5.2 Enthalpy

• Enthalpy: (H) is the total amount of thermal energy in a substance.

  • Can not be calculated but we have some theoretical values from observing changes and bond types.

• Enthalpy Change: (ΔH) is the energy absorbed from or released to the environment during a chemical or physical change.

  

If:

• ΔH > 0, endothermic reaction
• ΔH < 0, exothermic reaction

Molar Enthalpy Change

• Energy that it takes when 1 mol of a substance undergoes a physical/chemical/nuclear change.
• Measured in joules per mole (J/mol)
• ΔHr (r = reaction type)
• Ex: molar enthalpy of a physical change: vaporization of water
  • H2O(l) → H2O(g)
  • ΔHvap = 40.8 kJ/mol

Types of Molar Enthalpy Changes

Formula

• To obtain the change in energy involved in a given change, you must look up the molar enthalpy and apply it to a known amount of a substance.

  

Where:

• n is the amount in moles
• ΔHr is the molar enthalpy change of the reaction

Representing Molar Enthalpy Changes

• Endothermic and exothermic reactions can be represented by thermochemical equations.
• These are balanced chemical reactions, which include the energy changes that occur in the system.

Thermochemical Equations

• A chemical equation that describes the enthalpy change of a reaction is called a thermochemical equation

  

Potential Energy Diagrams