High School Chemistry/Thermochemistry & States of Matter

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Thermochemistry is a branch of chemistry that deals with how heat energy changes during chemical reactions.

The Law of Conversation of Energy is that energy, the capacity to do work or supply heat, cannot be created nor destroyed. It can only be changed from one state of matter to another (a hot object to a cooler object). Remember that in an endothermic process, heat is absorbed (feels cold), while in an exothermic process, heat is released (feels hot).

  • Heat energy: Represented by "q" is the TOTAL energy released/absorbed (UNITS: J/kJ/cal)
  • Enthalpy: Represented by "ΔH" is the energy released/absorbed PER MOLE (UNITS: kJ/mol)
  • Thermochemical Equation: A chemical equation that has an "ΔH" (or quantity of heat released/absorbed during a reaction per mole).
  • Heat can either be produced, therefore a product, or it can be absorbed, therefore a reactant.
  • The heat of reaction represents the quantity of energy is taken for the reaction to occur.

A way to calculate heat released/absorbed in a thermochemical equation is to use the Heat of Formation, which is the amount of energy released to create a substance. Most formations of substances commonly listed are exothermic.

A calorie is the amount of heat required to raise the temperature of 1 gram of pure water by 1 degree celsius. A joule is a SI unit of heat energy. (CONVERSION FACTOR: 1 cal/4.184 J; This is also water's specific heat value, which is very high)

Calculating q[edit | edit source]


  • q = heat energy (J)
  • m = mass (g or moles)
  • Cp = specific heat capacity (J/gC)
  • ΔT = Tf - Ti (C)

Phase Changes[edit | edit source]

  • ΔHfus (molar heat of fusion: melt (s --> l)) = -ΔHsolid (molar heat of solidifiction: freeze (l --> s)
  • ΔHvap (molar heat of vaporization: vaporize (l --> g)) = -ΔHcond (molar heat of condensation: condense (g --> l)