Heating and Cooling Curves

• Molar Enthalpies - changes in phases are accompanied by energy changes
• Endothermic Process - energy added to substance - q is positive - bonds broken
• Exothermic Process - energy released from substances - bonds form
• Phase changes occur at specific temperatures and pressures

  • freezing of water: 0°C and 1 atm
  • evaporation of water: 100°C and 1 atm
• Deposition - gas to solid
• Phase Change at constant pressure (P)

  q = n Δ H

  where;

  q = heat absorbed or released (J)

  n = amount of substance (moles)

   Δ H = molar heat associated with phase change (J/mol)

   Δ Hfus = melting 0°C = 6000 J/mol

   Δ Hvap = evaporation 100°C= 40400 J/mol
• Temperature Change

  q = mcΔT

  where;

  q = heat absorbed or released J

  m = mass (g)

  c = specific heat J/(g ⋅ °C -1) (depends on the phases of the substance)

  ΔT = final temp - initial temp
• c (specific heat)

  c = water = 4.181 J/g ⋅ °C -1

  c = ice = 2.11 J/g ⋅ °C -1

  c = water vapor = 2.08 J/g ⋅ °C -1
• The diagram process :

  • Solve each turning point or segments of the diagram using the two formulas
  • Add all q to get the total heat required