A2 Chemistry OCR Definitions: Rates, acids and enthalpy

A2 Chemistry OCR Definitions: Rates, acids and enthalpy

1. Enthalpy change

   • Enthalpy change of atomisation
     • The enthalpy change when 1 mole of gaseous atoms forms the element at 298K, 101kPa
   • 1st Ionisation energy
     • The energy needed to remove 1 electron from every atom in 1 mole of gaseous atoms, to form 1+ ions
     • 2nd Ionisation energy
       • Energy needed to remove the second electron from every atom in 1 mole of gaseous 1+ ions
   • Lattice enthalpy
     • The enthalpy change when 1 mole of an ionic compound is formed from its gaseous ions, at 298K, 101kPa
   • Enthalpy change of formation
     • The enthalpy change when 1 mole of a compound is formed from its elements, at 298K, 101 kPa
   • Standard enthalpy of electron affinity
     • Enthalpy change when 1 electron is gained by every atom in 1 mole of gaseous atoms, to form 1- ions
     • NB: 1st EA always exothermic, 2nd & 3rd always endothermic
   • Enthalpy change of solution
     • Enthalpy change when 1 mole of an ionic compound dissolves in sufficient water to make a very dilute solution, at 298K, 101kPa
     • NB: Exothermic = Soluble
   • Enthalpy change of hydration
     • The enthalpy change when 1 mole of gaseous ions dissolves in sufficient water to form a very dilute solution
   • Enthalpy of neutralisation
     • Enthalpy change when enough acid/alkali is neutralised to make 1 mole H2O

2. Hess' Law

   • The enthalpy change going from reactant to product is independent of the route taken

3. Acids/bases

   • Acid
     • Proton donor
   • Base
     • Proton acceptor
   • Strong acid
     • One that ionises fully
   • Weak acid
     • One that only partially ionises
   • Conjugate acid/base pairs
     • A conjugate acid becomes its own conjugate base when it gives a H+ to another base. Vice versa for base
     • NB: If conj. acid is v/ strong, conj. base is v/ weak

4. Kw

   • Ionic product of water
   • NB: 1x10-4

5. pH

   • -log(Concentration of H+)

6. Ka

   • Acid dissociation constant

7. Indicators

   • Weak acids which have 2 colours (one for acid, one for its conj. base)

8. Buffer

   • A system that minimises pH changes on addition of moderate amounts of acid/alkali

9. Rates

   • Rate of Reaction
     • How the concentration of a reactant decreases over time
   • Order
     • The power to which we raise the conc. of a particular reactant in a rate equation
   • Rate constant (k)
     • The proportionality constant in a rate equation
   • Rate Determining Step (RDS)
     • The slowest step in a reaction mechanism (series of reactions not shown in overall reaction)
     • Determines overall rate
   • Half-life
     • The time taken for the concentration of a reactant to halve

10. Free energy (ΔG)

    • The balance between enthalpy, entropy and temperature for a process
    • NB: Reaction is spontaneous is ΔG<0