Physical chemistry

• Perfect Ionic Model


  A theoretical model used to calculate lattice enthalpy
• To calculate a theoretical value for lattice enthalpy


  1. Know the geometry of the ionic solid

  2. Know the charge on the ions

  3. Know the distance between the ions
• The Perfect Ionic Model is based on the assumption that the substance is highly ionic
• Allows comparison between theoretical and experimental value


  Difference suggests the substance is not purely ionic and has some covalent character
• Covalent character


  • Caused by small highly charged ions
  • Caused by large negatively charged ions
• Covalent character is caused by the electron cloud being more easily distorted
• Enthalpy change

  Heat change at a constant pressure
• Enthalpy of formation: Enthalpy change when one mole of a substance is formed from its elements with all substances in their standard states
• Ionisation enthalpy

  enthalpy change when each atom in one mole of gaseous atoms loses an electron to form one mole of gaseous 1+ ions
• Enthalpy of atomisation

  enthalpy change when one mole of gaseous atoms is produced from an element in its standard state
• Bond enthalpy

  enthalpy change when one mole of covalent bonds is broken in the gaseous state
• Lattice enthalpy of association

  enthalpy change when one mole of a solid ionic compound is formed from its constituent ions in the gas phase
• Lattice enthalpy of dissociation

  enthalpy change when a mole of a solid ionic compound is broken up into its constituent ions in the gas phase
• Electron affinity

  enthalpy change when each atom in one mole of gaseous atoms gains an electron to form one mole of gaseous 1- ions
• Maxwell-Boltzmann Curves

  • Most probable energy
  • Average energy
  • Proportion of molecules with energy > Ea have more successful collisions
• Increase in temperature

  Increases average kinetic energy, more frequent collisions, more molecules with energy > Ea, more successful collisions
• concentration, and pressure increase

  The most probable or mean energy does not change, but more particles have energy > Ea, more successful collisions, average number of molecules per unit volume increases
• Testing for ions

  • NaOH - white ppt with Mg2+, Ca2+, Ba2+
  • NH4+ - turns damp red litmus paper blue
  • Cl-, Br-, I- - white, cream, yellow ppt with AgNO3
  • CO32- - effervescence with HCl
  • SO42- - white ppt with BaCl2
  • OH- - white ppt with MgCl2
  • Al3+ - white ppt with excess NaOH
• Bronsted-Lowry acid

  Proton donor
• Bronsted-Lowry base

  Proton acceptor
• Ka

  Acid dissociation constant = [H+][A-]/[HA]
• pKa

  • log Ka
• Ionic equations

  1. H+ + OH- → H2O

  2. 2H+ + CO32- → H2O + CO2

  3. H+ + HCO3- → H2O + CO2

  4. H+ + NH3 → NH4+
• Rate equation

  Rate of reaction = k[A]m[B]n
• Order of reaction

  Equal to the number of moles of that substance in the rate equation up to and including the rate-determining step
• Rate-determining step

  Slowest step in the reaction mechanism of a multi-step reaction
• Reaction order

  • Zero order, first order, second order
• Kw

  [H+][OH-] = 10-14 at room temperature
• Time-of-flight mass spectrometry

  1. Sample vapourised and injected into mass spectrometer

  2. Ionisation - removes electron leaving positively charged ions

  3. Acceleration - positively charged ions accelerated towards detection plate

  4. Ion drift - ions deflected by magnetic field into curved path, radius depends on charge and mass

  5. Detection - positive ions hit negatively charged plate, produce flow of charge proportional to abundance

  6. Analysis - values and time-of-flight used to produce spectrum with relative abundance of isotopes