Lattice enthalpy and entropy

• First ionisation energy

  Energy required to remove one mole of electrons from one mole of atoms in a gaseous state to produce one mole of gaseous 1+ ions

  Equations: $A\ (g)\ \rightarrow\ A^+$$\ (g)+$$\ e^-\ (g)$
• Factors effecting ionisation energy

  Nuclear Attraction -> More p+ in nucleus = greater attraction

  Atomic radius -> Bigger atom = e_ on outer shell are further away from nucleus = weaker attraction to nucleus

  Shielding -> e- in an outer shell is repelled by e- in complete inner shells (weakens nuclear attraction)
• Standard enthalpy change of formation

  When 1 mole of a compound is formed from its elements under standard condition

  1. q = mc$\Delta$t/1000
  2. Moles
  3. kJ/Moles = kJ mol$^{-1}$
• Standard enthalpy change of neutralisation

  When 1 mole of water is formed in an acid-base reaction under standard condition.

  1. q = mc△t/1000 -> m = vol. of acid + base
  2. Moles
  3. kJ/Moles = kJ mol$^{-1}$
• Standard Enthalpy change of combustion

  When 1 mole of a substance reacts completely with oxygen under standard condition

  1. q = mc△t -> m = mass of $H_2O$
  2. Moles of fuel
  3. kJ/Moles = kJ mol$^{-1}$
• Standard enthalpy change of reaction

  Enthalpy change that accompanies a reaction in the molar quantities expressed in the chemical equation under standard condition.

  (Total BE reactants) - (Total BE products)
• Enthalpy change of first electron affinity

  Enthalpy change when one electron is added to one mole of gaseous atoms to form one mole of gaseous 1- ions
• Enthalpy change of second electron affinity

  Enthalpy change when one electron is added to one mole of gaseous 1- ions to form one mole of gaseous 2- ions
• Enthalpy change of atomisation

  Enthalpy change that takes place for the formation of one mole of gaseous atoms from the element in its standard state under standard condition.
• Enthalpy change of bond dissociation

  Enthalpy change when one mole of covalent bonds of a gaseous covalent compound is broken to form products under standard conditions
• Standard lattice enthalpy

  Enthalpy change when one mole of ionic compound is formed from its gaseous ions under standard conditions
• Constructing a born haber cycle

  Formation = $\downarrow$

  Atomisation = Metal and non - metal $\uparrow$

  Ionisation = Metal $\uparrow$

  Electron affinity = 1) $\downarrow$ 2) $\uparrow$

  Lattice enthalpy = $\downarrow$
• Factors affecting lattice enthalpy: Size of ion


  • Ionic radius increases
  • Attraction between ions decreases
  • Lattice energy is less negative
  • Melting point decreases
• Factors affecting lattice enthalpy: Charge 


  • Ionic charge increases
  • Attraction between ions increases
  • Lattice energy becomes more negative
  • Melting point increases
• Dissolving ionic compounds

  • The forces of attraction between the oppositely charged ions are broken and new bonds are formed with water molecules
  • Energy is put in to break the ionic lattice = lattice enthalpy = endothermic
  • Energy is released when forces of attraction form with water molecules = enthalpy of hydration = exothermic
  • Difference between these enthalpies = enthalpy of solution = positive or negative
• Enthalpy change of solution

  Enthalpy change when 1 mole of an ionic compound dissolves in water
• Enthalpy change of hydration

  Enthalpy change when 1 mole of gaseous ions are dissolved in water
• Formula for enthalpy change of solution

  △H(le) + △H(sol) = $\Sigma$△H(hyd)
• Factors affecting enthalpy change of hydration: Ion size

  • Ionic radius increases
  • Attraction between ion and water molecules decreases
  • Hydration energy is less negative
• Factors affecting enthalpy change of hydration: Ion chatge

  • Ionic charge increases
  • Attraction with water molecules increases
  • Hydration energy is more negative
• Why first ionisation energy of Ca is less positive than the second ionisation energy?

  • When one electron is removed from Ca, number of protons stay the same
  • But number of electrons decreases
  • So there's a greater nuclear charge
  • So more energy is needed to remove the second electron
• Entropy

  Measure of disorder in a system

  More disorder = higher entropy

  Gases have the highest entropy and solids have the lowest

  Units = $JK^{-1}mol^{-1}$
• Changes in the system

  • Number of gas moles decreases (eg. 4 -> 2) = -△s
  • Number of gas moles increases (eg. 2 -> 4) = +△s
• Standard entropy data

  1 mole of substance under standard conditions

  All are positive because 0 is the most ordered state
• Entropy equation

  △s = $\Sigma$S products - $\Sigma$S reactants
• Gibbs free energy equation

  △G (kJmol$^{-1}$) = △H (kJmol$^{-1}$) - T(K)△S (JK$^{-1}$mol$^{-1}$)

  remember to △S/1000
• Feasibility

  More negative △G = more feasible the reaction is

  Has to be feasible for reaction to occur
• How do we know if a reaction is feasible or not?Step 1: prediction

  • △G must be = 0 or less than 0

  
• Step 2: Calculating minimum temp

  T = △H/△S

  Doesn't take into account:

  • Activation energy = could be high
  • Rate of reaction = could be very slow