1.1 Lattice Enthalpy Flashcards
Define ionic bonds
Strong electrostatic forces of attraction between oppositely charged ions
Define lattice enthalpy
the enthalpy change when one mole of an ionic compound is converted to gaseous ions
Give an equation to express the lattice enthalpy of sodium chloride
NaCl(s) → Na+ (g) + Cl- (g)
Describe the value of lattice enthalpy
- will be endothermic (+ve) as energy is required to break the ionic bonds
- a large value for lattice enthalpy shows there are strong electrostatic forces of attraction between the oppositely charged ions in the lattice.
Define the standard enthalpy change of formation
The energy change when one mole of a compound is formed from its constituent elements in their standard states under standard conditions.
Give an equation to express the standard enthalpy change of formation of potassium chloride
K(s) + ½ Cl2(g) → KCl(s)
Define standard enthalpy change of atomisation
The enthalpy change when one mole of gaseous atoms is formed from the element in its standard state
Give an equation to express the standard enthalpy change of atomisation of potassium
K(s) → K(g)
Give an equation to express the standard enthalpy change of atomisation of chlorine
½ Cl2(g) → Cl(g)
Define the standard bond dissociation enthalpy
The enthalpy change required to break 1 mol of a covalent bond, everything being in the gas state.
Give an equation to express the standard bond dissociation enthalpy of chlorine
Cl2(g) → 2Cl(g)
Define first ionisation energy
The enthalpy change accompanying the removal of one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions.
Give an equation to express the first ionisation energy of potassium
K(g) → K+(g) + e-
define second ionisation energy
The enthalpy change accompanying the removal of one electron from each ion in one mole of gaseous 1+ ions to form one mole of gaseous 2+ ions.
give an equation to express the second ionisation energy of calcium
Ca+(g) → Ca2+(g) + e-
Define first electron affinity
the enthalpy change when one mole of gaseous atoms is converted into gaseous ions with a single negative charge.
give an equation to express the first electron affinity of chlorine
Cl(g) + e- → Cl-(g)
Define second electron affinity
The enthalpy change when one mole of gaseous 1- ions is converted into one mole of gaseous ions with a double negative charge.
Give an equation to express the second electron affinity pf oxygen
O-(g) + e- → O2-(g)
In gaseous elements such as oxygen and chlorine, which two enthalpy values can be equated?
enthalpy of atomisation and bond dissociation enthalpy
What enthalpy changes are exothermic?
enthalpy of formation
first electron affinity
what enthalpy changes are endothermic?
lattice enthalpy enthalpy of atomisation bond dissociation enthalpy first ionisation energy second ionisation energy
what does a +ve ∆H imply?
endothermic enthalpy change
what goes a -ve ∆H imply?
exothermic enthalpy change
Describe the enthalpy changes found clockwise in an enthalpy diagram
TOP -> BOTTOM - Electron affinities - Lattice enthalpy BOTTOM -> TOP - Enthalpy of formation - Enthalpy of atomisation - Ionisation energies - Bond dissociation enthalpy (or atomisation)
what does a large endothermic enthalpy value mean?
there is a large electrostatic force of attraction between the oppositely charged ions, therefore the bonds are very strong
What factors govern the size of lattice enthalpy?
ionic size
ionic charge
how does ionic size govern the size of lattice enthalpy?
Lattice enthalpy becomes less endothermic (less positive) as the size of the negative ion increases.
This indicates weaker attraction between ions and hence weaker ionic bonding
how does ionic charge govern the size of lattice enthalpy?
Charge increases and produces a greater attraction between these positive ions and negative ions. The ionic radius decreases, resulting in the ions in the lattice being closer together, producing more attraction.
Therefore as charge increases, lattice enthalpies become more endothermic, therefore more positive.
what types of compounds would you expect to have the most positive lattice enthalpies?
those with small, highly charged ions
define enthalpy of solution
The enthalpy change when one mole of a solute dissolves in water.
define enthalpy of hydration
The enthalpy change when one mole of gaseous ions is converted to one mole of aqueous ions.
What processes occur when an ionic solid dissolves and why?
- breakdown of the ionic lattice - overcoming the attractive forces requires energy
- Hydration of the gaseous ions - gaseous ions bond with the water molecules
What equation connects enthalpy of solution and enthalpy of hydration?
Enthalpy of solution = lattice enthalpy + enthalpy of hydration
