole of ionic solid into gaseous cation and anion
is always endo

hydration enthalpy

enthalpy change occurs when 1 mole of gaseous cation and anion are dissolved in large amount of water to give an infinite dilute solution
is always exo bec electrostatic attraction is formed b/w ion and water molecule

hydration enthalpy can be highly exo or less exo

stronger the electrostatic attraction, more energy is released.
hydration enthalpy--->highly exo

hydration enthalpy depends on

charge of cation
size of cation
charge density

hydration enthalpy (overview with example)

magnesium ion has high magnitude of charge, smaller ionic radii, high charge density. strong electrostatic attraction exists b/w ion and water so more energy is released. if more energy is released, then hydration enthalpy is highly exo

dissolving ionic solids equation

sol=-le+hyd

lattice enthalpy equation

h1+le-hf=0

solubility can be explained on the basis of

lattice energy and hydration enthalpy

both lattice energy and hydration enthalpy depend on

charge density
also, both are exo

bigger ionic radius of cO3(2-) and so(2-) ions

electrostatic attraction b/w ions is not effective, hence lattice energy will not play a key role

le<hyd

more exo, sol is exo, compound is exo

le>hyd

more endo, sol is endo, compound is insoluble or partially soluble

solubility of group ii sulfates/carbonates

down the group, ionic radius increases so charge density decreases, both hydration and lattice enthalpy decrease, hydration enthalpy decreases more significantly than lattice energy, sol is endothermic, solubility decreases

mgso4

soluble

baso4

insoluble

solubility of group ii hydroxides

due to smaller ionic radii of oh- ion, electrostatic attraction b/w ions exist, so lattice energy will play a key role

solubility of group ii hydroxides (trend)

down the group ionic radius increases, charge density decreases, both hyd and le decreases, lattice energy decreases more significantly than hydration, sol is more exo, solubility increases down the group

thermal stability of group ii carbonates and nitrates

MCO3----->MO+CO2 (heat)
2M(NO3)2---->2MO+4NO2+O2 (heat)

experiment for thermal stability

place small amount of hydrated Mg(No3)2.6H2O in a boiling tube, heat the boiling tube gently.
Mg(No3)2.6H20---->Mg(No3)2+6h2o
water vapours are condensed onto the mouth of boiling tube
heat the boiling tube strongly
2Mg(No3)2---->2MgO+4No2+O2

NO2

dark brown gas

O2

colorless gas which relights a glowing splint

NO2 (structure)

double bond, dative bond
free radical

thermal stability depends on

charge density and polarisation

polarisation

distortion of electrons of anion

when does decomposition takes place at low temp

higher the charge density, higher the polarisation, easier the decomposition

thermal stability trend down group ii

down the group, ionic radius increases
charge density and polarisatio decrease
cation cannot distort the electrons of CO32- easily
for thermal decomposition, more energy is needed
decomposition occurs at a high temp
thermal stability increases down the group

carbonate ion

negative charge on the side oxygens

nitrate ion structure

sulfate ion structure

azide ion structure

dative bond and - sign on side N

ammonium ion structure