Gibbs free energy and entropy

• entropy is the degree or measure of disorder/ randomness (S)
• solid is more ordered than liquids. liquids are more ordered than gases - GASSES ARE THE MOST DISSORDERED

  • H20(s) = 48 JK-1 mol-1
  • H20(l) = 70 JK-1 mol-1
  • H20(g) = 189 JK mol-1
• the HIGHER the entropy value = the HIGHER degree of disorder of a substance
• crystalline solid = highly ordered arrangement; low entropy

  melts becoming less ordered = entropy increases

  • ordered metallic and ionic lattice = Low ENTROPY
• • melting point (s to l) there is a large increase in disorder
  • boiling point (l to g) there is a very large increase in disorder as the atoms are more far apart from each other

  
• the higher the mr value of the substance in the SAME state = the higher the entropy value

  • H2 (g) = 131 Jkmol CO2 (g) = 214 JKmol

  mr= 2 mr= 44
• entropy change = sum of entropy values of products - sum of entropy values of reactants
• determining entropy:

  1. how much DISORDER there is aka the state
  2. DISSOLUTION = if the solid is able to dissolve it increases its disorder so entropy increases
  3. AMOUNT OF PARTICLES = more particles mean more entropy as there are more ways their energy can be arranged
• free energy change (ΔG) is a measure used to predict whether a reaction is feasible
• feasible reaction is one that once started will carry on to completion without any energy being supplied to it
• feasibility of a reaction depends on:

  • enthalpy change of a reaction
  • entropy change of a reaction
  • temperature at which reaction is occurring
• If  free energy change (ΔG) is 0 or negative = reaction is FEASIBLE
• free energy change [ΔG] = enthalpy change [ΔH] – (temperature x entropy change [ΔS])
• units for: ∆G = ∆H - T∆S

  • ΔG = J mol-1
  • ΔH = J mol-1
  • T = K
  • ΔS = J K-1 mol-1

  ensure units are consistent so to convert KJ into J we x1000
• when working out minimum temperature for a reaction to become feasible you must convert ∆G = ∆H - T∆S

  • T = ∆ H / ∆ S

  we assume ∆G is 0
• for a reaction to be feasible (temp x entropy change) part of the equation must be BIGGER than (enthalpy change)
• free-energy graphs: ∆G vs T

  the graph is a straight line so can use y = mx + c rearranged

  • gradient (m) = -entropy change {-∆S}
  • y - intercept (c) = enthalpy change {∆H}

  equation

  ∆ G = (-∆ S x T) + ∆ H