chapter 22- enthalpy and entropy

why are ionic compounds so stable

the strength of the ionic bonds, electrostatic attraction between oppositely charged ions in the ionic lattice structure

what is lattice enthalpy

the enthalpy change that accompanies the formation of 1 mole of an ionic compound from it’s gaseous ions under standard conditions

what is lattice enthalpy used to measure

the strength of an ionic bond in a giant ionic lattice

what does a born-harber cycle show

2 routes for changing elements in their standard states into a ionic lattice

what are 3 processes in route 1 of the borne-harber cycle

• formation of gaseous atoms

• formation of gaseous ions

• lattice formation

how do you form gaseous atoms for the borne-harber cycle

changing the elements in their standard states into gas atoms

• endothermic (bond breaking)

how do you form gaseous ions for the borne-harber cycle

changing gaseous atoms into positive and negative gaseous ions

• endothermic change

how do you form a lattice in the borne-harber cycle

changing the gasesous ions into a solid ionic lattice

• exothermic change

what is route 2 of the borne-harber cycle

converting elements in their standard states directly to the ionic lattice (enthalpy of formation exothermic)

standard enthalpy change of formation

the enthalpy change that takes place when one mole of a compound is formed from its constituent elements under standard conditions

standard enthalpy change of atomisation

the enthalpy change that takes place for the formation of 1 mole of gaseous atoms from the element in it standard state under standard conditions

1st ionisation energy

the enthalpy change required to remove 1 electron from each atom in 1 mole of gaseous atoms in order to form 1 mole of gaseous 1+ ions

1st electron affinity

the enthalpy change required to add 1 electron to each atom in 1 mole of gaseous atoms in order to form 1 mole of gaseous 1- ions

steps of a route 1 borne harber cycle

• enthalpy change of atomisation for element 1

• enthalpy change of atomisation for element 2

• first ionisation energy for the cation

• first electron affinity for the anion

• lattice enthalpy of the ionic compound

steps for route 2 of the borne-harber cycle

• enthalpy change of formation of the ionic compound

how would you write 1st electron affinity

X_(g) + e^- —> X^- (g)

how would you write second electron affinity

X^-(g) + e^- —> X^2-_(g)

why is the second electron affinity endothermic

a second electron is gained by a negative ion, which repels the electron away. So, energy must be put in to force the negatively charged e^- into the negative ion

how can ionic compounds be dissolved

water molecules can break up the giant ionic lattice structure and overcome the strong electrostatic attractions between oppositely charged ions

standard enthalpy of solution

the enthalpy change that takes place when one mole of a solute dissolves in a solvent

is enthalpy change of solution ENDO or EXO

can be either

what part of the water molecule is attracted to the cation

the delta negative oxygen atom

what part of the water molecule is attracted to the anion

the delta positive hydrogen atoms

what happens when a solid ionic compound dissolves

• the ionic lattice breajs

• water molecules are attracted to, and surround, the io s

1st type of energy change in solution

the lattice must break down to form seperate gaseous ions

2nd type of energy change in solution

the seperate gaseous ions interact with polar water molecules to form hydrated aqeuous ions- enthalpy change of hydration

what is enthalpy change of hydration

the enthalpy change that accompanes the dissolving of gaseous ions in water to form one mole of aqeuous ions

if arrow points upwards

endothermic

if arrow points downwards

exothermic

properties of ionic compounds

• high mp and bp

• soluble in polar solvents

• conduct electricty when aqueous or molten

how does ionic size affect lattice enthalpy

• ionic radius increase

• attraction between ions decreases

• lattice enthalpy is less negative

• melting point decreases

how does ionic charge affect lattice enthalpy

• ionic charge increases

• attraction between ionic increase

• lattice enthalpy because more negative

• melting point increases

how does increasing ionic size affect hydration

• less attraction between ions and water molecules

• hydration is less negative

how does increasing ionic charge affect hydration

• increase attraction between ions and water molecules

• hydration is more negative

how to predict solubility

if the sum of hydration enthalpies is larger than the magnitude of the lattice enthalpy, the overall enthalpy change will be exothermic so it should dissolve