Chapter 22 - Enthalpy + Entropy

lattice enthalpy

enthalpy change of formation of 1 mole of ionic lattice from its gaseous ions

K+ (g) + F- (g) → KF (s)

-is exothermic = more exothermic → more stronger ionic bonds

standard enthalpy change of formation

enthalpy change when 1 mole of a compound is formed from its elements under standard conditions

Na (s) + ½ Cl2 (g) → NaCl (s)

first ionisation energy

enthalpy change when 1 electron is removed from each atom in 1 mole of gaseous atoms to form 1 mole of gaseous 1+ ions

-always endothermic = energy supplied to remove negative electron from the attraction to positive nucleus
-second ionisation energy is more endothermic as electron is closer to nucleus so greater attraction

standard enthalpy change of atomisation

enthalpy change of formation of 1 mole of gaseous atoms from element in its standard state under standard conditions

½ Cl2 (g) → Cl (g)

-is always endothermic = energy needs to be supplied to break bonds between molecules into atoms or into a gas

first electron affinity

enthalpy change when 1 electron is added to each atom in 1 mole of gaseous atoms to form gaseous 1- ions

-is exothermic = electron being added is attracted in towards the nucleus

-BUT second electron affinity is endothermic = negative electron being added to negative ion so energy required to overcome repulsion

method for Born-Haber Cycle

lattice enthalpy:

g = a-b-c-d-e-f

-need to multiply values by 2 if there are 2 moles of a substance

factors that affect LATTICE ENTHALPY + HYDRATION

-greater ionic charge

-smaller ionic size/radius

-both lead to stronger attractions between the ions so leads to a more negative/exothermic value

standard enthalpy change of solution

enthalpy change when 1 mole of a solute completely dissolves in water under standard conditions

MgBr2 (aq) + aq → Mg2+ (aq) + 2Br- (aq)

-energy is taken in to break ionic bonds, new attractions formed between ions and water, cations and S- dipole of H2O, anions and S+ dipole of H2O

-overall endothermic = more energy being taken in to break bonds than released to make bonds

-overall exothermic = vice versa

standard enthalpy of hydration

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

K+ (g) + aq → K+ (aq)

-always exothermic = energy is released when attractions to water molecules are made

calculation for enthalpy change of solution cycles + experiment

sum of lattice enthalpy + solution = sum of the enthalpy changes of hydration

Q = mcT