Chemistry lecture 12

• Thermochemical equation


  Contains energy component
• Properties of thermochemical equation

  • Balanced for charge/mass
  • States & sign of ΔH shown
  • ΔH increase by same factor as equation
• Enthalpy of Reaction (ΔrHº)

  Change in enthalpy (H) during a chemical reaction (r) where the mole quantities of the chemicals and their states are specified by the reaction equation
• Standard Enthalpy of Formation (ΔfHº)

  Energy released or absorbed when one mole of product is formed from its constituents at STP
• Standard Enthalpy of Combustion (ΔcHº)

  Energy released when one mole of a substance is completely burnt in oxygen at STP
• Hess's Law


  The overall reaction enthalpy is the sum of reaction enthalpies of individual reactions into which a reaction may be divided
• Nitrogen and oxygen gas combine to form nitrogen dioxide according to the following reaction: N2(g) + 2O2 → 2NO2(g)
• Calculating ΔH from Hess's Law

  Add up the two reactions keeping all the reactants on the left and all the products on the right to get the overall equation
• Calculating ΔH from Hess's Law (example 2)

  1. Multiply the whole reaction by a factor to get the required reactants

  2. Multiply the whole reaction by a factor to get the required products

  3. Flip the whole reaction and change sign of ΔH
• Bond Energy


  Minimum energy required to break one mole of a specified bond in a gaseous molecule
• Breaking chemical bonds is endothermic, forming chemical bonds is exothermic
• ΔH


  Sum of energy of bonds broken - sum of energy of bonds formed
• Calculating ΔrH from ΔfHº

  Total energy is sum of energy of products minus sum of energy of reactants
• Most spontaneous reactions are exothermic and occur rapidly because the loss of enthalpy energy (ΔH) drives these reactions
• Enthalpy is a function of internal energy (U) and product of Pressure-Volume (PV)
• Not all exothermic reactions are spontaneous, some endothermic reactions are spontaneous too
• Entropy (S)


  Measure of the disorder in the system expressed as k*lnW, where k is a proportionality constant equal to the ideal gas constant (R) divided by Avogadro's number (6.022 x 10^23) and lnW is the natural log of W, the number of equivalent ways of describing the state of a system
• Whenever a spontaneous change occurs in our universe, the total entropy of our universe increases (ΔStotal > 0)
• Many reactions are spontaneous at high temperatures (T), however some reactions are spontaneous only at low temperatures
• Gibbs Free Energy (G)


  Measure of the free energy in the system, expressed as H - TS
• Change in free energy (ΔG)


  ΔG = ΔH - TΔS
• Reaction classification based on ΔG

  • Endergonic - NON-SPONTANEOUS, ΔG > 0
  • Exergonic - SPONTANEOUS, ΔG < 0
  • Equilibrium, ΔG = 0
• Favorable conditions for spontaneous reactions: ΔH < 0, ΔS > 0
• Unfavorable conditions for spontaneous reactions: ΔH > 0, ΔS < 0