chem 3.6 - enthalpy changes for solids and solutions

what is lattice enthalpy?

a measure of strength of ionic bonding in a compounds - how strong the electrostatic forces are between + and - ions

what is the lattice enthalpy of an ionic compound?

the enthalpy change accompanying the formation of 1 mole of a solid ionic compound from its gaseous ions

symbol for lattice enthalpy

units for lattice enthalpy

KJmol-1

what does lattice enthalpy involve?

bond making

exothermic

negative

what value will lattice formation enthalpy have?

negative value

what value will lattice breaking enthalpy have?

positive

how can you calculate lattice enthalpy?

hess cycle

what is the standard enthalpy of formation?

the enthalpy change when one mole of a compound is formed from its constituent elements in their standard states

K(s) + 1/2Cl2(g) → KCl (s)

• negative value

what is the standard enthalpy of atomisation?

the enthalpy change when one mole of gaseous atoms is formed from its elements in their standard states

potassium (metallic bonds broken) - K(s) → K(g)

chlorine (covalent bonds broken) - 1/2Cl2(g) → Cl(g)

• positive value

what is first ionisation energy?

the energy required to remove the outer electron from each atom in one mole of gaseous atoms to form one mole of ions with a +1 charge

K(g) → K+ (g) + e-

• positive value

what is the second ionisation energy?

the energy required to remove the outer electron from each atom in one mole of gaseous atoms to form one mole of ions with a +2 charge

K+ (g) → K2+ (g) + e-

• positive value

what is the first electron affinity?

the energy change accompanying the addition of one electron to one mole of gaseous atoms to form one mole of gaseous ions with a -1 charge

Cl(g) + e- → Cl- (g)

• negative value

what is the second electron affinity?

the energy change accompanying the addition of one electron to one mole of -1 ions to form one mole of gaseous ion with a -2 charge

O- (g) + e- → O2- (g)

• positive value

how do you calculate lattice enthalpy? (step by step)

start with elements in standard states

convert elements into gaseous atoms (atomisation)

metal atoms need to lose their outer shell electrons to form ions (ionisation energy 1 and 2)

non metal atoms need to gain electrons to form ions (electron affinity 1 and 2)

gaseous ions join together to form ionic solid (lattice enthalpy)

what does the sum of all enthalpy changes equal?

the standard enthalpy change of formation of the ionic solid

in the born haber cycle what are the endothermic changes?

atomisation

ionisation energy 1 and 2

electron affinity 2

(positive values)

how are the endothermic changes of the haber cycle shown?

upwards arrows

what are the exothermic changes of the born haber cycle?

electron affinity 1

lattice enthalpy

(negative values)

how are the exothermic changes of the haber cycle shown?

downwards arrows

what does a large exothermic (negative) value indicate?

very stable compound

what does a smaller exothermic (negative) value indicate?

less stable compound

what does a large endothermic (positive) value indicate?

very unstable compound

what does the lattice enthalpy depend on?

the strength of attraction between ions → depends on their charge density

what does the charge density of ions depend on?

size of ions

charge of ions

how does the size of ions affect charge density and thus lattice enthalpy?

smaller ionic radii of ions

greater charge density

stronger attraction between ions

more exothermic (negative) lattice enthalpy

small ions close together in solid lattice → makes attraction stronger

how does the charge of ions affect the charge density and thus lattice enthalpy?

greater charge on ions

greater charge density

stronger attraction between ions

more exothermic (negative) lattice enthalpy

compounds with +2/-2 ions have more exothermic values of LE than those with +1 and -1 ions

what happens when ionic solids dissolve in water?

form aqueous solutions

what is the enthalpy change of a solution?

enthalpy change of a solution of an ionic compound is the enthalpy change when 1 mole of a solid ionic compound dissolves in water

when will the enthalpy change of a solution be positive?

if lattice enthalpy is greater than enthalpy of hydration

when will the enthalpy change of a solution be negative?

if lattice enthalpy is less than enthalpy of hydration

what are the 2 steps to an ionic solid dissolving?

1. lattice breaks up - if delta H is endothermic (positive) MAKE IT NEGATIVE.

2. separate ions are hydrated - exothermic - ALWAYS NEGATIVE (enthalpy change of hydration)

what is the enthalpy change of hydration?

the enthalpy change when 1 mole of gaseous ions dissolve in water to form 1 mole of aqueous ions under standard conditions

how do you calculate the enthalpy change of a solution?

enthalpy change of solution = lattice enthalpy + enthalpy change of hydration

• enthalpy change of solution - can be endo/exo

• lattice enthalpy - ALWAYS MAKE IT POSITIVE

• enthalpy change of hydration - ALWAYS EXO NEGATIVE

what does the standard enthalpy change of hydration depend on?

attraction between an ion and water molecules around it (ion-dipole forces) → depends on charge density of ions

how does charge density affect the attraction between an ion and water molecules (hydration)?

greater charge density

stronger attraction between ion and water molecules

more exothermic value

what does charge density depend on?

ionic charge

ionic size

how does ionic charge and ionic size affect charge density?

size of ions increase

charge stays same

charge density decreases

hydration → less exothermic

size of ions decrease

size of charge increases

charge density increases

hydration → more exothermic