Energetics II Flashcards
What is lattice energy?
Enthalpy change when 1 mole of a ionic compounds formed from its gaseous ions (under standard conditions)
- exothermic as energy released when ions combine to form ionic lattice so enthalpy change will be NEGATIVE
E.g . Na+ (g) + Cl- (g) —> NaCl (s)
What does a large negative value for lattice enthalpy suggest ?
Ionic compounds are much more stable than its gaseous ions
- due to strong electrostatic forces of attraction between oppositely charged ions in solid lattice
The more exothermic the value is , the stronger the ionic bonds within the lattice are
Enthalpy change of atomisation?
enthalpy change when 1 mole of gaseous atoms is formed from its element under standard conditions
- always endothermic as energy is taken in to break any bonds between atoms in element, to break into gaseous atoms (POSITIVE VALUE)
Eg. Na(s) —> Na (g)
Electron affinity?
energy change when 1 mole of electrons is gained by 1 mole of gaseous atoms of an element to form 1 mole of gaseous ions under standard conditions
E.g Cl(g) + e- —> Cl- (g)
First electron affinity is ALWAYS EXOTHERMIC as energy is released when electrons attracted to atoms
Second electron affinity can be ENDOTHERMIC :
O-(g) + e- —> O2-(g)
Because large force of repulsion must be overcome between negative ion /second electron
How does charge density effect polarising power of cations?
Cation with larger charge density will have larger polarising power
- cations with smaller ionic radius /higher charges have larger charge densities
What effects an anions Polarisability?
Ionic radius
Larger the ionic radius , more easily it will be distorted
Standard enthalpy of solution?
enthalpy change when 1 mole of an ionic substance dissolves in sufficient water to form an infinitely dilute solution
E.g: KCl (s) + aq —> KCl (aq) or K+ (aq) + Cl- (aq)
- can be both exo/endothermic
Enthalpy of hydration?
the enthalpy change when 1 mole of a specified gaseous ion dissolves in sufficient water to form an infinitely dilute solution
E.g Mg 2+ (g) + aq —> Mg 2+ (aq)
Measures energy released when attraction is formed between ions/water molecules —> EXOTHERMIC
Factors effecting lattice enthalpy?
IONIC RADIUS
CHARGE
How does ionic radius affect lattice enthalpy?
Lattice energy becomes less exothermic as ionic radius of ions INCREASES - lower charge density as ions spread out
Ions are also further from each other in lattice - electrostatic forces of attraction between centres of ions weaker as GREATER distance between centre of ions (when ions are larger)
How does ionic charge affect lattice enthalpy?
Lattice enthalpy more exothermic as ionic charge INCREASES
- as there is greater charge density
So stronger electrostatic attraction between oppositely charged ions in the lattice —> MORE EXOTHERMIC
Factors effecting hydration enthalpy ?
Higher charge density , the greater the hydration enthalpy (so in SMALLER IONS/IONS WITH LARGER CHARGE) because ions attract water molecules more strongly
Ionic radius
Why cant you use the C-C bond enthalpy data/x by 2 to get the C=C BOND ENTHALPY DATA?
C=C is weaker than 2x C-C bond
- as it consists of a pi/sigma bond instead of 2 sigma bonds
Explain why there is a significant difference in experimental/theoretical lattice energy values for MgI2 ?
Mg ion is small/highly charged (Mg2+) - high charge density
Iodide ions has large IONIC radius
Iodide ion is POLARISED/easily distorted by Mg ion
So MgI2 has partial covalent character —> so values are different
