Lesson 3: Phase and Chemical Changes

• Phase change

  A change in the state of matter without any change in the chemical composition of the system
• Types of phase changes

  • Exothermic (energy is released)
  • Endothermic (energy is absorbed)
• Phase changes always involve energy changes but never involve temperature changes during the actual change in state. For example, if liquid water is at 0 degrees, it starts to freeze and it only changes temperature until the freezing is complete
• A change in potential energy is taking place due to changes in the attractions between particles (IMF for molecular substances)
• Chemical change

  A change in the composition of the system as reactants are converted to new substances (products)
• Energy changes in chemical changes involve changes in bonding, thus the type of energy involved is potential energy
• Changes in potential energy are due to changes in the covalent and ionic bonds between atoms and ions, respectively
• Chemical changes involve more energy than phase changes because the ionic or covalent bonds are much stronger than IMF involved in phase changes
• For all reactions, energy is absorbed for bonds to break and is released when new bonds form
• Enthalpy (H)

  The total internal energy of a substance at constant pressure (measured in J or kJ)
• Enthalpy change (ΔH)

  A measure of the difference in potential energy of the products and the potential energy of the reactants in a phase or chemical change (H final - H initial)

  
• Molar enthalpy

  The enthalpy change involving one mole of a substance
• Enthalpy notation

  • ΔHfus (fusion)
  • ΔHsolid (solidification)
  • ΔHvap (vaporization)
  • ΔHcond (condensation)
  • ΔHsoln (solution)
  • ΔHcomb (combustion)
  • ΔHf (formation)
  • ΔHdecomp (decomposition)
• Positive ΔH value indicates an endothermic process, negative ΔH value indicates an exothermic process
• Ways to represent enthalpy changes

  • Thermochemical equations
  • ΔH notation
  • Enthalpy diagrams
• Thermochemical equations

  1. As a term in the equation

  2. In ΔH notation

  
• Addition of a catalyst will not affect the molar enthalpy of the reaction
• Enthalpy diagrams

  • Exothermic reaction (CaO + H2O → Ca(OH)2)
  • Endothermic reaction (H2O → H2 + 1/2 O2)

  
• a fusion phase change is endothermic (solid to liquid) because energy is absorbed to overcome intermolecular forces
• solidification is exothermic (liquid to solid) because energy is released to strengthen the intermolecular forces
• vaporization is endothermic (liquid to gas) because energy is required to break the intermolecular forces between molecules
• condensation is exothermic (gas to liquid) because energy is released as new intermolecular bonds form
• sublimation is endothermic (solid to gas) because energy is needed to break the strong intermolecular forces that hold the particles together
• sublimation from gas to solid is exothermic because the intermolecular forces are stronger and the system releases energy
• energy exchange is needed to disrupt intermolecular forces
• chemical changes focus on potential energy because energy is stored in bonds and a specific energy is needed to break it
• all systems release and absorb energy, but depending on the net energy change, it is endothermic or exothermic
• enthalpy is the measurement of internal energy in a system
• molar enthalpy is the energy it takes for one mole of substance to change state or react
• positive H = endothermic

  negative H = exothermic
• processes with positive molar enthalpy changes:

  • fusion
  • vaporization
  • solutions (sometimes)
  • decomposition (mostly)
• processes with negative molar enthalpy changes:

  • solidification
  • condensation
  • solution (sometimes)
  • combustion
  • formation (mostly)
• the molar enthalpy change for vaporization is positive since it is endothermic, because energy is needed to overcome the IMFs and change it from a liquid to a gas
• the molar enthalpy change for condensation is negative since it is exothermic, because energy is released as the IMFs are formed again when changing back into a liquid
• if there are two moles of a substance, there are twice as many atoms and bonds, which means more energy is absorbed or released to break or form bonds when there is more than 1 mole
• exothermic reactions mean the reactant bonds are stronger and contain more energy than the product bonds
• endothermic reactions mean the product bonds have more energy than the reactant bonds
• Label the parts of the energy diagram:

  A) H

  B) kJ

  C) reaction progress

  
• 1. ∆H°vap for water = 40.7 kJ/mol (+ve value = endothermic)

  2. ∆H°cond  for water = - 40.7 kJ/mol (-ve value = exothermic)